CVE-2024-7527: Mozilla Firefox Use-After-Free Vulnerability

CVE-2024-7527 Overview

CVE-2024-7527 is a use-after-free vulnerability in Mozilla Firefox, Firefox ESR, and Thunderbird. The flaw resides in the JavaScript engine's garbage collector, where unexpected marking work at the start of the sweeping phase could leave references to already-freed memory. An attacker who convinces a user to visit a crafted web page can trigger memory corruption and potentially execute arbitrary code in the content process. The issue is tracked under [CWE-416: Use After Free] and affects Firefox versions earlier than 129, Firefox ESR earlier than 115.14 and 128.1, and Thunderbird earlier than 115.14 and 128.1.

Critical Impact
Successful exploitation can lead to arbitrary code execution within the browser process, enabling sandbox-stage attacks against confidentiality, integrity, and availability of the host.

Affected Products

Mozilla Firefox versions prior to 129
Mozilla Firefox ESR versions prior to 115.14 and 128.1
Mozilla Thunderbird versions prior to 115.14 and 128.1

Discovery Timeline

2024-08-06 - CVE-2024-7527 published to NVD
2025-03-18 - Last updated in NVD database

Technical Details for CVE-2024-7527

Vulnerability Analysis

The vulnerability lives in SpiderMonkey, the JavaScript engine used by Firefox and Thunderbird. SpiderMonkey performs incremental garbage collection in distinct phases, including marking and sweeping. At the transition into the sweeping phase, the collector unexpectedly performed additional marking work on objects that were already scheduled for reclamation. This race between marking and sweeping desynchronized the collector's view of object liveness from the heap's actual state. The result is a dangling reference: code paths continued to operate on memory that the sweeper had freed or was about to free. Mozilla addressed the defect by ensuring marking completes before sweeping begins, restoring the invariants the collector relies on. The vulnerability requires user interaction, typically loading a malicious page or rendering hostile HTML email content in Thunderbird.

Root Cause

The root cause is improper sequencing in the garbage collector's state machine. Marking work executed after the sweep phase began allowed the engine to reference objects that the sweeper would free, producing a classic use-after-free condition tracked by Mozilla as bug 1871303.

Attack Vector

An attacker hosts a crafted web page that allocates and frees JavaScript objects in patterns designed to trigger the marking-sweeping race. When the victim visits the page, the engine reaches the vulnerable state and dereferences freed memory. In Thunderbird, equivalent content delivered through a remote-content email can reach the same code paths. Reliable exploitation typically chains the freed allocation with attacker-controlled data to redirect execution flow inside the content process.

No public proof-of-concept code or in-the-wild exploitation has been reported. Refer to the Mozilla Bug Report #1871303 for technical specifics.

Detection Methods for CVE-2024-7527

Indicators of Compromise

Unexpected crashes in firefox.exe, firefox-bin, or thunderbird processes with stack traces referencing js::gc marking or sweeping routines.
Creation of child processes or shell invocations from a browser or mail client content process.
Outbound network connections from the browser process to uncategorized or newly registered domains following the rendering of untrusted content.

Detection Strategies

Inventory Firefox, Firefox ESR, and Thunderbird installations and flag any build older than the fixed versions (Firefox 129, ESR 115.14, ESR 128.1, Thunderbird 115.14, Thunderbird 128.1).
Hunt for browser process anomalies such as unexpected memory region allocations marked RWX, code injection patterns, or unusual module loads inside firefox.exe or thunderbird.exe.
Correlate browser crash telemetry with web proxy logs to identify users who visited the same URL prior to repeated content-process failures.

Monitoring Recommendations

Forward EDR process telemetry and Windows Error Reporting or core_pattern crash dumps to a central data lake for retrospective hunting.
Alert on Mozilla application binaries spawning interpreters such as powershell.exe, cmd.exe, bash, or wscript.exe.
Track browser version strings in HTTP user-agent and endpoint inventory data to confirm patch coverage across the fleet.

How to Mitigate CVE-2024-7527

Immediate Actions Required

Upgrade Firefox to version 129 or later on all managed endpoints.
Upgrade Firefox ESR to 115.14 or 128.1 on systems pinned to the extended support channel.
Upgrade Thunderbird to 115.14 or 128.1 and restart the client to ensure the patched binary is loaded.
Validate update coverage through software inventory tooling and quarantine endpoints still running vulnerable builds.

Patch Information

Mozilla released fixes in the advisories MFSA-2024-33, MFSA-2024-34, MFSA-2024-35, MFSA-2024-37, and MFSA-2024-38. The patch corrects the garbage collector sequencing so that incremental marking finishes before sweeping starts.

Workarounds

Disable JavaScript in Thunderbird by ensuring javascript.enabled is set to false in the configuration editor, accepting reduced functionality for some message content.
Block rendering of remote content in Thunderbird until the client is updated.
Restrict browsing to trusted sites through web filtering or enterprise policy where patching is delayed.

bash

# Verify installed Firefox version on Linux
firefox --version

# Verify installed Thunderbird version on Linux
thunderbird --version

# Example Windows enterprise policy (policies.json) to disable JavaScript in PDFs
# Location: C:\Program Files\Mozilla Firefox\distribution\policies.json
{
  "policies": {
    "PDFjs": {
      "EnableScripting": false
    },
    "DisableAppUpdate": false
  }
}