critical CVSS 9

CVE-2026-27852·Dovecot vulnerability

An 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.

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

Affected versions

Until: 1:2.4.1+dfsg1-6+deb13u7

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

How to fix this CVE

Dovecot versions prior to 1:2.4.1+dfsg1-6+deb13u7 are vulnerable to a denial-of-service condition triggered by specially crafted email messages with malformed headers. Upgrade to version 1:2.4.1+dfsg1-6+deb13u7 or later to eliminate the memory exhaustion vulnerability. This fix prevents attackers from crashing mail server processes by sending emails with oversized header fields or MIME parameters.

sudo dnf update dovecot

Defensia detects this vulnerability

How to check if you are affected

  1. Check installed Dovecot version: dovecot --version | grep -oP '\d+\.\d+\.\d+'
  2. Confirm IMAP service is exposed: ss -tlnp | grep -E ':(143|993)' to verify IMAP ports are listening
  3. Search Dovecot logs for memory-related crashes: grep -i 'out of memory\|memory limit\|fatal' /var/log/dovecot/dovecot.log | head -20
  4. Verify patch installation: apt list --installed 2>/dev/null | grep dovecot or rpm -qa | grep dovecot to confirm version ≥ 1:2.4.1+dfsg1-6+deb13u7

FAQ

What is CVE-2026-27852?

This is a memory exhaustion vulnerability in Dovecot's IMAP message parser that allows unauthenticated attackers to crash mail server processes by sending emails with extremely large header fields or MIME parameters. The vulnerability denies service to legitimate users accessing their mailbox.

Is CVE-2026-27852 being actively exploited?

No, CVE-2026-27852 is not listed in the CISA Known Exploited Vulnerabilities catalog and no public exploits are currently available. However, the low barrier to exploitation (sending a crafted email) means affected systems should be patched promptly.

What versions of Dovecot are affected by CVE-2026-27852?

All Dovecot versions up to and including 1:2.4.1+dfsg1-6+deb13u6 are vulnerable. Version 1:2.4.1+dfsg1-6+deb13u7 and later contain the fix.

How do I check if my server is vulnerable to CVE-2026-27852?

Run 'dovecot --version' and compare the output to version 1:2.4.1+dfsg1-6+deb13u7; if your version is older, you are vulnerable. You can also run 'apt-cache policy dovecot-core' to check available updates.

Does Defensia detect CVE-2026-27852?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-27852 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-2026-33605CVSS 9An 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.
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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