Zedo (trademark styled as ZEDO ) is a US and India-based advertising technology company that provides several online advertising products and services to Internet publishers, advertisers, and agencies. The company was founded in 1999 by Roy de Souza .
86-397: The company works with publishers that sell space on their web pages to online advertisers. Zedo's servers send advertisements to users' browsers. Zedo uses an HTTP cookie to track users' browsing history resulting in targeted pop-up ads and pop-under ads. The cookie is often flagged by spyware and adware removal programs. In a 2013 case study written by Amazon, Amazon described ZEDO as
172-420: A web server while a user is browsing a website and placed on the user's computer or other device by the user's web browser . Cookies are placed on the device used to access a website, and more than one cookie may be placed on a user's device during a session. Cookies serve useful and sometimes essential functions on the web . They enable web servers to store stateful information (such as items added in
258-419: A 2021 blog post, Mozilla used the term supercookie to refer to the use of browser cache as a means of tracking users across sites. A zombie cookie is data and code that has been placed by a web server on a visitor's computer or other device in a hidden location outside the visitor's web browser 's dedicated cookie storage location, and that automatically recreates a HTTP cookie as a regular cookie after
344-581: A company that "develops innovative technology solutions to help publishers sell and deliver Internet ads". In December 2021, Zedo was acquired by Discovery, Inc . Zedo was founded in September 1999 by Roy de Souza . The company is headquartered in the North Beach district of San Francisco , California, and has four research and development centers in Russia and India. In 2001, it expanded by offering
430-423: A cookie excludes the same characters, as well as = , since that is the delimiter between the name and value. The cookie standard RFC 2965 is more restrictive but not implemented by browsers. The term cookie crumb is sometimes used to refer to a cookie's name–value pair. Cookies can also be set by scripting languages such as JavaScript that run within the browser. In JavaScript, the object document.cookie
516-447: A cookie set with the foo.com domain. In the former case, the cookie will only be sent for requests to foo.com , also known as a host-only cookie. In the latter case, all subdomains are also included (for example, docs.foo.com ). A notable exception to this general rule is Edge prior to Windows 10 RS3 and Internet Explorer prior to IE 11 and Windows 10 RS4 (April 2018), which always sends cookies to subdomains regardless of whether
602-409: A cookie that has a domain of foo.com because this would allow the website example.org to control the cookies of the domain foo.com . If a cookie's Domain and Path attributes are not specified by the server, they default to the domain and path of the resource that was requested. However, in most browsers there is a difference between a cookie set from foo.com without a domain, and
688-416: A cookie's data to be read by an attacker , used to gain access to user data , or used to gain access (with the user's credentials) to the website to which the cookie belongs (see cross-site scripting and cross-site request forgery for examples). Tracking cookies , and especially third-party tracking cookies , are commonly used as ways to compile long-term records of individuals' browsing histories —
774-412: A cookie, block a cookie or whether to send a cookie to the server. The Domain and Path attributes define the scope of the cookie. They essentially tell the browser what website the cookie belongs to. For security reasons, cookies can only be set on the current resource's top domain and its subdomains, and not for another domain and its subdomains. For example, the website example.org cannot set
860-475: A cross-site scripting flaw will ensue. A reflected attack is typically delivered via email or a neutral web site. The bait is an innocent-looking URL, pointing to a trusted site but containing the XSS vector. If the trusted site is vulnerable to the vector, clicking the link can cause the victim's browser to execute the injected script. The persistent (or stored ) XSS vulnerability is a more devastating variant of
946-406: A cross-site scripting flaw: it occurs when the data provided by the attacker is saved by the server, and then permanently displayed on "normal" pages returned to other users in the course of regular browsing, without proper HTML escaping. A classic example of this is with online message boards where users are allowed to post HTML formatted messages for other users to read. For example, suppose there
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#17327768077611032-659: A formal specification started in April 1995 on the www-talk mailing list . A special working group within the Internet Engineering Task Force (IETF) was formed. Two alternative proposals for introducing state in HTTP transactions had been proposed by Brian Behlendorf and David Kristol respectively. But the group, headed by Kristol himself and Lou Montulli, soon decided to use the Netscape specification as
1118-414: A fragment of JavaScript prepared by the attacker in the security context of the targeted domain (taking advantage of a reflected or non-persistent XSS vulnerability). The definition gradually expanded to encompass other modes of code injection, including persistent and non-JavaScript vectors (including ActiveX , Java , VBScript , Flash , or even HTML scripts), causing some confusion to newcomers to
