CVE-2023-29539 in Thunderbird
Summary
by MITRE • 06/02/2023
When handling the filename directive in the Content-Disposition header, the filename would be truncated if the filename contained a NULL character. This could have led to reflected file download attacks potentially tricking users to install malware. This vulnerability affects Firefox < 112, Focus for Android < 112, Firefox ESR < 102.10, Firefox for Android < 112, and Thunderbird < 102.10.
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Analysis
by VulDB Data Team • 05/09/2026
This vulnerability represents a critical security flaw in web browsers and email clients that handle file downloads through the Content-Disposition header mechanism. The issue stems from improper handling of NULL characters within filename directives, creating a potential attack vector for malicious actors to manipulate file download behavior. When a web application or email client encounters a filename containing a NULL character in the Content-Disposition header, the system truncates the filename at that point, effectively stripping away potentially critical portions of the intended file name. This truncation behavior creates a pathway for attackers to craft malicious download scenarios that could deceive users into executing harmful files under seemingly benign names. The vulnerability specifically affects multiple Mozilla products including Firefox versions prior to 112, Firefox ESR versions prior to 102.10, Firefox for Android versions prior to 112, Focus for Android versions prior to 112, and Thunderbird versions prior to 102.10, indicating a widespread impact across the Mozilla ecosystem.
The technical implementation of this vulnerability falls under the category of improper input validation and handling of special characters in HTTP headers. When browsers process the Content-Disposition header, they typically parse the filename parameter to determine how to name downloaded files. The presence of a NULL character (0x00) within this parameter causes the parsing routine to terminate prematurely, resulting in truncated filenames. This behavior creates a mismatch between the intended file name and the actual file name that gets saved on the user's system. Security researchers have classified this type of vulnerability according to CWE-121, which deals with buffer overflow conditions, and CWE-122, which addresses buffer overflow in heap-based buffers. The vulnerability also aligns with ATT&CK technique T1195.001, which covers the use of malicious file downloads to execute payloads on target systems, as the truncated filename could be manipulated to appear legitimate while actually delivering malicious content.
The operational impact of this vulnerability extends beyond simple filename truncation, creating potential for sophisticated social engineering attacks. Attackers could exploit this behavior by crafting Content-Disposition headers with carefully placed NULL characters to truncate filenames in ways that make malicious downloads appear as legitimate files. For instance, a malicious file named "update.exe" could be truncated to "update" or "upda" potentially tricking users into executing the file when they expect to see a benign file like "update.zip" or "patch.exe". This manipulation capability allows threat actors to bypass simple user verification mechanisms that rely on file extensions or names. The vulnerability particularly affects environments where users download files from untrusted sources or where automatic execution of downloaded files is enabled, as the truncated filename could lead to confusion about the actual file type and purpose. The risk is amplified in mobile environments where users may be less cautious about file downloads due to the limited visual cues available on smaller screens.
Organizations and users should immediately update all affected Mozilla products to their latest versions to remediate this vulnerability. System administrators should implement network-level controls to monitor and filter Content-Disposition headers for suspicious NULL character sequences, though this approach may not be comprehensive as the vulnerability can be exploited through various attack vectors. Security teams should consider implementing user awareness training to educate personnel about the risks of downloading files from untrusted sources and the importance of verifying file integrity before execution. The vulnerability also underscores the importance of proper input validation in web applications, particularly when handling user-supplied data in HTTP headers. Organizations should conduct security assessments to identify any custom applications or services that might be vulnerable to similar issues in their own codebases. Additionally, browser security teams should consider implementing additional safeguards in their parsing routines to prevent premature termination of filename processing, potentially by implementing stricter validation of HTTP header parameters or by normalizing special characters before processing. The vulnerability serves as a reminder that seemingly minor parsing issues can create significant security risks when combined with user interaction patterns and the trust models inherent in web browsing and email environments.