Mozilla Firefox ESR and Thunderbird: Mitigation bypass in File Handling component
Summary
A mitigation bypass in the File Handling component of Mozilla Firefox ESR and Thunderbird weakens a security protection of the application. According to the CVSS vector (8.3, HIGH), an attacker who gets a user to interact with malicious content could affect confidentiality, integrity and availability, with a scope change. Mozilla has fixed the issue in Firefox ESR 153.5 and Thunderbird 153.5.
Affected
- Mozilla Firefox ESR before 153.5
- Mozilla Thunderbird before 153.5
- Third-party data (VulDB) gives a Firefox range up to 153.4; not confirmed by Mozilla.
Fixed in
- Mozilla Firefox ESR 153.5
- Mozilla Thunderbird 153.5
- Newer builds are available; deploy the latest build of your branch.
Workaround
Unknown. No vendor workaround is documented; updating is the remedy.
Exploitation
- Not known to be exploited.
- CISA SSVC: exploitation none, automatable no, technical impact total.
- No public proof of concept known.
Our recommendation
- Update Firefox ESR and Thunderbird to version 153.5 or later on all endpoints.
- Check which installations are on the ESR channel or pinned to older builds, and prioritise managed endpoints.
- Remind users to be cautious with untrusted links and attachments until patching is complete.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Mozilla MFSA 2026-108 www.mozilla.org
- Mozilla MFSA 2026-110 www.mozilla.org
- Mozilla Bugzilla 2075533 bugzilla.mozilla.org
- Ubuntu CVE tracker ubuntu.com
- VulDB (third-party) vuldb.com
- HOL (third-party) hol.org
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 1 Nov 2026, 13:54.
Original description (NVD)
Mitigation bypass in the File Handling component. This vulnerability was fixed in Firefox ESR 153.5 and Thunderbird 153.5.
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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