# CVE-2026-109783: Mozilla Firefox ESR mitigation bypass in File Handling

Source: https://junglewise.ai/threats/cve-2026-109783-mitigation-bypass-in-the-file-handling-component-this  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-13. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

CVE-2026-109783 · Severity: info · Published 2026-10-13

Firefox ESR is a web browser used by organizations and individuals for secure web browsing. A mitigation bypass in the File Handling component could allow an attacker to circumvent security protections designed to prevent malicious file operations, potentially leading to unauthorized file access or execution.

Technologies: [Mozilla Firefox ESR](https://junglewise.ai/threats/technologies/firefox-esr), [Mozilla Thunderbird](https://junglewise.ai/threats/technologies/thunderbird). Vendors: [Mozilla](https://junglewise.ai/threats/vendors/mozilla).

## Quick answers

**Is CVE-2026-109783 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-109783 being exploited in the wild.

**How severe is CVE-2026-109783?** Informational severity, published 13 October 2026.

**Which products and versions are affected?** Mozilla Firefox ESR; Mozilla Thunderbird.

**Is there a fix for CVE-2026-109783?** Yes: fixed in Firefox ESR 153.5, Thunderbird 153.5. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**Am I affected by CVE-2026-109783?** Check whether you run Mozilla Firefox ESR or Mozilla Thunderbird. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in Firefox ESR 153.5, Thunderbird 153.5

## Technical details

This is a mitigation bypass vulnerability in the File Handling component of Firefox ESR. The advisory does not specify the attack vector, required authentication, or preconditions. The vulnerability affects the security mitigations that protect against unsafe file handling, allowing an attacker to potentially bypass these protections to access or manipulate files.

## Affected products

- Mozilla Firefox ESR
- Mozilla Thunderbird

## Timeline

- 2026-10-13: advisory

## References

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2075533](https://bugzilla.mozilla.org/show_bug.cgi?id=2075533)
- [https://www.mozilla.org/security/advisories/mfsa2026-108/](https://www.mozilla.org/security/advisories/mfsa2026-108/)

## Related threats

- [Thunderbird OAuth2 access token leak over HTTP with Exchange accounts](https://junglewise.ai/threats/cve-2026-126136-for-exchange-accounts-using-ews-or-microsoft-graph-with-oauth2)
- [Mozilla Thunderbird SMTP/POP3 response injection via STARTTLS](https://junglewise.ai/threats/cve-2026-126135-if-an-attacker-was-able-to-perform-a-man-in-the-middle-attack-on)
- [Thunderbird NTLM credential leak via feed subscription to file URL](https://junglewise.ai/threats/cve-2026-126134-thunderbird-allowed-feed-subscriptions-to-file-urls-that-point)
- [Thunderbird calendar server credentials sent to HTTP proxy](https://junglewise.ai/threats/cve-2026-126133-when-a-caldav-or-ics-calendar-was-accessed-through-an-http-proxy)
- [Thunderbird attachment replacement disclosure via link files](https://junglewise.ai/threats/cve-2026-126132-when-a-message-was-forwarded-inline-edited-as-new-or-had-an)