CVE-2025-6435 in Firefox
Summary
by MITRE • 06/24/2025
If a user saved a response from the Network tab in Devtools using the Save As context menu option, that file may not have been saved with the `.download` file extension. This could have led to the user inadvertently running a malicious executable. This vulnerability affects Firefox < 140 and Thunderbird < 140.
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Analysis
by VulDB Data Team • 11/14/2025
This vulnerability represents a critical file extension handling flaw in Mozilla Firefox and Thunderbird browsers that could enable arbitrary code execution through social engineering attacks. The issue stems from improper file naming conventions when users save network responses through the browser's developer tools interface, specifically the Network tab context menu. When users right-click on network requests and select the "Save As" option, the browser fails to automatically append the appropriate `.download` file extension to saved files, creating a potential security risk that could be exploited by attackers.
The technical flaw manifests in the browser's file handling mechanism within the developer tools component, where the Save As functionality does not properly validate or enforce file extensions for downloaded content. This behavior creates a dangerous precedent where saved files could appear as legitimate documents or scripts but actually contain malicious executables. The vulnerability specifically affects versions prior to Firefox 140 and Thunderbird 140, indicating that this was a long-standing issue that required significant browser updates to address properly. The flaw operates at the application-level file handling rather than at the network protocol level, making it particularly insidious as it exploits user trust in the browser's developer tools functionality.
The operational impact of this vulnerability extends beyond simple file naming issues and represents a serious escalation path for attackers who could craft malicious network responses designed to exploit this behavior. An attacker could serve a malicious file through a network request that appears to be a legitimate document or script, but when saved by a user through the browser's developer tools, it would lack the proper file extension that would typically trigger security warnings or prevent automatic execution. This creates a scenario where users might inadvertently execute malicious code simply by using legitimate browser functionality, making the attack surface significantly broader than typical file extension-based exploits.
Security researchers have classified this vulnerability under the broader category of improper input validation and file handling issues that can lead to privilege escalation or arbitrary code execution. The flaw aligns with common attack patterns described in the attack tree framework where user interaction is required to complete the exploit chain, making it particularly relevant to the attack techniques catalogued in the ATT&CK framework under initial access and execution phases. Organizations using affected browser versions should immediately implement mitigation strategies including browser updates, user education about the risks of saving network responses, and network-level controls to prevent the delivery of potentially malicious content. The vulnerability also highlights the importance of proper file extension validation in web applications and browser components, as outlined in various security standards including those related to secure coding practices and input sanitization techniques.
The fix for this vulnerability required modifications to the browser's developer tools implementation to ensure that all saved files automatically receive appropriate file extensions, particularly the `.download` suffix that would alert users to the nature of the file and trigger appropriate security mechanisms. This update represents a fundamental improvement in how browsers handle file downloads through developer interfaces, addressing a gap that could have been exploited in targeted attacks against users who regularly use browser developer tools for debugging or analysis purposes. The vulnerability demonstrates the critical importance of maintaining security hygiene in all browser components, particularly those that provide powerful debugging capabilities that users might employ in security-sensitive contexts.