CVE-2011-3656 in Firefox
Summary
by MITRE • 06/02/2021
Cross-site scripting (XSS) vulnerability in Mozilla Firefox before 3.6.24 and 4.x through 7 allows remote attackers to inject arbitrary web script or HTML via vectors involving HTTP 0.9 errors, non-default ports, and content-sniffing.
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Analysis
by VulDB Data Team • 03/04/2025
The CVE-2011-3656 vulnerability represents a significant cross-site scripting flaw affecting Mozilla Firefox versions prior to 3.6.24 and 4.x through 7. This vulnerability stems from the browser's handling of HTTP 0.9 error responses combined with content-sniffing mechanisms, creating a dangerous intersection that allows remote attackers to execute arbitrary web scripts or HTML code within the context of a user's browser session. The flaw specifically exploits how Firefox processes malformed HTTP responses and attempts to determine content types through aggressive content-sniffing behavior, which can be manipulated by malicious actors to bypass security restrictions.
The technical implementation of this vulnerability involves multiple attack vectors that work in concert to compromise browser security. When Firefox encounters HTTP 0.9 errors, it processes these responses in a manner that can be manipulated to trigger unintended content-sniffing behavior. The vulnerability specifically leverages the browser's content-type detection mechanisms, which are designed to identify the correct MIME type of received content. However, when combined with non-default port scenarios and malformed HTTP responses, these mechanisms can be tricked into interpreting malicious payloads as legitimate content, effectively allowing attackers to inject scripts that execute with the privileges of the user's browsing session.
The operational impact of CVE-2011-3656 extends beyond simple script injection, as it can enable sophisticated attacks including session hijacking, credential theft, and redirection to malicious sites. Attackers can craft HTTP responses that appear to be legitimate content but contain embedded malicious scripts, which execute automatically when the browser attempts to render the content. This vulnerability particularly affects users who access websites on non-standard ports, as the browser's content-sniffing behavior becomes more aggressive and less predictable in these scenarios. The attack surface is further expanded by the fact that the vulnerability can be exploited across different versions of Firefox, making it a widespread concern for organizations relying on these browser versions.
This vulnerability maps directly to CWE-79, which describes cross-site scripting flaws where untrusted data is incorporated into web pages without proper validation or encoding. The ATT&CK framework categorizes this as a technique for code injection, specifically targeting the browser's rendering engine and content processing capabilities. The exploitation requires minimal user interaction beyond visiting a malicious website, making it particularly dangerous in phishing campaigns or compromised websites. Security researchers have noted that the vulnerability's exploitation is particularly effective in environments where users frequently visit websites on non-standard ports or when web applications fail to properly validate HTTP responses. The combination of HTTP 0.9 error handling, content-sniffing behavior, and port-based attack vectors creates a complex exploit scenario that demonstrates the importance of proper input validation and secure content handling in web browsers.
Organizations should immediately upgrade to Firefox versions 3.6.24 or later, or 4.x through 7, which contain patches addressing this vulnerability. System administrators should implement network-level protections including web application firewalls and content filtering systems to detect and block malicious HTTP responses. Users should be educated about the risks of visiting untrusted websites, particularly those on non-standard ports, and should be encouraged to maintain updated browser versions. The vulnerability highlights the critical importance of content-type validation and proper error handling in web browsers, as well as the need for comprehensive security testing of HTTP response processing mechanisms. Regular security audits should include testing for similar content-sniffing vulnerabilities in other browser components and web applications to prevent similar exploitation scenarios.