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DovecotApplication

CVE-2026-27855

MEDIUM · 5.9 CVSS v3.1 Published 2026-03-27
Fix available
A fix is available. Upgrade to 2.4.3 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Dovecot OTP authentication is vulnerable to replay attack under specific conditions. If auth cache is enabled, and username is altered in passdb, then OTP credentials can be cached so that same OTP reply is valid. An attacker able to observe an OTP exchange is able to log in as the user. If authentication happens over unsecure connection, switch to SCRAM protocol. Alternatively ensure the communcations are secured, and if possible switch to OAUTH2 or SCRAM. No publicly available exploits are known.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · moderate confidence

Dovecot OTP authentication is vulnerable to replay attacks when auth caching is enabled and the username is modified in the passdb. Under these conditions, OTP credentials can be improperly cached, allowing the same OTP reply to be reused. An attacker who observes an OTP exchange can replay the credentials to authenticate as the user.

MitigationSwitch to SCRAM or OAUTH2 protocols and ensure all authentication communications are secured over TLS. Disable auth caching for OTP authentication or ensure usernames remain consistent in the passdb.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
DovecotApplication
Affected:< 2.4.3
DovecotApplication
Affected:<= 2.3.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check your installed Dovecot version
    Run 'dovecot --version' or check the package manager for the installed dovecot package version
    Affected if The version is less than 2.4.3 for standard Dovecot, or 2.3.0 or earlier for Open Xchange Dovecot
  2. Verify if auth caching is enabled
    Inspect your dovecot configuration files (typically in /etc/dovecot/conf.d/ or dovecot.conf) for 'auth_cache_size', 'auth_cache_ttl', or 'auth_cache_negative_ttl' settings
    Affected if Auth caching is enabled with any non-zero TTL value (auth_cache_size is set and TTL values are configured)
  3. Confirm OTP authentication is in use
    Check your passdb configuration in dovecot.conf or 10-auth.conf for 'otp', 'otps', or 'digests' authentication mechanisms, and verify the auth scheme includes OTP-based methods
    Affected if OTP, OTPS, or digest-based authentication mechanisms are configured in the authentication stack
  4. Check for username modification in passdb
    Review your passdb configuration (in 10-passdb.conf or auth-passdb.ext) for any 'user' filter, 'username' transformation, or passdb rewrite rules that modify the username between authentication stages
    Affected if The passdb configuration contains username filtering, modification, or rewrite rules that alter the username from what the client sent to what is looked up in the backend
  5. Determine if OTP credentials are being cached
    Check if auth caching applies to the OTP authentication path by examining whether 'auth_cache_ttl' or 'auth_cache_negative_ttl' are set while OTP is enabled in the passdb
    Affected if Auth caching is active (non-zero TTL) AND OTP authentication is configured in the passdb

You are affected if you run a vulnerable Dovecot version with auth caching enabled, use OTP authentication, and have username modification rules in your passdb configuration.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 2.4.3 or later
Fixed in 2.4.3
Interim mitigation

Switch to SCRAM or OAUTH2 protocols and ensure all authentication communications are secured over TLS. Disable auth caching for OTP authentication or ensure usernames remain consistent in the passdb.

Recommended fix High confidence

Dovecot 2.4.3 or later (or Dovecot 2.3.1+ on the 2.3.x branch)

  1. 1. Identify the current installed Dovecot version using 'dovecot --version' or checking the package manager
  2. 2. Back up current Dovecot configuration files in /etc/dovecot/ and related directories
  3. 3. For systems using package managers (apt/yum/dnf), update to the latest available Dovecot package
  4. 4. Verify the upgraded version is 2.4.3 or later (or 2.3.1+ if staying on the 2.3.x branch)
  5. 5. Review and test authentication configuration, especially OTP-related settings
  6. 6. Consider disabling auth cache for OTP authentication if not needed, or ensure username consistency in passdb
  7. 7. Restart Dovecot service after upgrade: 'systemctl restart dovecot'
  8. 8. Verify the service is running correctly: 'systemctl status dovecot'
Caveat Minor - upgrading within the same major version should be low risk; always test in non-production first

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Dovecot Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-27855 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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