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

CVE-2026-6757 Overview

CVE-2026-6757 is an invalid pointer vulnerability in the JavaScript WebAssembly component shared by Mozilla Firefox and Mozilla Thunderbird. The flaw is tracked under CWE-824 (Access of Uninitialized Pointer) and affects the JIT and execution path used when parsing or running WebAssembly code. Mozilla addressed the issue in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. Exploitation requires the targeted user to load attacker-controlled web content, making drive-by web pages and HTML email the primary delivery channels.

Critical Impact
A remote attacker who lures a user to a crafted page can trigger memory corruption in the WebAssembly engine, leading to limited compromise of confidentiality, integrity, and availability within the browser process.

Affected Products

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

Discovery Timeline

2026-04-21 - CVE-2026-6757 published to NVD
2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-6757

Vulnerability Analysis

The vulnerability resides in the WebAssembly (Wasm) implementation inside SpiderMonkey, the JavaScript engine shared by Firefox and Thunderbird. The component dereferences a pointer that has not been correctly initialized or validated before use. Mozilla classifies the issue as an invalid pointer access, mapped to CWE-824.

Exploitation requires user interaction. The victim must navigate to a malicious page or open HTML email content that delivers a crafted WebAssembly module. Once the engine compiles and executes the module, the invalid dereference occurs in the renderer process, where Mozilla's sandbox limits the immediate scope of impact.

Refer to the Mozilla Bug Report #2013588 and the consolidated Mozilla Security Advisory MFSA-2026-30 for further technical context.

Root Cause

The root cause is an uninitialized or improperly tracked pointer within the WebAssembly execution path. When the engine reaches a state where it expects a valid object reference, it instead operates on memory with undefined contents. The resulting access produces predictable corruption that an attacker can leverage during exploitation chains.

Attack Vector

The attack vector is network based and triggered through standard browser navigation or message rendering. An attacker hosts a page containing a malicious WebAssembly payload and induces the victim to visit it. In Thunderbird, the same code path is reachable through HTML message content when remote resources or scripted content are processed. No prior authentication or local access is required.

No verified public exploit, proof-of-concept, or in-the-wild exploitation has been reported. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-6757

Indicators of Compromise

Unexpected crashes of firefox.exe, firefox-bin, or thunderbird processes with WebAssembly-related stack frames in crash telemetry.
Outbound connections from browser or mail client processes to newly observed domains immediately after page load or message preview.
Browser child process spawning unusual child processes such as command shells or scripting hosts.

Detection Strategies

Inventory installed Firefox, Firefox ESR, and Thunderbird versions across managed endpoints and flag any below the fixed builds (150 / 140.10).
Hunt for renderer-process crashes correlated with WebAssembly stack traces in endpoint telemetry and browser crash reports.
Correlate proxy and DNS logs with browser process activity to identify visits to suspicious WebAssembly-hosting sites preceding crash events.

Monitoring Recommendations

Enable extended process and module-load logging for browser and mail client binaries.
Monitor for child process creation chains originating from Firefox or Thunderbird that deviate from baseline behavior.
Track patch deployment status against Mozilla's advisory IDs MFSA-2026-30, 32, 33, and 34.

How to Mitigate CVE-2026-6757

Immediate Actions Required

Upgrade Firefox to version 150 or later and Firefox ESR to 140.10 or later on all managed endpoints.
Upgrade Thunderbird to version 150 or later, or to ESR 140.10 or later, on all user systems.
Restart browser and mail client sessions after patching to ensure the vulnerable code path is unloaded from memory.

Patch Information

Mozilla released fixes in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. Details are documented in Mozilla Security Advisory MFSA-2026-30, MFSA-2026-32, MFSA-2026-33, and MFSA-2026-34.

Workarounds

Configure Thunderbird to render messages as plain text to reduce exposure of the WebAssembly code path through HTML email.
Restrict execution of untrusted web content using enterprise policies, content filtering, or browser isolation for high-risk users.
Disable JavaScript on untrusted sites through site-specific permissions until patching is complete.

bash

# Configuration example: enforce minimum Firefox version via enterprise policy (policies.json)
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true,
    "ExtensionUpdate": true,
    "OverrideFirstRunPage": "",
    "BlockAboutConfig": false
  }
}