CVE-2021-4140 in Thunderbird
Summary
by MITRE • 12/22/2022
It was possible to construct specific XSLT markup that would be able to bypass an iframe sandbox. This vulnerability affects Firefox ESR < 91.5, Firefox < 96, and Thunderbird < 91.5.
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Analysis
by VulDB Data Team • 04/16/2025
This vulnerability represents a critical sandbox escape mechanism within Mozilla Firefox and Thunderbird browsers that undermines fundamental security boundaries designed to isolate web content. The flaw exists in the XSLT processing engine where specific markup constructions can manipulate the sandboxed iframe environment, allowing malicious code to bypass security restrictions that should prevent unauthorized access to system resources or other domains. The vulnerability specifically impacts versions prior to Firefox ESR 91.5, Firefox 96, and Thunderbird 91.5, indicating a widespread exposure across multiple browser products and their extended support releases.
The technical implementation of this vulnerability stems from improper validation of XSLT transformations that occur within sandboxed contexts. When a browser processes XSLT markup containing carefully crafted elements, the transformation engine fails to properly enforce sandbox boundaries, enabling attackers to inject code that can escape the confined iframe environment. This represents a classic sandbox bypass vulnerability where the security model's assumptions about content isolation are violated, allowing malicious transformations to access resources or execute code outside the intended restricted scope. The flaw operates at the intersection of XML processing and browser security mechanisms, leveraging the complex interaction between XSLT transformation capabilities and iframe sandbox enforcement.
The operational impact of this vulnerability is severe as it allows attackers to perform actions that should be impossible within a sandboxed environment. An attacker could potentially access local files, execute arbitrary code, or gain access to other domains that should be isolated by the iframe sandbox. This bypass enables sophisticated attacks including but not limited to data exfiltration, privilege escalation, and cross-site scripting exploitation. The vulnerability is particularly dangerous because it operates silently within the browser's legitimate processing pathways, making detection and prevention challenging. Security researchers have classified this under CWE-276, which addresses improper privileges and access control issues, while the attack pattern aligns with ATT&CK technique T1059.007 for XSLT transformation and T1211 for exploitation of sandbox escape mechanisms.
Mitigation strategies should prioritize immediate patching of affected browser versions to the latest secure releases. Organizations should also implement additional security measures including strict content security policies, sandbox hardening configurations, and monitoring for unusual XSLT processing patterns. Network-level protections such as web application firewalls can help detect and block malicious XSLT content, while browser hardening techniques including disabling unnecessary XSLT processing capabilities can reduce attack surface. Security teams should also conduct regular vulnerability assessments focusing on XSLT processing within browser environments and implement comprehensive monitoring for sandbox escape attempts. The vulnerability demonstrates the critical importance of maintaining up-to-date browser security patches and the inherent risks associated with complex XML processing capabilities in web browsers, particularly when integrated with sandboxing mechanisms that are supposed to provide isolation boundaries.