CVE-2026-27857: Sending "NOOP (((...)))" command with 4000 parenthesis open+close results in ~1MB extra memory usage

Routine

MEDIUM 4.3UPGRADEUPGRADE

CVE-2026-27857 is a medium-severity flaw (CVSS 4.3) affecting dovecot, dovecot. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Sending "NOOP (((...)))" command with 4000 parenthesis open+close results in ~1MB extra memory usage. Longer commands will result in client disconnection. This 1 MB can be left allocated for longer time periods by not sending the command ending LF. So attacker could connect possibly from even a single IP and create 1000 connections to allocate 1 GB of memory, which would likely result in reaching VSZ limit and killing the process and its other pr

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	4.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.7% (top 48%)
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 22:10 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-2026-27857?

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-2026-27857 being exploited?

CVE-2026-27857 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-2026-27857 affect?

CVE-2026-27857 affects dovecot, dovecot. Severity is medium (CVSS 4.3). Sending "NOOP (((...)))" command with 4000 parenthesis open+close results in ~1MB extra memory usage. Longer commands will result in client disconnection. This 1 MB can be left allocated for longer time periods by not sending the command en

How urgent is CVE-2026-27857?

Priority 28/100. No elevated risk signals yet.

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-2026-27858 HIGH 7.5 UPGRADE UPGRADE Attacker can send a specifically crafted message before authentication that causes managesieve to allocate lar
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

← Back to latest CVEs