CVE-2026-27651: When the ngx_mail_auth_http_module module is enabled on NGINX Plus or NGINX Open Source, undisclosed requests

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HIGH 7.5REMOTEWATCHLISTUPGRADEUPGRADE

CVE-2026-27651 is a high-severity flaw (CVSS 7.5) affecting nginx open source, nginx plus. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

High severity
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.0% (top 39%)
CISA exploited list	No
Weakness	CWE-476
Published	2026-03-24
Last modified	2026-07-15

Publisher (CNA): F5 Networks · Vendor: f5

Last updated 2026-07-15 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-24 15:16 UTCPublished CVE record published
2026-07-15 02:19 UTCUpdated Record last modified

f5 nginx open source UPGRADE

Affected: >= 0.5.15 and <= 0.9.7 · >= 1.0.0 and < 1.28.3 · >= 1.29.0 and < 1.29.7

Fixed in: 1.28.3, 1.29.7

Safe to downgrade to: a release before 0.5.15

1. Check installed version

apt-cache policy nginx-open-source || rpm -q nginx-open-source || nginx-open-source --version

2. UPGRADE to 1.28.3 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade nginx-open-source

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade nginx-open-source

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 0.5.15 (Debian / Ubuntu)

apt-cache madison nginx-open-source sudo apt install nginx-open-source=<VERSION_BEFORE_0.5.15> sudo apt-mark hold nginx-open-source

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list nginx-open-source sudo dnf downgrade nginx-open-source-<VERSION_BEFORE_0.5.15> sudo dnf versionlock add nginx-open-source

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

f5 nginx plus UPGRADE

Affected: >= r33 and < r35 · r32 · r32 p1 · r32 p2 · r32 p3 · r32 p4 · r35 · r35 p1

Fixed in: r35

Safe to downgrade to: a release before r32

1. Check installed version

apt-cache policy nginx-plus || rpm -q nginx-plus || nginx-plus --version

2. UPGRADE to r35 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade nginx-plus

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade nginx-plus

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before r32 (Debian / Ubuntu)

apt-cache madison nginx-plus sudo apt install nginx-plus=<VERSION_BEFORE_r32> sudo apt-mark hold nginx-plus

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list nginx-plus sudo dnf downgrade nginx-plus-<VERSION_BEFORE_r32> sudo dnf versionlock add nginx-plus

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-27651?

nginx open source: upgrade to 1.28.3, 1.29.7; nginx plus: upgrade to r35. Exact commands for each OS are on this page.

Is CVE-2026-27651 being exploited?

CVE-2026-27651 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-27651 affect?

CVE-2026-27651 affects nginx open source, nginx plus. Severity is high (CVSS 7.5). 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 aut

How urgent is CVE-2026-27651?

Priority 52/100. High severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

nginx vulnerabilities · F5 BIG-IP vulnerabilities

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References

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