1204-435: A given page while disallowing others on the same page. For example, scripts from example.com could be allowed, while scripts from advertisingagency.com that are attempting to run on the same page could be disallowed. Content Security Policy (CSP) allows HTML documents to opt in to disabling some scripts while leaving others enabled. The browser checks each script against a policy before deciding whether to run it. As long as
1290-470: A hole. Any data received by the web application (via email, system logs, IM etc.) that can be controlled by an attacker could become an injection vector. XSS vulnerabilities were originally found in applications that performed all data processing on the server side. User input (including an XSS vector) would be sent to the server, and then sent back to the user as a web page. The need for an improved user experience resulted in popularity of applications that had
1376-541: A majority of the presentation logic (maybe written in JavaScript ) working on the client-side that pulled data, on-demand, from the server using AJAX . As the JavaScript code was also processing user input and rendering it in the web page content, a new sub-class of reflected XSS attacks started to appear that was called DOM -based cross-site scripting . In a DOM-based XSS attack, the malicious data does not touch
1462-631: A packet of data a program receives and sends back unchanged, used by Unix programmers. Magic cookies were already used in computing when computer programmer Lou Montulli had the idea of using them in web communications in June 1994. At the time, he was an employee of Netscape Communications , which was developing an e-commerce application for MCI . Vint Cerf and John Klensin represented MCI in technical discussions with Netscape Communications. MCI did not want its servers to have to retain partial transaction states, which led them to ask Netscape to find
1548-399: A potential privacy concern that prompted European and U.S. lawmakers to take action in 2011. European law requires that all websites targeting European Union member states gain " informed consent " from users before storing non-essential cookies on their device. The term cookie was coined by web-browser programmer Lou Montulli . It was derived from the term magic cookie , which is
1634-463: A remote server and the page or frame does not need to be reloaded). Another problem with script blocking is that many users do not understand it, and do not know how to properly secure their browsers. Yet another drawback is that many sites do not work without client-side scripting, forcing users to disable protection for that site and opening their systems to vulnerabilities. The Firefox NoScript extension enables users to allow scripts selectively from
1720-538: A separate company called Zinc with headquarters in Mumbai. On December 8, 2021, it was announced that Discovery had acquired Zedo, its real-time bidding and supply-side platforms to sell advertising programmatically. Zedo employees joined Discovery as part of the acquisition deal. Zedo uses HTTP cookies to track users' browsing and advertisement viewing history. A writer for The Independent called pop-unders from Zedo and other providers "annoying" while also describing
1806-509: A starting point. In February 1996, the working group identified third-party cookies as a considerable privacy threat. The specification produced by the group was eventually published as RFC 2109 in February 1997. It specifies that third-party cookies were either not allowed at all, or at least not enabled by default. At this time, advertising companies were already using third-party cookies. The recommendation about third-party cookies of RFC 2109
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#17327768077611892-542: A target domain even it is different from the origin domain, but only for safe requests such as GET (POST is unsafe) and not third-party cookies (inside iframe). Attribute SameSite=None would allow third-party (cross-site) cookies, however, most browsers require secure attribute on SameSite=None cookies. The Same-site cookie is incorporated into a new RFC draft for "Cookies: HTTP State Management Mechanism" to update RFC 6265 (if approved). Chrome, Firefox, and Edge started to support Same-site cookies. The key of rollout
1978-425: A user is logged in, and with which account they are logged in. Without the cookie, users would need to authenticate themselves by logging in on each page containing sensitive information that they wish to access. The security of an authentication cookie generally depends on the security of the issuing website and the user's web browser, and on whether the cookie data is encrypted . Security vulnerabilities may allow
2064-436: A user's shopping cart are usually stored in a database on the server, rather than in a cookie on the client. To keep track of which user is assigned to which shopping cart, the server sends a cookie to the client that contains a unique session identifier (typically, a long string of random letters and numbers). Because cookies are sent to the server with every request the client makes, that session identifier will be sent back to
2150-426: A user's web browsing habits over an extended period of time. Persistent cookies are also used for reasons such as keeping users logged into their accounts on websites, to avoid re-entering login credentials at every visit. (See § Uses , below.) A secure cookie can only be transmitted over an encrypted connection (i.e. HTTPS ). They cannot be transmitted over unencrypted connections (i.e. HTTP ). This makes
