CVE-2026-27858: Attacker can send a specifically crafted message before authentication that causes managesieve to allocate lar

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HIGH 7.5REMOTEUPGRADEUPGRADE

CVE-2026-27858 is a high-severity flaw (CVSS 7.5) affecting dovecot, dovecot. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Attacker can send a specifically crafted message before authentication that causes managesieve to allocate large amount of memory. Attacker can force managesieve-login to be unavailable by repeatedly crashing the process. Protect access to managesieve protocol, or install fixed version. No publicly available exploits are known.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.8% (top 44%)
CISA exploited list	No
Weakness	CWE-400, CWE-770
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 21:10 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: < 2.3.22.1 · >= 3.0.0 and < 3.0.5 · >= 3.1.0 and < 3.1.4

Fixed in: 2.3.22.1, 3.0.5, 3.1.4

Safe to downgrade to: a release before 3.0.0

1. Check installed version

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

2. UPGRADE to 2.3.22.1 (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 3.0.0 (Debian / Ubuntu)

apt-cache madison dovecot sudo apt install dovecot=<VERSION_BEFORE_3.0.0> 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_3.0.0> 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.

Advisories and coverage

Official advisories

AlmaLinux Errata: [ALSA-2026:19364] Important: dovecot security update 2026-05-26

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-2026-27858?

dovecot: upgrade to 2.4.3; dovecot: upgrade to 2.3.22.1, 3.0.5, 3.1.4. Exact commands for each OS are on this page.

Is CVE-2026-27858 being exploited?

CVE-2026-27858 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-2026-27858 affect?

CVE-2026-27858 affects dovecot, dovecot. Severity is high (CVSS 7.5). Attacker can send a specifically crafted message before authentication that causes managesieve to allocate large amount of memory. Attacker can force managesieve-login to be unavailable by repeatedly crashing the process. Protect access t

How urgent is CVE-2026-27858?

Priority 45/100. High severity; Remotely exploitable, no login, no user action

Software pages

Dovecot vulnerabilities

Related CVEs

CVE-2026-27855 MEDIUM 6.8 UPGRADE UPGRADE Dovecot OTP authentication is vulnerable to replay attack under specific conditions
CVE-2026-27857 MEDIUM 4.3 UPGRADE UPGRADE Sending "NOOP (((...)))" command with 4000 parenthesis open+close results in ~1MB extra memory usage
CVE-2026-24031 HIGH 7.7 UPGRADE UPGRADE Dovecot SQL based authentication can be bypassed when auth_username_chars is cleared by admin
CVE-2026-27856 HIGH 7.4 UPGRADE UPGRADE Doveadm credentials are verified using direct comparison which is susceptible to timing oracle attack
CVE-2026-0394 MEDIUM 5.3 UPGRADE UPGRADE When dovecot has been configured to use per-domain passwd files, and they are placed one path component above

References

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