high CVSS 7 EPSS 0.35%

CVE-2026-42395·Dovecot vulnerability

A host listed as a trusted proxy can send forwarding information containing a NUL byte, which crashes the login process on the following login attempt. The login process is terminated, which can cause degradation or denial of service for logins. Deployments that do not configure trusted proxies are not affected. Restrict the list of trusted proxy networks to hosts that are fully under your control. Update to non-vulnerable version. No publicly available exploits are known.

Severity
high
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 0.35% — higher than 27% 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-2026-42395 scores 7 on CVSS, so a successful attack would be serious — but at 0.35%, 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

Organizations running Dovecot with configured trusted proxy networks should upgrade to version 1:2.4.1+dfsg1-6+deb13u7 or later to prevent login process crashes triggered by malformed proxy headers. This vulnerability allows a compromised or untrusted proxy to inject NUL bytes into forwarding information, causing subsequent authentication attempts to fail. Immediately audit your proxy configuration to ensure only internal, fully-controlled hosts are listed as trusted, and apply the security update to restore stable login functionality.

sudo dnf update dovecot

Defensia detects this vulnerability

How to check if you are affected

  1. Check installed Dovecot version: dpkg -l | grep dovecot (Debian/Ubuntu) or rpm -qa | grep dovecot (RHEL/CentOS)
  2. Verify if trusted proxies are configured: grep -r 'trusted_proxy' /etc/dovecot/ — if no results, your system is not vulnerable
  3. Search login logs for repeated authentication failures: grep -i 'login.*fatal\|segfault\|crash' /var/log/mail.log or /var/log/dovecot/dovecot.log
  4. Confirm the fix: dovecot --version and verify it reports 2.4.1+dfsg1-6+deb13u7 or higher

FAQ

What is CVE-2026-42395?

This vulnerability affects Dovecot's proxy header parsing logic, where a trusted proxy can inject NUL bytes into forwarding metadata, triggering a crash in the login process on the next authentication attempt. Only installations explicitly configuring trusted proxies are at risk.

Is CVE-2026-42395 being actively exploited?

No, this vulnerability is not listed on CISA's Known Exploited Vulnerabilities catalog and no public exploits have been published.

What versions of Dovecot are affected by CVE-2026-42395?

All versions prior to 1:2.4.1+dfsg1-6+deb13u7 are affected. The vulnerability exists in all Dovecot releases up to and including 2.4.1 on Debian 13.

How do I check if my server is vulnerable to CVE-2026-42395?

Run 'dovecot --version' and compare the output to 1:2.4.1+dfsg1-6+deb13u7. If your version is older and you have trusted_proxy configured in /etc/dovecot/dovecot.conf, your server is vulnerable.

Does Defensia detect CVE-2026-42395?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-42395 will appear in your dashboard with remediation steps.

Related Dovecot CVEs

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.
CVE-2026-40018CVSS 9None None None 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-2026-42391CVSS 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.

References

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