2236-469: A variety of other information maintained by the browser on behalf of the user. Cross-site scripting attacks are a case of code injection . Microsoft security-engineers introduced the term "cross-site scripting" in January 2000. The expression "cross-site scripting" originally referred to the act of loading the attacked, third-party web application from an unrelated attack-site, in a manner that executes
2322-427: A way to store that state in each user's computer instead. Cookies provided a solution to the problem of reliably implementing a virtual shopping cart . Together with John Giannandrea, Montulli wrote the initial Netscape cookie specification the same year. Version 0.9beta of Mosaic Netscape , released on October 13, 1994, supported cookies. The first use of cookies (out of the labs) was checking whether visitors to
2408-502: Is a cookie with an origin of a top-level domain (such as .com ) or a public suffix (such as .co.uk ). Ordinary cookies, by contrast, have an origin of a specific domain name, such as example.com . Supercookies can be a potential security concern and are therefore often blocked by web browsers. If unblocked by the browser, an attacker in control of a malicious website could set a supercookie and potentially disrupt or impersonate legitimate user requests to another website that shares
2494-633: Is a cross-vendor initiative that aims to provide an accurate and up-to-date list of domain name suffixes. Older versions of browsers may not have an up-to-date list, and will therefore be vulnerable to supercookies from certain domains. The term supercookie is sometimes used for tracking technologies that do not rely on HTTP cookies. Two such supercookie mechanisms were found on Microsoft websites in August 2011: cookie syncing that respawned MUID (machine unique identifier) cookies, and ETag cookies. Due to media attention, Microsoft later disabled this code. In
2580-422: Is a dating website where members scan the profiles of other members to see if they look interesting. For privacy reasons, this site hides everybody's real name and email. These are kept secret on the server. The only time a member's real name and email are in the browser is when the member is signed in , and they can't see anyone else's. Suppose that Mallory, an attacker, joins the site and wants to figure out
2666-472: Is a form of XSS vulnerability that relies on social engineering in order to trick the victim into executing malicious JavaScript code in their browser. Although it is technically not a true XSS vulnerability due to the fact it relies on socially engineering a user into executing code rather than a flaw in the affected website allowing an attacker to do so, it still poses the same risks as a regular XSS vulnerability if properly executed. Mutated XSS happens when
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2752-476: Is an HttpOnly flag which allows a web server to set a cookie that is unavailable to client-side scripts. While beneficial, the feature can neither fully prevent cookie theft nor prevent attacks within the browser. While Web 2.0 and Ajax developers require the use of JavaScript, some web applications are written to allow operation without the need for any client-side scripts. This allows users, if they choose, to disable scripting in their browsers before using
2838-431: Is by far the most basic type of web vulnerability. These holes show up when the data provided by a web client, most commonly in HTTP query parameters (e.g. HTML form submission), is used immediately by server-side scripts to parse and display a page of results for and to that user, without properly sanitizing the content. Because HTML documents have a flat, serial structure that mixes control statements, formatting, and
2924-669: Is derived from the request domain). The other two cookies, HSID and SSID , would be used when the browser requests any subdomain in .foo.com on any path (for example www.foo.com/bar ). The prepending dot is optional in recent standards, but can be added for compatibility with RFC 2109 based implementations. Cross-site scripting Cross-site scripting ( XSS ) is a type of security vulnerability that can be found in some web applications . XSS attacks enable attackers to inject client-side scripts into web pages viewed by other users. A cross-site scripting vulnerability may be used by attackers to bypass access controls such as
3010-561: Is given this characteristic by adding the HttpOnly flag to the cookie. In 2016 Google Chrome version 51 introduced a new kind of cookie with attribute SameSite with possible values of Strict , Lax or None . With attribute SameSite=Strict , the browsers would only send cookies to a target domain that is the same as the origin domain. This would effectively mitigate cross-site request forgery (CSRF) attacks. With SameSite=Lax , browsers would send cookies with requests to
3096-458: Is granted permission to access resources (like cookies etc.) on a web browser, then content from any URL with the same (1) URI scheme (e.g. ftp, http, or https), (2) host name , and (3) port number will share these permissions. Content from URLs where any of these three attributes are different will have to be granted permissions separately. Cross-site scripting attacks use known vulnerabilities in web-based applications , their servers , or
3182-563: Is her script to steal names and emails. If the script is enclosed inside a <script> element, it won't be shown on the screen. Then suppose that Bob, a member of the dating site, reaches Mallory's profile, which has her answer to the First Date question. Her script is run automatically by the browser and steals a copy of Bob's real name and email directly from his own machine. Persistent XSS vulnerabilities can be more significant than other types because an attacker's malicious script
