CVE-2026-27859: A mail message containing excessive amount of RFC 2231 MIME parameters causes LMTP to use too much CPU

Routine

MEDIUM 5.3REMOTEUPGRADEUPGRADE

CVE-2026-27859 is a medium-severity flaw (CVSS 5.3) affecting dovecot, dovecot. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A mail message containing excessive amount of RFC 2231 MIME parameters causes LMTP to use too much CPU. A suitably formatted mail message causes mail delivery process to consume large amounts of CPU time. Use MTA capabilities to limit RFC 2231 MIME parameters in mail messages, or upgrade to fixed version where the processing is limited. No publicly available exploits are known.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 68%)
CISA exploited list	No
Weakness	CWE-400
Published	2026-03-27
Last modified	2026-10-26

Publisher (CNA): Open-Xchange · Vendor: dovecot · open-xchange

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-27 09:16 UTCPublished CVE record published
2026-10-26 00:10 UTCUpdated Record last modified

dovecot dovecot UPGRADE

Affected: < 2.4.3

Fixed in: 2.4.3

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy dovecot || rpm -q dovecot || dovecot --version

2. UPGRADE to 2.4.3 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade dovecot

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade dovecot

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

open-xchange dovecot UPGRADE

Affected: < 3.0.5 · >= 3.1.0 and < 3.1.4

Fixed in: 3.0.5, 3.1.4

Safe to downgrade to: a release before 3.1.0

1. Check installed version

apt-cache policy dovecot || rpm -q dovecot || dovecot --version

2. UPGRADE to 3.0.5 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade dovecot

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade dovecot

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.1.0 (Debian / Ubuntu)

apt-cache madison dovecot sudo apt install dovecot=<VERSION_BEFORE_3.1.0> sudo apt-mark hold dovecot

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list dovecot sudo dnf downgrade dovecot-<VERSION_BEFORE_3.1.0> sudo dnf versionlock add dovecot

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

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-2026-27859?

dovecot: upgrade to 2.4.3; dovecot: upgrade to 3.0.5, 3.1.4. Exact commands for each OS are on this page.

Is CVE-2026-27859 being exploited?

CVE-2026-27859 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-2026-27859 affect?

CVE-2026-27859 affects dovecot, dovecot. Severity is medium (CVSS 5.3). A mail message containing excessive amount of RFC 2231 MIME parameters causes LMTP to use too much CPU. A suitably formatted mail message causes mail delivery process to consume large amounts of CPU time. Use MTA capabilities to limit RFC 2

How urgent is CVE-2026-27859?

Priority 32/100. Remotely exploitable, no login, no user action

Software pages

Dovecot vulnerabilities

Related CVEs

CVE-2026-27860 LOW 3.7 UPGRADE UPGRADE If auth_username_chars is empty, it is possible to inject arbitrary LDAP filter to Dovecot's LDAP authenticati

References

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