CVE-2026-113866: When setting up an account, Thunderbird's Exchange AutoDiscover lookup also queried the domain's AutoDiscover

Routine

MEDIUM 5.9REMOTE

CVE-2026-113866 is a medium-severity flaw (CVSS 5.9) affecting affected software. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.9 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 71%)
CISA exploited list	No
Weakness	CWE-523
Published	2026-10-20
Last modified	2026-10-22

Publisher (CNA): Mozilla Corporation

Last updated 2026-10-22 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Timeline

2026-10-20Published CVE record published
2026-10-22 17:36 UTCUpdated Record last modified

Exact fixes by distribution

Fedora 46 · sugar-clock · FIXED · 22.1-17.fc46 · FEDORA-2026-57948849df · source

sudo dnf upgrade --advisory=FEDORA-2026-57948849df

Fedora ELN · selinux-policy · FIXED · 45.23-1.eln159 · FEDORA-2026-3668c969a2 · source

sudo dnf upgrade --advisory=FEDORA-2026-3668c969a2

Debian 12 · thunderbird · NO FIX YET

Debian 13 · thunderbird · NO FIX YET

RHEL 10 · thunderbird · DEFERRED

RHEL 8 · thunderbird · DEFERRED

RHEL 9 · thunderbird · DEFERRED

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-113866?

No fixed release is listed yet. Watch the vendor advisory and apply any mitigation shown on this page.

Is CVE-2026-113866 being exploited?

CVE-2026-113866 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-113866 affect?

CVE-2026-113866 affects affected software. Severity is medium (CVSS 5.9). 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

How urgent is CVE-2026-113866?

Priority 33/100. Remotely exploitable, no login, no user action

References

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