3268-402: Is no single, standardized classification of cross-site scripting flaws, but most experts distinguish between at least two primary flavors of XSS flaws: non-persistent and persistent . Some sources further divide these two groups into traditional (caused by server-side code flaws) and DOM -based (in client-side code). The non-persistent (or reflected ) cross-site scripting vulnerability
3354-607: Is not always sufficient to prevent many forms of XSS attacks, security encoding libraries are usually easier to use. Some web template systems understand the structure of the HTML they produce and automatically pick an appropriate encoder. Many operators of particular web applications (e.g. forums and webmail) allow users to utilize a limited subset of HTML markup. When accepting HTML input from users (say, <b>very</b> large ), output encoding (such as <b>very</b> large ) will not suffice since
3440-443: Is rendered automatically, without the need to individually target victims or lure them to a third-party website. Particularly in the case of social networking sites, the code would be further designed to self-propagate across accounts, creating a type of client-side worm . The methods of injection can vary a great deal; in some cases, the attacker may not even need to directly interact with the web functionality itself to exploit such
3526-425: Is the open source NoScript add-on which, in addition to the ability to enable scripts on a per-domain basis, provides some XSS protection even when scripts are enabled. The most significant problem with blocking all scripts on all websites by default is substantial reduction in functionality and responsiveness (client-side scripting can be much faster than server-side scripting because it does not need to connect to
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3612-624: Is the treatment of existing cookies without the SameSite attribute defined, Chrome has been treating those existing cookies as if SameSite=None, this would let all website/applications run as before. Google intended to change that default to SameSite=Lax in Chrome 80 planned to be released in February 2020, but due to potential for breakage of those applications/websites that rely on third-party/cross-site cookies and COVID-19 circumstances, Google postponed this change to Chrome 84. A supercookie
3698-430: Is the use of additional security controls when handling cookie -based user authentication. Many web applications rely on session cookies for authentication between individual HTTP requests, and because client-side scripts generally have access to these cookies, simple XSS exploits can steal these cookies. To mitigate this particular threat (though not the XSS problem in general), many web applications tie session cookies to
3784-588: Is to use automated tools that will remove XSS malicious code in web pages, these tools use static analysis and/or pattern matching methods to identify malicious codes potentially and secure them using methods like escaping. When a cookie is set with the SameSite=Strict parameter, it is stripped from all cross-origin requests. When set with SameSite=Lax , it is stripped from all non-"safe" cross-origin requests (that is, requests other than GET, OPTIONS, and TRACE which have read-only semantics). The feature
3870-539: Is too late. Functionality that blocks all scripting and external inclusions by default and then allows the user to enable it on a per-domain basis is more effective. This has been possible for a long time in Internet Explorer (since version 4) by setting up its so called "Security Zones", and in Opera (since version 9) using its "Site Specific Preferences". A solution for Firefox and other Gecko -based browsers
3956-409: Is used for this purpose. For example, the instruction document.cookie = "temperature=20" creates a cookie of name temperature and value 20 . In addition to a name and value, cookies can also have one or more attributes. Browsers do not include cookie attributes in requests to the server—they only send the cookie's name and value. Cookie attributes are used by browsers to determine when to delete
4042-464: The Financial Times published an article about them on February 12, 1996. In the same year, cookies received a lot of media attention, especially because of potential privacy implications. Cookies were discussed in two U.S. Federal Trade Commission hearings in 1996 and 1997. The development of the formal cookie specifications was already ongoing. In particular, the first discussions about
4128-478: The iframe tag , link and the script tag. There are several issues with this approach, for example sometimes seemingly harmless tags can be left out which when utilized correctly can still result in an XSS Another popular method is to strip user input of " and ' however this can also be bypassed as the payload can be concealed with obfuscation . Besides content filtering, other imperfect methods for cross-site scripting mitigation are also commonly used. One example
4214-404: The same-origin policy . During the second half of 2007, XSSed documented 11,253 site-specific cross-site vulnerabilities, compared to 2,134 "traditional" vulnerabilities documented by Symantec . XSS effects vary in range from petty nuisance to significant security risk, depending on the sensitivity of the data handled by the vulnerable site and the nature of any security mitigation implemented by
