CVE-2024-4769 in Thunderbird
Summary
by MITRE • 05/14/2024
When importing resources using Web Workers, error messages would distinguish the difference between `application/javascript` responses and non-script responses. This could have been abused to learn information cross-origin. This vulnerability affects Firefox < 126, Firefox ESR < 115.11, and Thunderbird < 115.11.
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Analysis
by VulDB Data Team • 01/21/2026
This vulnerability in Firefox and Thunderbird stems from an information disclosure issue within the web worker resource import mechanism. The flaw exists in how the browser handles error messages when processing different content types during web worker operations. Specifically, when a web worker attempts to import resources, the error messages returned by the browser contain distinguishable characteristics between javascript responses and non-javascript responses. This differential handling creates a side-channel information leak that can be exploited by malicious actors to infer cross-origin resource characteristics.
The technical implementation of this vulnerability involves the browser's response handling within web worker contexts. When a web worker processes a resource import operation, the system generates error messages that vary based on whether the response is of content type application/javascript or other content types. This distinction in error message formatting or structure provides attackers with a means to determine the nature of responses from cross-origin sources without direct access to the content itself. The vulnerability essentially creates a timing or structural side-channel that reveals metadata about the underlying response types.
The operational impact of this vulnerability extends beyond simple information disclosure as it enables potential attackers to perform cross-origin reconnaissance attacks. An attacker could potentially determine whether a cross-origin resource is a javascript file or other content type, which might reveal information about the target application's architecture or expose patterns in how resources are organized and served. This information could then be leveraged to craft more sophisticated attacks or to identify additional vulnerabilities in the target system. The vulnerability affects not only the browser's security model but also undermines the fundamental principle of cross-origin isolation that web applications rely upon for security.
The vulnerability aligns with CWE-200 (Information Exposure) and represents a classic example of a side-channel attack that exploits implementation details rather than direct protocol flaws. From an ATT&CK framework perspective, this vulnerability maps to T1566 (Phishing with Social Engineering) and T1071.004 (Application Layer Protocol: DNS) as attackers could use this information to refine their reconnaissance efforts. The affected versions include Firefox versions prior to 126, Firefox ESR versions prior to 115.11, and Thunderbird versions prior to 115.11, indicating this was a widespread issue affecting the core browser security model. Mitigation strategies should focus on ensuring that error messages returned during resource import operations do not contain distinguishable characteristics between different content types, requiring consistent error handling regardless of response content type. Organizations should immediately update to the patched versions and consider implementing additional network-level controls to prevent exploitation of this information disclosure channel.