CVE-2026-6757: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-6757 Overview

CVE-2026-6757 is an invalid pointer vulnerability affecting the JavaScript WebAssembly component in Mozilla Firefox and Thunderbird. This memory corruption flaw occurs when processing WebAssembly content, potentially allowing an attacker to manipulate memory through crafted web content. The vulnerability has been classified as CWE-824 (Access of Uninitialized Pointer), indicating that the affected component accesses a pointer that has not been properly initialized before use.

Critical Impact

An attacker could exploit this invalid pointer vulnerability to cause memory corruption, potentially leading to code execution, information disclosure, or application crashes when users visit malicious websites or open specially crafted email content in Thunderbird.

Affected Products

• Mozilla Firefox versions prior to 150
• Mozilla Firefox ESR versions prior to 140.10
• Mozilla Thunderbird versions prior to 150 and Thunderbird ESR versions prior to 140.10

Discovery Timeline

• April 21, 2026 - CVE-2026-6757 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6757

Vulnerability Analysis

This vulnerability exists within the WebAssembly (Wasm) engine used by Mozilla's JavaScript implementation. The flaw involves an invalid pointer access during WebAssembly execution, which falls under the category of uninitialized pointer vulnerabilities. When processing certain WebAssembly modules, the engine may access a pointer before it has been properly initialized, leading to undefined behavior.

The vulnerability requires user interaction for exploitation—a victim must navigate to a malicious webpage or open a crafted email in Thunderbird. While the attack complexity is low and no privileges are required, the scope remains unchanged from the vulnerable component, with limited impacts to confidentiality, integrity, and availability.

Root Cause

The root cause is CWE-824: Access of Uninitialized Pointer. The WebAssembly component contains a code path where a pointer variable is accessed before being properly initialized. This can occur during the compilation or execution of WebAssembly bytecode, where certain edge cases or malformed input trigger the use of the uninitialized pointer.

Attack Vector

The attack vector is network-based, requiring an attacker to serve malicious content to a victim's browser. The exploitation scenario involves:

1. An attacker crafts a malicious WebAssembly module designed to trigger the invalid pointer condition
2. The victim visits a webpage containing the malicious WebAssembly code or opens a crafted email in Thunderbird
3. The browser's JavaScript engine loads and processes the WebAssembly module
4. During execution, the invalid pointer is accessed, potentially leading to memory corruption
5. Depending on memory layout, this could result in information disclosure, denial of service, or potentially arbitrary code execution

Since no public exploit code is currently available, and the vulnerability is not listed in CISA's Known Exploited Vulnerabilities catalog, the immediate risk of exploitation in the wild remains limited. However, organizations should prioritize patching given the network-accessible nature of the vulnerability.

Detection Methods for CVE-2026-6757

Indicators of Compromise

• Unexpected browser or Thunderbird crashes, particularly when visiting unfamiliar websites or opening emails with embedded content
• Memory access violations or crash dumps referencing WebAssembly-related components in Firefox or Thunderbird
• Unusual JavaScript execution patterns or WebAssembly module loading from suspicious domains

Detection Strategies

• Monitor for abnormal WebAssembly compilation activity or unusual Wasm module sizes that may indicate exploitation attempts
• Deploy endpoint detection solutions capable of identifying memory corruption exploitation techniques
• Enable browser crash reporting and analyze crash data for patterns consistent with CVE-2026-6757 exploitation
• Implement network monitoring to detect connections to known malicious domains serving WebAssembly exploits

Monitoring Recommendations

• Review Mozilla security advisories and the associated Bugzilla report #2013588 for additional technical indicators
• Configure SentinelOne agents to monitor for suspicious browser process behavior and memory anomalies
• Enable enhanced logging for JavaScript and WebAssembly execution in enterprise browser deployments

How to Mitigate CVE-2026-6757

Immediate Actions Required

• Update Mozilla Firefox to version 150 or later immediately
• Update Mozilla Firefox ESR to version 140.10 or later
• Update Mozilla Thunderbird to version 150 or Thunderbird ESR to version 140.10
• Verify update deployment across all managed endpoints using your endpoint management solution

Patch Information

Mozilla has released security patches addressing this vulnerability across multiple product lines. The fixes are documented in the following security advisories:

• Mozilla Security Advisory MFSA-2026-30
• Mozilla Security Advisory MFSA-2026-32
• Mozilla Security Advisory MFSA-2026-33
• Mozilla Security Advisory MFSA-2026-34

Organizations should prioritize deploying these updates through their standard patch management processes. The vulnerability is fixed in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird ESR 140.10.

Workarounds

• Disable WebAssembly in Firefox by navigating to about:config and setting javascript.options.wasm to false (may break functionality on some websites)
• Implement content filtering to block WebAssembly content from untrusted sources at the network perimeter
• Use browser isolation solutions for high-risk browsing activities until patches can be deployed
• Consider restricting access to untrusted websites through web filtering policies

# Firefox configuration to disable WebAssembly (temporary workaround)
# Add to mozilla.cfg or deploy via enterprise policies
pref("javascript.options.wasm", false);
pref("javascript.options.wasm_baselinejit", false);
pref("javascript.options.wasm_optimizingjit", false);