4300-433: The HTTP response in order to instruct the browser to add new cookies, modify existing cookies, or remove existing cookies. To remove a cookie, the server must include a Set-Cookie header field with an expiration date in the past. The value of a cookie may consist of any printable ASCII character ( ! through ~ , Unicode \u0021 through \u007E ) excluding , and ; and whitespace characters . The name of
4386-653: The IP address of the user who originally logged in, then only permit that IP to use that cookie. This is effective in most situations (if an attacker is only after the cookie), but obviously breaks down in situations where an attacker is behind the same NATed IP address or web proxy as the victim, or the victim is changing his or her mobile IP . Another mitigation present in Internet Explorer (since version 6), Firefox (since version 2.0.0.5), Safari (since version 4), Opera (since version 9.5) and Google Chrome ,
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#17327768077614472-513: The Netscape website had already visited the site. Montulli applied for a patent for the cookie technology in 1995, which was granted in 1998. Support for cookies was integrated with Internet Explorer in version 2, released in October 1995. The introduction of cookies was not widely known to the public at the time. In particular, cookies were accepted by default, and users were not notified of their presence. The public learned about cookies after
4558-434: The actual content, any non-validated user-supplied data included in the resulting page without proper HTML encoding, may lead to markup injection. A classic example of a potential vector is a site search engine: if one searches for a string, the search string will typically be redisplayed verbatim on the result page to indicate what was searched for. If this response does not properly escape or reject HTML control characters,
4644-616: The ad-serving technology to large websites. By 2004, the use of filters to limit pop-ups and pop-unders increased, and Zedo began using intromercials—advertisements served before the requested content—as an alternate method. Zedo has also experimented with creating its own social networking sites. In 2006, the company launched Zebo.com, a social networking site, where users get shopping advice from friends who own products. In 2011, Zedo began partnering with newspaper publishers. In October 2011, Zedo spun out its ad exchange platform in India into
4730-447: The advertisements' windows as a "seemingly endless barrage". Technologist Danny Sullivan has stated that Zedo carries misleading "junk" ads linking to fake news sites. Zedo offers an option to opt out of targeted advertisements and says that it has an anti-spyware policy. HTTP cookie HTTP cookies (also called web cookies , Internet cookies , browser cookies , or simply cookies ) are small blocks of data created by
4816-422: The application. In this way, even potentially malicious client-side scripts could be inserted unescaped on a page, and users would not be susceptible to XSS attacks. Some browsers or browser plugins can be configured to disable client-side scripts on a per-domain basis. This approach is of limited value if scripting is allowed by default, since it blocks bad sites only after the user knows that they are bad, which
4902-427: The attacker injects something that is seemingly safe but is rewritten and modified by the browser while parsing the markup. This makes it extremely hard to detect or sanitize within the website's application logic. An example is rebalancing unclosed quotation marks or even adding quotation marks to unquoted parameters on parameters to CSS font-family. There are several escaping schemes that can be used depending on where
4988-590: The browser by the absence of an expiration date assigned to them. A persistent cookie expires at a specific date or after a specific length of time. For the persistent cookie's lifespan set by its creator, its information will be transmitted to the server every time the user visits the website that it belongs to, or every time the user views a resource belonging to that website from another website (such as an advertisement). For this reason, persistent cookies are sometimes referred to as tracking cookies because they can be used by advertisers to record information about
5074-473: The browser to delete the cookie at a specific date and time. Next, the browser sends another request to visit the spec.html page on the website. This request contains a Cookie header field, which contains the two cookies that the server instructed the browser to set: This way, the server knows that this HTTP request is related to the previous one. The server would answer by sending the requested page, possibly including more Set-Cookie header fields in
5160-470: The browser will not run programs from untrusted authors. Some large application providers report having successfully deployed nonce-based policies. Trusted types changes Web APIs to check that values have been trademarked as trusted. As long as programs only trademark trustworthy values, an attacker who controls a JavaScript string value cannot cause XSS. Trusted types are designed to be auditable by blue teams . Another defense approach
5246-429: The contents of the website's homepage. But it also instructs the browser to set two cookies. The first, theme , is considered to be a session cookie since it does not have an Expires or Max-Age attribute. Session cookies are intended to be deleted by the browser when the browser closes. The second, sessionToken , is considered to be a persistent cookie since it contains an Expires attribute, which instructs
