CVE-2025-3031: Mozilla Firefox Information Disclosure Bug

CVE-2025-3031 Overview

CVE-2025-3031 is an information disclosure vulnerability in the Just-In-Time (JIT) compiler used by Mozilla Firefox and Thunderbird. An attacker can read 32 bits of values spilled onto the stack during execution of a JIT compiled function. The flaw allows web content to leak fragments of process memory through crafted JavaScript that triggers specific JIT compilation paths. Mozilla resolved the issue in Firefox 137 and Thunderbird 137. The vulnerability is classified under [CWE-200] Information Exposure and requires no authentication or user interaction beyond visiting a malicious page.

Critical Impact
Remote attackers can leak 32-bit fragments of stack memory from JIT compiled functions, potentially exposing sensitive process data through crafted web content.

Affected Products

Mozilla Firefox versions prior to 137
Mozilla Thunderbird versions prior to 137
Mozilla Firefox ESR builds sharing the affected JIT code paths

Discovery Timeline

2025-04-01 - CVE-2025-3031 published to the National Vulnerability Database
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-3031

Vulnerability Analysis

The vulnerability resides in the SpiderMonkey JIT compiler used by Firefox and Thunderbird. When the JIT compiles a function, it allocates register slots for intermediate values. Values that exceed available registers are spilled to the stack. The JIT emits code that reads and writes these spilled values during execution.

Under the vulnerable code path, the compiler reads 32 bits of a spilled value even when the logical operand width or initialization state does not guarantee that all 32 bits are defined. Attacker controlled JavaScript can shape the operand types and control flow to induce the compiler to emit code that returns previously written or uninitialized stack contents. The leaked bits reach the JavaScript runtime, where scripts can inspect them numerically.

Root Cause

The defect stems from improper handling of value widths during stack spill and reload operations in the JIT backend. The compiler assumes the full 32-bit slot is defined by the producing instruction, but certain operand paths write only a narrower portion. Subsequent reloads then include adjacent bits belonging to earlier computations or uninitialized memory.

Attack Vector

Exploitation requires an attacker to serve malicious JavaScript to a targeted browser or email client that renders remote content. The script crafts operations that trigger JIT compilation, forces register spills, and reads the resulting values. Repeated invocations can be used to sample stack memory across many function calls. The vulnerability does not permit direct code execution or heap corruption, but the leaked data can weaken address space layout randomization or expose secrets held on the stack.

The vulnerability is described in the Mozilla advisories MFSA-2025-20 and MFSA-2025-23, with implementation details tracked in Mozilla Bug 1947141.

Detection Methods for CVE-2025-3031

Indicators of Compromise

Browser or Thunderbird processes running versions below 137 while executing JavaScript from untrusted origins
Repeated JIT tier-up events on script-controlled functions that perform arithmetic on narrow integer types
Outbound network requests correlating with leaked numeric values encoded as query parameters or POST bodies

Detection Strategies

Inventory endpoints for Firefox and Thunderbird builds and flag any release earlier than 137
Monitor web proxy logs for repeated requests to previously unseen domains delivering JavaScript-heavy payloads
Correlate email client telemetry with links or remote content that trigger Thunderbird's JavaScript engine

Monitoring Recommendations

Track browser version telemetry through endpoint management and alert on hosts pinned to unpatched builds
Enable extended process telemetry to capture child process creation and network egress from firefox.exe and thunderbird.exe
Route browser and mail client logs into a centralized data lake for retrospective hunting once new indicators emerge

How to Mitigate CVE-2025-3031

Immediate Actions Required

Upgrade Mozilla Firefox to version 137 or later on all managed endpoints
Upgrade Mozilla Thunderbird to version 137 or later, including any ESR channels sharing the affected code
Restrict Thunderbird from loading remote content in HTML messages until patching is complete
Enforce automatic updates through enterprise policy so future JIT fixes deploy without user action

Patch Information

Mozilla fixed the flaw in Firefox 137 and Thunderbird 137. Refer to Mozilla Security Advisory MFSA-2025-20 and Mozilla Security Advisory MFSA-2025-23 for release notes and download locations.

Workarounds

Disable the JavaScript JIT by setting javascript.options.baselinejit and javascript.options.ion to false in about:config where compatibility permits
Block untrusted script execution by deploying content filtering or enterprise policy controls on Firefox and Thunderbird
Configure Thunderbird to render messages as plain text to prevent JavaScript execution triggered by remote HTML content

bash

# Configuration example: enforce Firefox auto-update via enterprise policy on Linux
sudo tee /etc/firefox/policies/policies.json > /dev/null <<'EOF'
{
  "policies": {
    "DisableAppUpdate": false,
    "AppAutoUpdate": true,
    "OverrideFirstRunPage": "",
    "Preferences": {
      "javascript.options.ion": { "Value": false, "Status": "locked" },
      "javascript.options.baselinejit": { "Value": false, "Status": "locked" }
    }
  }
}
EOF