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CVE Record

CVE-2026-27651: NGINX ngx_mail_auth_http_module vulnerability

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.

HighCVSS 8.7Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

This flaw can let remote unauthenticated requests crash NGINX worker processes in a specific mail-authentication configuration. The business impact is availability: mail proxy service disruption, degraded authentication handling, and possible repeated outages if the vulnerable path is reachable.

Executive priority

Treat this as a high-priority availability risk for environments using NGINX as a mail proxy. Prioritize internet-exposed or business-critical mail authentication paths, but avoid emergency assumptions unless your configuration matches the documented prerequisites.

Technical view

CVE-2026-27651 is a CWE-476 NULL pointer dereference in NGINX ngx_mail_auth_http_module. It requires the module to be enabled, CRAM-MD5 or APOP authentication enabled, and an auth server that allows retry using the Auth-Wait response header. CVSS 4.0 is 8.7 with high availability impact.

Likely exposure

Exposure appears limited to NGINX mail proxy deployments using ngx_mail_auth_http_module with CRAM-MD5 or APOP and Auth-Wait retry behavior. The bundle lists NGINX Open Source and NGINX Plus references, but version status details are incomplete and should be confirmed against vendor advisories.

Exploitation context

The CVE is not listed as KEV in the supplied bundle, and no provided source states active exploitation. The issue is network-reachable and unauthenticated, but the triggering conditions are configuration-specific.

Researcher notes

The public bundle identifies crash conditions but withholds request details. Do not assume broad NGINX HTTP exposure. Focus validation on mail module configuration, auth methods, Auth-Wait behavior, package lineage, and vendor version status. Evidence for exploitation and exact fixed versions is incomplete in the supplied material.

Mitigation direction

  • Check F5 advisory K000160383 for fixed or unaffected versions.
  • Review Red Hat errata for distribution-specific package updates.
  • Disable CRAM-MD5 or APOP if not required.
  • Avoid Auth-Wait retry behavior where operationally feasible.
  • Restrict external access to affected mail proxy services.
  • Monitor NGINX workers for abnormal termination or restart loops.

Validation and detection

  • Inventory NGINX hosts exposing mail proxy services.
  • Confirm whether ngx_mail_auth_http_module is enabled.
  • Check whether CRAM-MD5 or APOP authentication is configured.
  • Verify whether the auth server returns Auth-Wait.
  • Compare installed package versions against F5 and Red Hat guidance.
  • Review logs for repeated worker exits during mail authentication.
Prepared
Confidence
medium
Sources
9

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-476: Exact CWE lookup

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cve · low confidence lookup

CVE-2026-27651 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
8.7 (4.0)
Known Exploited
No
Published

Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

3CVSS vectors
5Timeline events
2ADP providers
20Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: yesTechnical Impact: partial

CVSS vector scores

3 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
8.7CVSS 4.0HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N——f5
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6f5
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6redhat-SADP

Vulnerability scoring details

Base CVSS 4.0 score

8.7High
CVSS 4.0 vector shape for CVE-2026-27651Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. ADP timelineredhat-SADP

    Made public.

  3. CVE publishedCVE Program

    The CVE record was published.

  4. ADP timelineredhat-SADP

    Reported to Red Hat.

  5. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
redhat-SADPNGINX: NGINX: Denial of Service via undisclosed requests when ngx_mail_auth_http_module is enabled
other:Red Hat severity ratingcvssV3_1
  • 2026-03-24T15:02:32.414Z: Reported to Red Hat.
  • 2026-03-24T14:13:27.295Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
F5NGINX Open Source1.29.0, 0.5.15unknown
F5NGINX PlusR36, R35, R34, R33, R32unaffected
Weakness

CWE details

CWE-476

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-476 · source CWE mapping

NULL Pointer Dereference

NULL Pointer Dereference represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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