# CVE-2026-113866: Mozilla Thunderbird account password disclosure in Exchange AutoDiscover

Source: https://junglewise.ai/threats/cve-2026-113866-when-setting-up-an-account-thunderbird-s-exchange-autodiscover  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-20. Updated: 2026-10-20  
License: CC BY 4.0 with a link back

CVE-2026-113866 · Severity: medium · CVSS 5.9 · Published 2026-10-20

Thunderbird, an email client used by millions to manage corporate and personal email accounts, transmitted user passwords over unencrypted HTTP during Exchange account setup. A network attacker positioned on the user's connection could intercept these credentials and gain full access to the user's email account and any systems it protects.

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

## Quick answers

**Is CVE-2026-113866 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-113866 being exploited in the wild.

**How severe is CVE-2026-113866?** Medium severity, with a CVSS score of 5.9, published 20 October 2026.

**Which products and versions are affected?** Mozilla Thunderbird before 158.0.1 and before 153.5.1.

**Is there a fix for CVE-2026-113866?** Yes: fixed in Thunderbird 158.0.1, Thunderbird 153.5.1. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-113866 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-113866?** 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 158.0.1, Thunderbird 153.5.1

## Technical details

During Exchange AutoDiscover account configuration, Thunderbird queries the domain's AutoDiscover service over HTTP without encryption. Due to a bug, user-supplied credentials were sent in plaintext within this unencrypted request, allowing network-based attackers to obtain passwords. The vulnerability requires no user interaction beyond initial account setup and affects all users configuring Exchange accounts on vulnerable Thunderbird versions.

## Affected products

- Mozilla Thunderbird before 158.0.1 and before 153.5.1

## Timeline

- 2026-10-20: disclosed
- 2026-10-20: patched

## References

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2080121](https://bugzilla.mozilla.org/show_bug.cgi?id=2080121)
- [https://www.mozilla.org/security/advisories/mfsa2026-114/](https://www.mozilla.org/security/advisories/mfsa2026-114/)

## 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)