high CVSS 8 EPSS 0.21%

CVE-2026-40205·Dovecot vulnerability

An attacker that holds an OAuth2 token granting only part of the required scopes can authenticate, because when more than one scope is required in the configuration, the remote token validation paths accept a token that carries only one of them, while the local token validation path correctly requires all of them. The configured authorization policy is not enforced, so a token that was granted only part of the required permissions is accepted where it should have been rejected. Use local token validation where tokens can be validated locally. 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 0.21% — higher than 10% 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-2026-40205 scores 8 on CVSS, so a successful attack would be serious — but at 0.21%, 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

Dovecot versions prior to 1:2.4.1+dfsg1-6+deb13u7 contain a scope validation bypass in OAuth2 token handling that allows attackers with partial token permissions to authenticate as if they held full authorization. Update Dovecot immediately to version 1:2.4.1+dfsg1-6+deb13u7 or later, and prioritize deploying local token validation where possible rather than relying on remote validation endpoints. Review all OAuth2 authentication policies in your Dovecot configuration to ensure scope requirements are properly enforced.

sudo dnf update dovecot

Defensia detects this vulnerability

How to check if you are affected

  1. Check installed Dovecot version: dovecot --version
  2. Verify OAuth2 configuration is active: grep -r 'oauth2' /etc/dovecot/conf.d/ | grep -v '^#'
  3. Search mail logs for OAuth2 authentication attempts: sudo grep -i 'oauth2\|scope' /var/log/mail.log* | grep -i 'auth\|failed\|denied'
  4. Confirm patch installation: dpkg -l | grep dovecot && dovecot --version | grep -E '2\.4\.1.*deb13u7'

FAQ

What is CVE-2026-40205?

This is a scope validation bypass in Dovecot's OAuth2 authentication handler. When multiple OAuth2 scopes are required by policy, the remote token validation path incorrectly accepts tokens with only partial scope coverage instead of enforcing all required scopes, bypassing the configured authorization policy.

Is CVE-2026-40205 being actively exploited?

No, CVE-2026-40205 is not currently tracked as actively exploited by CISA, and no public exploit code has been released as of this advisory.

What versions of Dovecot are affected by CVE-2026-40205?

All Dovecot versions up to and including 1:2.4.1+dfsg1-6+deb13u6 are vulnerable. The fix is available in version 1:2.4.1+dfsg1-6+deb13u7 and later.

How do I check if my server is vulnerable to CVE-2026-40205?

Run 'dovecot --version' and compare the output to 1:2.4.1+dfsg1-6+deb13u7. If your version is older and contains 'oauth2' in your Dovecot configuration (check /etc/dovecot/conf.d/), your server is vulnerable.

Does Defensia detect CVE-2026-40205?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Dovecot is installed on a monitored server, CVE-2026-40205 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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