CVE-2026-91768: The IPv6 branch of the FastCGI client access check compares only the first 12 bytes of a 16-byte IPv6 address,

Routine

MEDIUM 6.5UPGRADE

CVE-2026-91768 is a medium-severity flaw (CVSS 6.5) affecting php. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The IPv6 branch of the FastCGI client access check compares only the first 12 bytes of a 16-byte IPv6 address, so listen.allowed_clients matches on a /96 prefix instead of the exact address. An attacker who can source an address sharing the first 96 bits with an allowed one passes the check and reaches the FastCGI endpoint.

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	6.5 (Medium)
Attack vector	Adjacent network
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 69%)
CISA exploited list	No
Weakness	CWE-1023
Published	2026-09-25
Last modified	2026-10-27

Publisher (CNA): PHP Group · Vendor: php

Last updated 2026-10-27 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-25 22:18 UTCPublished CVE record published
2026-10-27 15:40 UTCUpdated Record last modified

php php UPGRADE

Affected: >= 8.2.0 and < 8.2.34 · >= 8.3.0 and < 8.3.35 · >= 8.4.0 and < 8.4.26 · >= 8.5.0 and < 8.5.11

Fixed in: 8.2.34, 8.3.35, 8.4.26, 8.5.11

Safe to downgrade to: a release before 8.2.0

1. Check installed version

apt-cache policy php || rpm -q php || php --version

2. UPGRADE to 8.2.34 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade php

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade php

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 8.2.0 (Debian / Ubuntu)

apt-cache madison php sudo apt install php=<VERSION_BEFORE_8.2.0> sudo apt-mark hold php

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

dnf --showduplicates list php sudo dnf downgrade php-<VERSION_BEFORE_8.2.0> sudo dnf versionlock add php

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-2026-91768?

php: upgrade to 8.2.34, 8.3.35, 8.4.26, 8.5.11. Exact commands for each OS are on this page.

Is CVE-2026-91768 being exploited?

CVE-2026-91768 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-2026-91768 affect?

CVE-2026-91768 affects php. Severity is medium (CVSS 6.5). The IPv6 branch of the FastCGI client access check compares only the first 12 bytes of a 16-byte IPv6 address, so listen.allowed_clients matches on a /96 prefix instead of the exact address. An attacker who can source an address sharing the

How urgent is CVE-2026-91768?

Priority 31/100. No elevated risk signals yet.

Software pages

PHP vulnerabilities

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References

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