Mozilla Firefox: Mitigation bypass in File Handling component
Summary
A mitigation bypass in the File Handling component of Mozilla Firefox and Thunderbird allows security protections to be circumvented. Mozilla fixed it in Firefox 157.0.1 and Thunderbird 158. The NVD score of 9.8 (Critical) comes from a worst-case CISA assessment, while Mozilla rates the issue as moderate, and it is unclear whether user interaction such as opening or downloading a file is required.
Affected
- Firefox versions before 157.0.1 (per Mozilla MFSA 2026-104)
- Thunderbird versions before 158 (per Tenable plugin; Mozilla's MFSA 2026-109 confirmation not verified)
- Firefox ESR: unknown
- Linux distribution packages (e.g. Debian thunderbird): unknown, check your distribution tracker
- Bundled copies, e.g. the Playwright v1.64.1 Docker image with Firefox 157.0
Fixed in
- Firefox 157.0.1 or later
- Thunderbird 158 or later (158.0.1 or later recommended)
- Firefox ESR and distribution packages: unknown
Workaround
- No workaround published. Install the fixed version.
Exploitation
- No known exploitation in the wild (not verified on a CISA page).
- No public proof of concept found; Mozilla's bug details are restricted.
- CISA SSVC: Exploitation none, Automatable yes, Technical Impact total.
Our recommendation
- Confirm Firefox is 157.0.1 or later and Thunderbird is 158 or later, ideally 158.0.1 or later.
- Check Firefox ESR, Linux distribution packages and bundled copies in test or CI images (e.g. Playwright containers), as these may not update automatically.
- Use endpoint management or enterprise policy tooling to verify versions across all workstations.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Mozilla MFSA 2026-104 (Firefox 157.0.1) www.mozilla.org
- Mozilla MFSA 2026-109 (Thunderbird) www.mozilla.org
- Mozilla Bugzilla 2067465 bugzilla.mozilla.org
- OSV entry DEBIAN-CVE-2026-106016 osv.dev
- Playwright issue 43518 (bundled Firefox) github.com
- Tenable Nessus plugin 365350 (Thunderbird) www.tenable.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 30 Oct 2026, 21:07.
Original description (NVD)
Mitigation bypass in the File Handling component. This vulnerability was fixed in Firefox 157.0.1 and Thunderbird 158.
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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