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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.8 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

EPSS

• CVE-2026-55766: 0.23% estimated exploitation activity in the next 30 days; percentile 13.81 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact 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 is not the normal request-sending path used by guzzlehttp/guzzle;…

Published by the source

19 Jun 2026, 14:35 UTC

Last modified by the source

19 Jun 2026, 14:35 UTC

Source last checked

26 Oct 2026, 14:49 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-vm85-hxw5-5432

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# Patch Next advisory brief: CVE-2026-55766

Advisory: guzzlehttp/psr7: CRLF Injection in HTTP Start-Line Serialization
IDs: CVE-2026-55766, GHSA-vm85-hxw5-5432
Record: https://patchnext.com/advisories/cve-2026-55766 (revision 1, source checked 26 Oct 2026, 14:49 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: guzzlehttp/psr7 2.12.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist guzzlehttp/psr7: affected < 2.12.1; fixed in 2.12.1

## Suggested inspection (read-only)
- Packagist guzzlehttp/psr7: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on guzzlehttp/psr7.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-vm85-hxw5-5432 (GitHub-reviewed advisory GHSA-vm85-hxw5-5432 lists affected ranges: Packagist guzzlehttp/psr7 >=0 <2.12.1)

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