Critical

Use-after-free in Mozilla Firefox JavaScript Engine

IdentifiersCVE-2026-0884CWE-416· Use After Free

CVE-2026-0884 is a use-after-free vulnerability in the JavaScript Engine component of Mozilla Firefox. The flaw affects Firefox versions prior to 147, Firefox ESR versions prior to 140.7, Thunderbird versions prior to 147, and Thunderbird ESR versions prior to 140.7. A use-after-free condition indicates that the JavaScript engine can access memory after it has been released, creating a memory corruption condition that may be exploitable. The available source material does not identify the specific vulnerable function or code path, but Mozilla classifies the issue in the JavaScript Engine and tracks it in Bugzilla bug 2003588.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation can result in memory corruption in the browser process. Based on the provided CVSS v3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H), the vulnerability may allow a remote attacker to achieve high impact to confidentiality, integrity, and availability, potentially including arbitrary code execution in the browser context. The broader advisory context for the release notes that multiple browser flaws could enable arbitrary code execution if exploited. No confirmed in-the-wild exploitation is provided in the supplied content.

Mitigation

If you can’t patch tonight, do this now.

If immediate patching is not possible, reduce exposure to untrusted web content. Practical interim measures from the provided advisories include limiting browsing to trusted sites, using web or network filtering to block malicious domains, and in high-risk environments considering temporary restriction or disabling of JavaScript where operationally feasible. Additional host-level containment such as OS sandboxing or mandatory access controls may reduce risk, but no complete workaround is provided; patching remains the effective mitigation.

Remediation

Patch, then assume compromise.

Upgrade affected products to fixed versions. Mozilla fixed the issue in Firefox 147, Firefox ESR 140.7, Thunderbird 147, and Thunderbird ESR 140.7. On Debian systems, apply the relevant security updates for firefox-esr and thunderbird as provided by Debian security advisories. This is the primary and recommended remediation.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

MozillaFirefoxapplication

MozillaThunderbirdapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

13 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

13 SOURCESView more in app

cyber security newsNews

Jan 15, 2026

Firefox 147 Released With Fixes for 16 Vulnerabilities that Enable Arbitrary Code Execution

Use-after-free in the JavaScript Engine (moderate severity) that could lead to memory corruption and potentially code execution depending on exploitability.

cvefeed high severityNews

Jan 13, 2026

CVE-2026-0884 - Use-after-free in the JavaScript Engine component

Use-after-free vulnerability (CWE-416) in Mozilla Firefox’s JavaScript Engine component affecting Firefox versions prior to 147 and Firefox ESR prior to 140.7.

What this page doesn’t show

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This page is what’s public. Mallory adds the parts that aren’t: which of your assets are affected, which adversaries are exploiting it right now, which detections to deploy, and what to do tonight.

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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity9