CVE-2026-55766: guzzlehttp/psr7 is a PSR-7 HTTP message library implementation in PHP

Q: How do I fix CVE-2026-55766?

psr-7: upgrade to 2.12.1. Exact commands for each OS are on this page.

Q: Is CVE-2026-55766 being exploited?

CVE-2026-55766 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-55766 affect?

CVE-2026-55766 affects psr-7. Severity is medium (CVSS 4.8). guzzlehttp/psr7 is a PSR-7 HTTP message library implementation in PHP. Prior to 2.12.1, guzzlehttp/psr7 did not reject CR/LF characters in certain first-party HTTP start-line fields: the request method, protocol version, and response reason

Q: How urgent is CVE-2026-55766?

Priority 25/100. Remotely exploitable, no login, no user action

Routine

MEDIUM 4.8REMOTEUPGRADE

CVE-2026-55766 is a medium-severity flaw (CVSS 4.8) affecting psr-7. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

guzzlehttp/psr7 is a PSR-7 HTTP message library implementation in PHP. Prior to 2.12.1, guzzlehttp/psr7 did not reject CR/LF characters in certain first-party HTTP start-line fields: the request method, protocol version, and response reason phrase. If an application placed attacker-controlled data into one of those fields and later serialized the PSR-7 message as raw HTTP/1.x, for example with Message::toString() or an equivalent serializer, the

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	4.8 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 86%)
CISA exploited list	No
Weakness	CWE-93, CWE-113
Published	2026-06-23
Last modified	2026-10-25

Publisher (CNA): GitHub, Inc. · Vendor: guzzlephp

Last updated 2026-10-25 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-23 16:17 UTCPublished CVE record published
2026-10-25 23:10 UTCUpdated Record last modified

guzzlephp psr-7 UPGRADE

Affected: < 2.12.1

Fixed in: 2.12.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy psr-7 || rpm -q psr-7 || psr-7 --version

2. UPGRADE to 2.12.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade psr-7

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade psr-7

Note

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

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-2026-55766?

psr-7: upgrade to 2.12.1. Exact commands for each OS are on this page.

Is CVE-2026-55766 being exploited?

CVE-2026-55766 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-2026-55766 affect?

CVE-2026-55766 affects psr-7. Severity is medium (CVSS 4.8). guzzlehttp/psr7 is a PSR-7 HTTP message library implementation in PHP. Prior to 2.12.1, guzzlehttp/psr7 did not reject CR/LF characters in certain first-party HTTP start-line fields: the request method, protocol version, and response reason

How urgent is CVE-2026-55766?

Priority 25/100. Remotely exploitable, no login, no user action

Software pages

PHP vulnerabilities

Related CVEs

CVE-2026-49214 MEDIUM 5.3 UPGRADE guzzlehttp/psr7 is a PSR-7 HTTP message library implementation in PHP
CVE-2026-59882 MEDIUM 4.2 UPGRADE guzzlehttp/psr7 is a PSR-7 HTTP message library implementation in PHP

References

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