Mozilla has urgently patched a critical zero-interaction vulnerability in its Firefox browser (CVE-2025-5263), identified as MFSA-TMP-2025-0001, which allows remote attackers to execute arbitrary code without user interaction.
The flaw, fixed in Firefox 139 released on May 27, 2025, stems from a memory corruption bug in the libvpx video codec library used for WebRTC communications.
Alongside this critical fix, Mozilla addressed eight additional security issues ranging from cross-origin data leaks to local code execution risks, underscoring the importance of immediate updates for all Firefox users.
The most severe patched flaw, a double-free vulnerability in the libvpx encoder (vpx_codec_enc_init_multi), occurs when initializing WebRTC video sessions.
During failed memory allocations, the encoder improperly frees memory blocks twice, corrupting the browser’s heap and creating opportunities for attackers to crash or execute malicious code.
According to Mozilla’s advisory, this vulnerability requires no user interaction—attackers could exploit it simply by forcing the victim to load a malicious WebRTC session, such as via a compromised website or phishing link.
Randell Jesup, a Mozilla security engineer, discovered the flaw during routine code audits (Bug 1962421).
The critical severity underscores its potential for widespread exploitation, particularly in environments where Firefox is used for real-time communication tools.
Memory corruption vulnerabilities in libvpx are historically high-value targets due to their proximity to media processing pipelines, which often handle untrusted content.
Mozilla has since refactored the encoder’s error-handling routines to prevent double-free scenarios.
Security Patches Address Multiple Attack Vectors
Firefox 139 also resolves two moderate-severity flaws in the “Copy as cURL” developer tool feature.
Insufficient escaping of newline (CVE-2025-5264) and ampersand (CVE-2025-5265) characters could allow attackers to trick users into pasting malicious commands into terminals, leading to local code execution.
For example, a malicious website might prompt users to copy a manipulated cURL command that, when executed, runs arbitrary shell scripts. The ampersand vulnerability specifically affects Windows users due to differences in command-line parsing.
Cross-origin information leaks were another focal point. A scripting error isolation flaw (CVE-2025-5263) and script element event disclosures (CVE-2025-5266) could enable cross-site leaking (XS–Leaks) attacks, allowing attackers to infer sensitive data across origins.
Meanwhile, a clickjacking vulnerability (CVE-2025-5267) in the payment card autofill UI risked exposing saved credit card details through deceptive overlays.
Memory safety bugs, a perennial challenge in complex codebases, were also addressed.
Mozilla’s fuzzing team identified multiple memory corruption issues (CVE-2025-5268, CVE-2025-5272) in Firefox, Thunderbird, and ESR branches, some of which showed signs of exploitability.
These fixes complement low-severity patches for encrypted SNI bypasses (CVE-2025-5270) and DevTools’ CSP header ignorance (CVE-2025-5271).
Enterprise Implications and Urgency of Updates
The libvpx vulnerability’s zero-interaction nature makes it particularly dangerous for enterprises, where Firefox is often deployed across thousands of endpoints.
Attackers could integrate this flaw into exploit chains to bypass perimeter defenses, especially in sectors reliant on WebRTC for conferencing or customer support.
Mozilla’s advisory urges all users to update to Firefox 139 immediately, while enterprise administrators should expedite deployment through centralized management tools.
While no active exploits have been confirmed, the public disclosure of technical details increases the likelihood of rapid weaponization.
Organizations using Firefox ESR must apply version 128.11 to mitigate parallel memory safety risks. As browser-based attacks grow increasingly sophisticated, proactive patch management remains critical to thwarting advanced persistent threats.
Mozilla’s continued investment in fuzzing and static analysis tools highlights the evolving battlefield of browser security.
For developers, this incident reinforces the importance of rigorous error handling in memory-sensitive operations—a lesson that extends far beyond Firefox’s codebase.
Find this Story Interesting! Follow us on LinkedIn and X to Get More Instant Updates.