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#17327768077615332-495: The cookie less likely to be exposed to cookie theft via eavesdropping . A cookie is made secure by adding the Secure flag to the cookie. An http-only cookie cannot be accessed by client-side APIs, such as JavaScript . This restriction eliminates the threat of cookie theft via cross-site scripting (XSS). However, the cookie remains vulnerable to cross-site tracing (XST) and cross-site request forgery (CSRF) attacks. A cookie
5418-469: The cookie was set with or without a domain. Below is an example of some Set-Cookie header fields in the HTTP response of a website after a user logged in. The HTTP request was sent to a webpage within the docs.foo.com subdomain: The first cookie, LSID , has no Domain attribute, and has a Path attribute set to /accounts . This tells the browser to use the cookie only when requesting pages contained in docs.foo.com/accounts (the domain
5504-456: The field of information security . XSS vulnerabilities have been reported and exploited since the 1990s. Prominent sites affected in the past include the social-networking sites Twitter and Facebook . Cross-site scripting flaws have since surpassed buffer overflows to become the most common publicly reported security vulnerability, with some researchers in 2007 estimating as many as 68% of websites are likely open to XSS attacks. There
5590-516: The form to the server. The server encodes the preferences in a cookie and sends the cookie back to the browser. This way, every time the user accesses a page on the website, the server can personalize the page according to the user's preferences. For example, the Google search engine once used cookies to allow users (even non-registered ones) to decide how many search results per page they wanted to see. Also, DuckDuckGo uses cookies to allow users to set
5676-408: The original cookie had been deleted. The zombie cookie may be stored in multiple locations, such as Flash Local shared object , HTML5 Web storage , and other client-side and even server-side locations, and when absence is detected in one of the locations, the missing instance is recreated by the JavaScript code using the data stored in other locations. A cookie wall pops up on a website and informs
5762-513: The plug-in systems on which they rely. Exploiting one of these, attackers fold malicious content into the content being delivered from the compromised site. When the resulting combined content arrives at the client-side web browser, it has all been delivered from the trusted source, and thus operates under the permissions granted to that system. By finding ways of injecting malicious scripts into web pages, an attacker can gain elevated access-privileges to sensitive page content, to session cookies, and to
5848-412: The policy only allows trustworthy scripts and disallows dynamic code loading , the browser will not run programs from untrusted authors regardless of the HTML document's structure. Modern CSP policies allow using nonces to mark scripts in the HTML document as safe to run instead of keeping the policy entirely separate from the page content. As long as trusted nonces only appear on trustworthy scripts,
5934-400: The real names of the people she sees on the site. To do so, she writes a script designed to run from other users' browsers when they visit her profile. The script then sends a quick message to her own server, which collects this information. To do this, for the question "Describe your Ideal First Date", Mallory gives a short answer (to appear normal), but the text at the end of her answer
6020-404: The same top-level domain or public suffix as the malicious website. For example, a supercookie with an origin of .com , could maliciously affect a request made to example.com , even if the cookie did not originate from example.com . This can be used to fake logins or change user information. The Public Suffix List helps to mitigate the risk that supercookies pose. The Public Suffix List
6106-465: The server every time the user visits a new page on the website, which lets the server know which shopping cart to display to the user. Another popular use of cookies is for logging into websites. When the user visits a website's login page, the web server typically sends the client a cookie containing a unique session identifier. When the user successfully logs in, the server remembers that that particular session identifier has been authenticated and grants
6192-474: The shopping cart in an online store ) on the user's device or to track the user's browsing activity (including clicking particular buttons, logging in , or recording which pages were visited in the past ). They can also be used to save information that the user previously entered into form fields , such as names, addresses, passwords , and payment card numbers for subsequent use. Authentication cookies are commonly used by web servers to authenticate that
6278-460: The site's owner network . OWASP considers the term cross-site scripting to be a misnomer . It initially was an attack that was used for breaching data across sites, but gradually started to include other forms of data injection attacks. Security on the web depends on a variety of mechanisms, including an underlying concept of trust known as the same-origin policy . This states that if content from one site (such as https://mybank.example1.com )
