CVE-2024-7524 in Firefox
Summary
by MITRE • 08/06/2024
Firefox adds web-compatibility shims in place of some tracking scripts blocked by Enhanced Tracking Protection. On a site protected by Content Security Policy in "strict-dynamic" mode, an attacker able to inject an HTML element could have used a DOM Clobbering attack on some of the shims and achieved XSS, bypassing the CSP strict-dynamic protection. This vulnerability affects Firefox < 129, Firefox ESR < 115.14, and Firefox ESR < 128.1.
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Analysis
by VulDB Data Team • 03/15/2025
This vulnerability represents a sophisticated bypass of Firefox's Enhanced Tracking Protection mechanisms that leverages the intersection of web compatibility shims and Content Security Policy restrictions. The core issue arises from Firefox's approach to maintaining web compatibility while enforcing strict tracking protection measures. When websites employ Content Security Policy in strict-dynamic mode, they establish a security model that relies on dynamic script loading while maintaining strict control over script execution. However, Firefox's implementation of web-compatibility shims creates an unexpected attack surface that undermines this protection.
The technical flaw manifests through DOM Clobbering attacks that exploit the shims added by Firefox to maintain compatibility with blocked tracking scripts. These shims are essentially placeholder elements that Firefox injects into web pages to ensure that legacy scripts continue to function despite being blocked by Enhanced Tracking Protection. When an attacker can inject HTML elements into a page, they can manipulate the DOM in such a way that these shims become vulnerable to clobbering attacks. The vulnerability occurs because the shims are not properly isolated from user-controlled input, allowing attackers to manipulate their properties and achieve cross-site scripting execution.
The operational impact of this vulnerability is significant as it demonstrates how browser security mechanisms can be bypassed through seemingly benign compatibility features. The attack vector requires only HTML injection capability, which is often achievable through various web application vulnerabilities such as reflected XSS, stored XSS, or server-side injection flaws. This makes the vulnerability particularly dangerous because it can be exploited in environments where CSP strict-dynamic is properly configured but the browser's compatibility shims create unexpected execution paths. The vulnerability affects multiple Firefox versions, including regular releases and extended support releases, indicating a widespread impact across different deployment scenarios.
The security implications extend beyond simple XSS execution as this vulnerability demonstrates how browser compatibility features can inadvertently create security holes that bypass well-established security controls. This attack pattern aligns with ATT&CK technique T1211 which involves manipulating loaded scripts to bypass security controls. From a CWE perspective, this vulnerability relates to CWE-79 Cross-site Scripting and CWE-1234 Web Application Compatibility Features that introduce security vulnerabilities. Organizations should implement immediate mitigation strategies including updating to affected Firefox versions, reviewing Content Security Policy configurations, and considering additional security layers such as Subresource Integrity checks. The vulnerability also highlights the importance of proper sandboxing and isolation of compatibility features within browsers, as well as the need for more comprehensive testing of security controls in the presence of browser compatibility mechanisms.
This vulnerability serves as a critical reminder of the complex security landscape that modern browsers must navigate, where the need to maintain web compatibility creates potential security risks that may not be immediately apparent. The interplay between Enhanced Tracking Protection, Content Security Policy strict-dynamic mode, and web compatibility shims creates a particularly challenging attack surface that requires careful consideration when implementing security controls. The fix for this vulnerability required modifications to how Firefox handles these compatibility shims and their interaction with DOM elements, demonstrating the complexity of modern browser security implementations where seemingly isolated features can create unexpected security implications.