TI-2026-40255 Aliases CVE-2026-55766 GHSA-vm85-hxw5-5432

guzzlehttp/psr7: CRLF Injection in HTTP Start-Line Serialization

LOW 21CWE-113Published 2026-06-23 · Updated 2026-06-23

Key facts

Attack vector: Network
Privileges required: None
User interaction: None
Attack complexity: High
Impact type: —
Confidentiality · integrity · availability: Low · Low · None
Fix status: Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub.

Summary

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 serialized message could contain attacker-controlled header lines. The issue can also be reached through Message::parseRequest() or Message::parseResponse() when malformed raw messages are parsed into first-party PSR-7 objects and then serialized again. Creating or modifying a Request, Response, or other PSR-7 object alone is not sufficient. The issue requires the malformed message to be serialized and written to the network, forwarded, replayed, or otherwise processed by software that does not independently reject the malformed start line. This vulnerability is fixed in 2.12.1.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV

EPSS 0.00229 13th percentileScored 2026-11-07

Fix Fixed psr7 (guzzle): 2.12.1guzzlehttp/psr7 (Packagist): 2.12.1

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: None
Automatable: No
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
guzzle	psr7	before 2.12.1	2.12.1	Patch available	CNA
—	guzzlehttp/psr7 (Packagist)	before 2.12.1	2.12.1	Patch available	GHSA-vm85-hxw5-5432

psr7 Vendorguzzle Affected before 2.12.1 Fixed in 2.12.1 Patch available CNA
guzzlehttp/psr7 (Packagist) Vendor— Affected before 2.12.1 Fixed in 2.12.1 Patch available GHSA-vm85-hxw5-5432

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:guzzlephp:psr-7::::::::	before 2.12.1	Vulnerable	guzzle

cpe:2.3:a:guzzlephp:psr-7:*:*:*:*:*:*:*:* Versions before 2.12.1 Vulnerable ·guzzle

Remediation

Fix · CNA psr7 (guzzle) Fixed in 2.12.1
Fix · GHSA-vm85-hxw5-5432 guzzlehttp/psr7 (Packagist) Fixed in 2.12.1 Advisory →

Vendor assessments

Red Hat

Red Hat's page for this CVE →

Not affected: 1 product

Red Hat

Timeline

2026-10-02 Threatist record created from CVE-2026-55766
2026-10-03 Alias added: GHSA-vm85-hxw5-5432

References

Advisory 1

https://github.com/guzzle/psr7/security/advisories/GHSA-vm85-hxw5-5432

Other 1

https://github.com/guzzle/psr7

TI-2022-08589 Improper Input Validation in guzzlehttp/psr7 CVSS 3.1HIGH 7.5
TI-2023-13898 Laminas Diactoros vulnerable to HTTP Multiline Header Termination CVSS 3.1HIGH 7.5
TI-2023-13625 Improper header name validation in guzzlehttp/psr7 CVSS 3.1HIGH 7.5
TI-2026-42696 guzzlehttp/psr7: Host Confusion via Weak URI Host Validation CVSS 3.1MEDIUM 6.5
TI-2026-36415 guzzlehttp/psr7 has CRLF Injection via URI Host Component CVSS 3.1MEDIUM 5.3
TI-2026-36252 guzzlehttp/psr7 has Host Confusion via Authority Reinterpretation CVSS 3.1MEDIUM 5.3

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences