What is CVE-2026-55766?

CVE-2026-55766 is a medium-severity vulnerability in Guzzle Psr7, classified under CRLF Injection. CVSS score: 4.8/10. Published 2026-06-23.

How severe is CVE-2026-55766?

Medium severity. CVSS v3 base score is 4.8 out of 10.

Vulnerability in Guzzle Psr7

CVE-2026-55766

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 reaso…

Vulnerability class: CRLF Injection

CVSS v3 metric

CVSS v3 base score 4.8 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N.

Affected products

Guzzle Psr7 — versions < 2.12.1

Weakness classification (CWE)

CWE-93 — CRLF Injection
CWE-113 — HTTP Response Splitting

References

security-advisories@github.com (x_refsource_CONFIRM)

Frequently asked questions

What is CVE-2026-55766?: CVE-2026-55766 is a medium-severity vulnerability in Guzzle Psr7, classified under CRLF Injection. CVSS score: 4.8/10. Published 2026-06-23.
How severe is CVE-2026-55766?: Medium severity. CVSS v3 base score is 4.8 out of 10.