# CVE-2026-118176: Thunderbird mailbox: URL UNC path restriction bypass on Windows

Source: https://junglewise.ai/threats/cve-2026-118176-thunderbird-s-check-that-prevents-mailbox-urls-from-accessing  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-27. Updated: 2026-10-27  
License: CC BY 4.0 with a link back

CVE-2026-118176 · Severity: info · Published 2026-10-27

Thunderbird, an email client used for reading and managing corporate messages, has a security check designed to prevent attackers from accessing files on Windows network shares. This check can be bypassed by crafting a specially designed email that tricks Thunderbird into connecting to an attacker's server, which can expose the user's Windows username and authentication credentials. An attacker could use these exposed credentials for offline password attacks or network relay attacks.

Technologies: [Mozilla Thunderbird](https://junglewise.ai/threats/technologies/thunderbird). Vendors: [Mozilla](https://junglewise.ai/threats/vendors/mozilla).

## Quick answers

**Is CVE-2026-118176 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-118176 being exploited in the wild.

**How severe is CVE-2026-118176?** Informational severity, published 27 October 2026.

**Which products and versions are affected?** Mozilla Thunderbird before 159 and 153.6.

**Is there a fix for CVE-2026-118176?** Yes: fixed in Thunderbird 159, Thunderbird 153.6. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-118176 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-118176?** Check whether you run Mozilla Thunderbird at an affected version. 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 Thunderbird 159, Thunderbird 153.6

## Technical details

The vulnerability is a path traversal bypass in Thunderbird's mailbox: URL handler on Windows, where extended-length UNC path formats can circumvent the check that blocks access to remote network shares. Displaying a specially crafted email triggers an SMB connection to an attacker-chosen server without user interaction, leaking NTLM credentials. The issue is Windows-specific; other operating systems are unaffected.

## Affected products

- Mozilla Thunderbird before 159 and 153.6

## Timeline

- 2026-10-27: disclosed
- 2026-10-27: patched

## References

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2081141](https://bugzilla.mozilla.org/show_bug.cgi?id=2081141)
- [https://www.mozilla.org/security/advisories/mfsa2026-121/](https://www.mozilla.org/security/advisories/mfsa2026-121/)

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