What is CVE-2026-113866?

CVE-2026-113866 is a medium-severity vulnerability, classified under Unprotected Transport of Credentials. CVSS score: 5.9/10. Published 2026-10-20.

How severe is CVE-2026-113866?

Medium severity. CVSS v3 base score is 5.9 out of 10.

CVE-2026-113866

CVE-2026-113866

When setting up an account, Thunderbird's Exchange AutoDiscover lookup also queried the domain's AutoDiscover service over plain HTTP. Due to a bug, that unencrypted request included the credentials the user had entered, allowing a network attacker to obtain the user's password. This vulnerability was fixed in Thunderbird 158.0.1 and Thunderbird 153.5.1.

Published 2026-10-20.

CVSS v3 metric

CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N.

EPSS exploit prediction

No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.

Weakness classification (CWE)

CWE-523 — Unprotected Transport of Credentials

References

Frequently asked questions

What is CVE-2026-113866?: CVE-2026-113866 is a medium-severity vulnerability, classified under Unprotected Transport of Credentials. CVSS score: 5.9/10. Published 2026-10-20.
How severe is CVE-2026-113866?: Medium severity. CVSS v3 base score is 5.9 out of 10.