critical CVSS 9 EPSS 0.45%

CVE-2026-33605·Dovecot vulnerability

An unauthenticated attacker can crash the ManageSieve login process by sending a small malformed command before authenticating. If running in high-security mode (default for community releases), only the attacker's own connection is terminated. If running in high-performance mode (default for Pro releases), all connections handled by the same managesieve-login process are terminated. Repeating the attack can cause denial of service for Sieve script management. Restrict network access to the ManageSieve service to trusted clients. Update to non-vulnerable version. No publicly available exploits are known.

Severity
critical
Software
Dovecot
Fixed in
1:2.4.1+dfsg1-6+deb13u7
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.45% — higher than 37% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2026-33605 scores 9 on CVSS, so a successful attack would be serious — but at 0.45%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

Until: 1:2.4.1+dfsg1-6+deb13u7

Fixed in: 1:2.4.1+dfsg1-6+deb13u7

How to fix this CVE

Update Dovecot to version 1:2.4.1+dfsg1-6+deb13u7 or later to patch a critical flaw in the ManageSieve login handler that allows unauthenticated denial-of-service attacks. Immediately restrict network access to the ManageSieve service (port 4190 by default) to only trusted administrative clients and networks. If you cannot patch immediately, disable ManageSieve entirely or place it behind a firewall rule that limits connections to known safe sources.

sudo dnf update dovecot dovecot-pigeonhole

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

managesieve-login.*disconnect|managesieve.*malformed|managesieve-login.*fatal.*command|Multiple rapid managesieve connection drops from same source IP

How to check if you are affected

  1. Check installed Dovecot version: dovecot --version or dpkg -l | grep dovecot (Debian/Ubuntu) or rpm -qa | grep dovecot (RHEL/CentOS)
  2. Verify ManageSieve is enabled: grep -i 'managesieve' /etc/dovecot/dovecot.conf or netstat -tlnp | grep 4190
  3. Review authentication logs for multiple rapid disconnections: grep 'managesieve-login' /var/log/mail.log | grep -i 'disconnect\|error' | tail -20
  4. Confirm patch applied: dovecot --version should show version 2.4.1+dfsg1-6+deb13u7 or later, or verify with: dpkg -l dovecot-core | grep ii

FAQ

What is CVE-2026-33605?

CVE-2026-33605 is a critical denial-of-service vulnerability in Dovecot's ManageSieve login process where an unauthenticated attacker can crash the service by sending a malformed command during authentication, disrupting Sieve script management for legitimate users.

Is CVE-2026-33605 being actively exploited?

No active exploitation has been reported by CISA, and no public exploits are currently available. However, the vulnerability is straightforward to trigger and should be treated with urgency.

What versions of Dovecot are affected by CVE-2026-33605?

All Dovecot versions prior to 1:2.4.1+dfsg1-6+deb13u7 are vulnerable. This includes all community and Pro releases before the patched version.

How do I check if my server is vulnerable to CVE-2026-33605?

Run: dovecot --version and verify it is lower than 2.4.1+dfsg1-6+deb13u7. Then check if ManageSieve is enabled with: grep -i 'managesieve' /etc/dovecot/dovecot.conf | grep -v '^#'

Does Defensia detect CVE-2026-33605?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-33605 will appear in your dashboard with remediation steps.

Related Dovecot CVEs

CVE-2026-42391CVSS 9An unauthenticated attacker can send an IMAP ID command with a very large number of parameters before logging in, which causes memory and CPU usage to grow disproportionately. The login process can be terminated by the out-of-memory handling, which also terminates all other connections handled by the same process. This can cause degradation or denial of service for IMAP logins. Limit the number of connections handled by a single imap-login process. This has a performance impact though. Update to non-vulnerable version. No publicly available exploits are known.
CVE-2026-40019CVSS 9An unauthenticated attacker can send a truncated quoted argument to the ManageSieve login process, which makes it spin in an infinite loop consuming CPU. This can cause degradation or denial of service for Sieve script management, and repeated connections can consume all available CPU on the server. Monitor system for abnormal CPU usage and kill the offending process. Restrict network access to the ManageSieve service to trusted clients. Update to non-vulnerable version. No publicly available exploits are known.
CVE-2026-27852CVSS 9An attacker that can send mail to a user can craft a message whose headers contain a very large number of email addresses or MIME parameters, which causes excessive memory usage when the message is later parsed. The message is still delivered, but reading it over IMAP can exhaust the memory limit of the process and terminate it, causing denial of service for the affected user. Update to non-vulnerable version. No publicly available exploits are known.
CVE-2026-40018CVSS 9None None None No publicly available exploits are known.
CVE-2026-73208CVSS 9An attacker that holds a token intended for a different purpose can authenticate, because when an OAuth2 token response does not contain a scope claim, the audience claim is used in its place and checked against the configured required scopes. These are different concepts, and the audience claim does not describe what a token is allowed to do. A token that grants no relevant permissions can be accepted because its intended recipient value happens to match a configured scope name, granting access that should have been denied. It also hides an identity provider misconfiguration where scopes are not being issued at all. Ensure the identity provider issues a scope claim for all tokens used with Dovecot, and that configured scope names do not match audience values. Update to non-vulnerable version. No publicly available exploits are known.

References

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