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

CVE-2026-55766 is fixed in php-guzzlehttp-psr7 on 1 platform, first by DEBIAN-CVE-2026-55766, released June 24, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-55766
- Priority: Routine
- CVSS: 4.8 (medium)
- EPSS: 0.0023
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jun 23, 2026

## Description

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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | php-guzzlehttp-psr7 Debian 13.5 (trixie) | 2.7.1-1+deb13u2 | [DEBIAN-CVE-2026-55766](https://patchblox.com/security/data/fix/linux/debian-cve-2026-55766.md) | Jun 24, 2026 |
| Linux | php-guzzlehttp-psr7 Debian 14 | 2.12.1-1 | [DEBIAN-CVE-2026-55766](https://patchblox.com/security/data/fix/linux/debian-cve-2026-55766.md) | Jun 24, 2026 |

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Data as of 2026-11-08T19:12:23Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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