CVE-2026-8390 Overview
CVE-2026-8390 is a use-after-free vulnerability in the JavaScript WebAssembly component of Mozilla Firefox. The flaw is tracked under CWE-416 and is exploitable over the network through crafted web content. A remote attacker can trigger memory corruption by serving a malicious page that invokes the affected WebAssembly code paths. Mozilla addressed the issue in Firefox 150.0.3 and documented the fix in the Mozilla Security Advisory MFSA-2026-45.
Critical Impact
Remote attackers can trigger memory corruption in the browser through malicious WebAssembly content, potentially leading to crashes or further exploitation.
Affected Products
- Mozilla Firefox versions prior to 150.0.3
- Firefox builds incorporating the vulnerable JavaScript WebAssembly component
- Desktop Firefox distributions on Windows, macOS, and Linux
Discovery Timeline
- 2026-05-12 - CVE-2026-8390 published to the National Vulnerability Database (NVD)
- 2026-05-14 - Last updated in NVD database
Technical Details for CVE-2026-8390
Vulnerability Analysis
The vulnerability is a use-after-free condition within Firefox's JavaScript WebAssembly subsystem. A use-after-free occurs when code continues to reference memory after it has been released, allowing attacker-controlled data to occupy the freed region. In a browser context, this typically results in a crash and may enable code execution if the attacker can shape the heap predictably. The flaw is reachable from web content because WebAssembly modules can be loaded and instantiated by any origin the browser visits. Mozilla classified the issue as high impact and corrected it in Firefox 150.0.3. Additional engineering context is available in the Mozilla Bug Report #2038081.
Root Cause
The defect stems from the JavaScript WebAssembly component releasing an object while another execution path still holds a reference to it. When the dangling reference is dereferenced, the engine operates on memory that may have been reallocated for unrelated data. This violates object lifetime guarantees inside the JavaScript engine.
Attack Vector
Exploitation requires a victim to load attacker-controlled web content in a vulnerable Firefox build. No authentication or user interaction beyond normal browsing is required. A malicious site can deliver a WebAssembly module that drives the engine into the vulnerable state. The attack scope is limited to the browser process, but successful exploitation can corrupt heap memory accessible to the renderer.
Vulnerability mechanism (prose description):
1. A WebAssembly module is instantiated by attacker-controlled JavaScript.
2. An internal object backing the module is freed during a transition or error path.
3. A surviving reference is later dereferenced by the engine.
4. Heap content that replaced the freed allocation is processed as the original object type.
Detection Methods for CVE-2026-8390
Indicators of Compromise
- Unexpected Firefox renderer crashes with stack traces involving WebAssembly internals.
- Crash reporter submissions referencing js::wasm modules immediately after visiting an untrusted site.
- Outbound connections from Firefox processes to domains serving large or obfuscated .wasm payloads.
Detection Strategies
- Inventory installed Firefox versions across managed endpoints and flag any build earlier than 150.0.3.
- Correlate browser process crash events with web proxy logs to identify the page that triggered the failure.
- Monitor for unusual child process creation or memory anomalies originating from firefox.exe or equivalent binaries.
Monitoring Recommendations
- Forward browser crash telemetry and endpoint process events to a centralized analytics platform for correlation.
- Alert on Firefox processes spawning shells, scripting hosts, or LOLBins shortly after browsing activity.
- Track HTTP responses with Content-Type: application/wasm from low-reputation domains in web gateway logs.
How to Mitigate CVE-2026-8390
Immediate Actions Required
- Update Firefox to version 150.0.3 or later on all managed endpoints.
- Enforce browser update policies through enterprise management tooling to prevent rollback to vulnerable builds.
- Restrict access to untrusted sites via web filtering until patching is verified across the fleet.
Patch Information
Mozilla fixed the vulnerability in Firefox 150.0.3. The patch is described in the Mozilla Security Advisory MFSA-2026-45 and tracked in Mozilla Bug Report #2038081. Administrators should validate the installed version on each endpoint after deployment.
Workarounds
- Block or restrict WebAssembly execution through enterprise browser policies where supported.
- Use web content filtering to limit exposure to untrusted origins until patches are applied.
- Run Firefox under standard user privileges to reduce post-exploitation impact.
# Verify installed Firefox version on Linux/macOS
firefox --version
# Windows: query installed version from the registry
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
