CVE-2026-12303: Firefox Information Disclosure Vulnerability

CVE-2026-12303 Overview

CVE-2026-12303 is an information disclosure vulnerability in the Graphics: WebGPU component shared by Mozilla Firefox and Mozilla Thunderbird. The flaw stems from incorrect boundary conditions in WebGPU operations, classified under [CWE-125] (Out-of-Bounds Read). A remote attacker can serve crafted web content that triggers the boundary error and reads adjacent memory the page should not access. Exploitation requires user interaction, such as visiting a malicious page or rendering attacker-controlled content in Thunderbird. Mozilla addressed the issue in Firefox 152 and Thunderbird 152, documented in advisories MFSA-2026-57 and MFSA-2026-60.

Critical Impact
A remote attacker can disclose limited in-process memory contents from Firefox or Thunderbird users who render attacker-controlled WebGPU content.

Affected Products

Mozilla Firefox versions prior to 152
Mozilla Thunderbird versions prior to 152
Graphics: WebGPU component in both products

Discovery Timeline

2026-06-16 - CVE-2026-12303 published to NVD
2026-06-17 - Last updated in NVD database
Mozilla advisories - Fixes documented in MFSA-2026-57 and MFSA-2026-60

Technical Details for CVE-2026-12303

Vulnerability Analysis

The vulnerability resides in the WebGPU implementation within Mozilla's Graphics subsystem. WebGPU exposes a low-level GPU programming interface to web content, including buffer creation, texture sampling, and compute shader dispatch. Incorrect boundary checks within this component allow operations to read past the intended end of a buffer or texture region. The result is an out-of-bounds read that returns adjacent process memory to the calling script context. Exposed data may include rendering state, shader inputs, or other content held in the renderer process at the time of the read.

Root Cause

The root cause is improper validation of size or offset parameters supplied to WebGPU operations. The bounds calculation does not correctly account for all input combinations, producing a read range that extends beyond the allocated object. Refer to Mozilla Bug 2034608 for the underlying implementation details.

Attack Vector

An attacker hosts a page or HTML email that issues crafted WebGPU calls when rendered. In Firefox, a user must visit the page. In Thunderbird, the content must be processed by the rendering pipeline. The attacker's script then reads back out-of-bounds memory through legitimate WebGPU return paths and exfiltrates it to a controlled endpoint. No privilege or authentication is required, but user interaction is necessary, limiting drive-by reach.

No public proof-of-concept is available. The vulnerability mechanism is described in the Mozilla advisories and the linked Bugzilla entry rather than reproduced here.

Detection Methods for CVE-2026-12303

Indicators of Compromise

Firefox or Thunderbird process versions below 152 observed in endpoint inventory after the patch release date
Outbound HTTP/HTTPS requests from browser processes carrying unusually large or binary payloads following WebGPU-heavy page loads
Browser telemetry showing repeated WebGPU buffer or texture operations from low-reputation domains

Detection Strategies

Inventory installed Firefox and Thunderbird builds across managed endpoints and flag versions earlier than 152
Correlate web proxy logs with browser process telemetry to identify sessions loading WebGPU content from untrusted origins
Alert on child process or network anomalies originating from firefox.exe, firefox, or thunderbird binaries following media-heavy navigation events

Monitoring Recommendations

Forward browser version data and process telemetry into a centralized analytics platform for continuous version compliance checks
Track Mozilla security advisory feeds, including MFSA-2026-57 and MFSA-2026-60, to detect new WebGPU-related issues
Monitor egress traffic patterns from end-user browsers for anomalous data volumes after GPU-intensive page activity

How to Mitigate CVE-2026-12303

Immediate Actions Required

Update Mozilla Firefox to version 152 or later on all endpoints
Update Mozilla Thunderbird to version 152 or later, including managed enterprise deployments
Validate patch deployment through software inventory and endpoint management tooling

Patch Information

Mozilla released fixes in Firefox 152 and Thunderbird 152. The corresponding advisories are MFSA-2026-57 for Firefox and MFSA-2026-60 for Thunderbird. Implementation notes are tracked in Mozilla Bug 2034608.

Workarounds

Disable WebGPU by setting dom.webgpu.enabled to false in about:config until patches are applied
Restrict Thunderbird remote content rendering for untrusted senders through enterprise policy
Apply web proxy or DNS controls to block access to known-malicious domains hosting WebGPU exploitation content

bash

# Configuration example: disable WebGPU via Firefox enterprise policy
# /etc/firefox/policies/policies.json
{
  "policies": {
    "Preferences": {
      "dom.webgpu.enabled": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}