6364-400: The untrusted string needs to be placed within an HTML document including HTML entity encoding, JavaScript escaping, CSS escaping, and URL (or percent) encoding . Most web applications that do not need to accept rich data can use escaping to largely eliminate the risk of XSS attacks in a fairly straightforward manner. Performing HTML entity encoding only on the five XML significant characters
6450-485: The user access to its services. Because session cookies only contain a unique session identifier, this makes the amount of personal information that a website can save about each user virtually limitless—the website is not limited to restrictions concerning how large a cookie can be. Session cookies also help to improve page load times, since the amount of information in a session cookie is small and requires little bandwidth. Cookies can be used to remember information about
6536-496: The user has visited, in what sequence, and for how long. Corporations exploit users' web habits by tracking cookies to collect information about buying habits. The Wall Street Journal found that America's top fifty websites installed an average of sixty-four pieces of tracking technology onto computers, resulting in a total of 3,180 tracking files. The data can then be collected and sold to bidding corporations. Cookies are arbitrary pieces of data, usually chosen and first sent by
6622-410: The user in order to show relevant content to that user over time. For example, a web server might send a cookie containing the username that was last used to log into a website, so that it may be filled in automatically the next time the user logs in. Many websites use cookies for personalization based on the user's preferences. Users select their preferences by entering them in a web form and submitting
6708-442: The user input needs to be rendered as HTML by the browser (so it shows as " very large", instead of "<b>very</b> large"). Stopping an XSS attack when accepting HTML input from users is much more complex in this situation. Untrusted HTML input must be run through an HTML sanitization engine to ensure that it does not contain XSS code. Many validations rely on parsing out (blacklisting) specific "at risk" HTML tags such as
6794-400: The user of the website's cookie usage. It has no reject option, and the website is not accessible without tracking cookies. A cookie consists of the following components: Cookies were originally introduced to provide a way for users to record items they want to purchase as they navigate throughout a website (a virtual shopping cart or shopping basket ). Today, however, the contents of
6880-518: The viewing preferences like colors of the web page. Tracking cookies are used to track users' web browsing habits. This can also be done to some extent by using the IP address of the computer requesting the page or the referer field of the HTTP request header, but cookies allow for greater precision. This can be demonstrated as follows: By analyzing this log file, it is then possible to find out which pages
6966-445: The web server, and stored on the client computer by the web browser. The browser then sends them back to the server with every request, introducing states (memory of previous events) into otherwise stateless HTTP transactions. Without cookies, each retrieval of a web page or component of a web page would be an isolated event, largely unrelated to all other page views made by the user on the website. Although cookies are usually set by
7052-472: The web server, they can also be set by the client using a scripting language such as JavaScript (unless the cookie's HttpOnly flag is set, in which case the cookie cannot be modified by scripting languages). The cookie specifications require that browsers meet the following requirements in order to support cookies: Cookies are set using the Set-Cookie header field , sent in an HTTP response from
7138-567: The web server. Rather, it is being reflected by the JavaScript code, fully on the client side. An example of a DOM-based XSS vulnerability is the bug found in 2011 in a number of jQuery plugins. Prevention strategies for DOM-based XSS attacks include very similar measures to traditional XSS prevention strategies but implemented in JavaScript code and contained in web pages (i.e. input validation and escaping). Some JavaScript frameworks have built-in countermeasures against this and other types of attack — for example AngularJS . Self-XSS
7224-439: The web server. This header field instructs the web browser to store the cookie and send it back in future requests to the server (the browser will ignore this header field if it does not support cookies or has disabled cookies). As an example, the browser sends its first HTTP request for the homepage of the www.example.org website: The server responds with two Set-Cookie header fields: The server's HTTP response contains
7310-459: Was not followed by Netscape and Internet Explorer. RFC 2109 was superseded by RFC 2965 in October 2000. RFC 2965 added a Set-Cookie2 header field , which informally came to be called "RFC 2965-style cookies" as opposed to the original Set-Cookie header field which was called "Netscape-style cookies". Set-Cookie2 was seldom used, however, and was deprecated in RFC 6265 in April 2011 which
7396-488: Was written as a definitive specification for cookies as used in the real world. No modern browser recognizes the Set-Cookie2 header field. A session cookie (also known as an in-memory cookie , transient cookie or non-persistent cookie ) exists only in temporary memory while the user navigates a website. Session cookies expire or are deleted when the user closes the web browser. Session cookies are identified by
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