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Nginx Open SourceWeb server / proxy · F5

CVE-2026-27651

HIGH · 7.5 CVSS v3.1 Published 2026-03-24
Fix available
A fix is available. Upgrade to 1.28.3 / 1.29.7 or later.
See remediation →
84/100
Remediation priority · High
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
When the ngx_mail_auth_http_module module is enabled on NGINX Plus or NGINX Open Source, undisclosed requests can cause worker processes to terminate. This issue may occur when (1) CRAM-MD5 or APOP authentication is enabled, and (2) the authentication server permits retry by returning the Auth-Wait response header. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.

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

Denial of service vulnerability in NGINX ngx_mail_auth_http_module where specially crafted requests cause worker process termination when CRAM-MD5 or APOP authentication is enabled and the authentication server returns an Auth-Wait response header allowing retry.

MitigationDisable CRAM-MD5 and APOP authentication methods in the mail authentication configuration, or apply vendor-supplied patches when available.

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
Nginx Open SourceWeb server / proxy
Affected:>= 0.5.15, <= 0.9.7>= 1.0.0, < 1.28.3>= 1.29.0, < 1.29.7
Nginx PlusWeb server / proxy
Affected:>= r33, < r35= r32= r35= r36

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
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

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 NGINX version
    Run 'nginx -v' or examine package metadata to obtain the installed version
    Affected if Version is >= 0.5.15 through <= 0.9.7, OR >= 1.0.0 through < 1.28.3, OR >= 1.29.0 through < 1.29.7 for F5 Nginx Open Source; or is r32, r33, r34, r35, or r36 for F5 Nginx Plus
  2. Verify mail authentication module is in use
    Review nginx configuration files for a 'mail' block containing the 'auth_http' directive, which enables ngx_mail_auth_http_module
    Affected if Configuration contains a mail { } block with an auth_http directive pointing to an upstream authentication server
  3. Confirm CRAM-MD5 or APOP authentication is enabled
    Search nginx mail configuration for 'auth_method' directive or check for 'cram_md5' or 'apop' in the authentication methods configuration
    Affected if CRAM-MD5 or APOP authentication methods are explicitly enabled in the mail module configuration

Environment is affected only if NGINX version is within the vulnerable ranges, the mail auth_http module is configured with an upstream server, AND CRAM-MD5 or APOP authentication is enabled.

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 1.28.3 / 1.29.7 or later
Fixed in 1.28.31.29.7
Interim mitigation

Disable CRAM-MD5 and APOP authentication methods in the mail authentication configuration, or apply vendor-supplied patches when available.

Recommended fix High confidence

Nginx Open Source: 1.28.3 or 1.29.7+ | Nginx Plus: r37+

  1. 1. Identify the current installed version using 'nginx -v' or 'nginx-plus -v'
  2. 2. For Nginx Open Source: upgrade to version 1.28.3 or later (if on 1.28.x branch), or 1.29.7 or later (if on 1.29.x branch)
  3. 3. For Nginx Plus: upgrade to version r37 or later
  4. 4. After upgrade, verify the worker processes no longer terminate unexpectedly with CRAM-MD5 or APOP authentication
  5. 5. Test mail authentication functionality to confirm normal operation
Caveat Review release notes for any changes to mail module behavior; minor version upgrades typically have low risk

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

Fix this in Nginx Open Source Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing3.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,210
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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-2026-27651 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data