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Fencer Vulns
medium 4.8

CVE-2026-55766 json

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

published 2026-06-19 4 mo ago modified 2026-10-25 14 d ago also EUVD-2026-38458 , GHSA-vm85-hxw5-5432

cvss
4.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 13%
public exploits
0
none known

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.

Patch / fix

  • packagist guzzlehttp/psr7 2.12.1
  • guzzlephp psr-7 2.12.1
  • php-guzzlehttp-psr7 Debian 13 2.7.1-1+deb13u2
  • php-guzzlehttp-psr7 Debian 14 2.12.1-1

debian advisories DEBIAN-CVE-2026-55766

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-55766 updated cvss 4.8

guzzlehttp/psr7: guzzlehttp/psr7: Information disclosure due to improper handling of CR/LF characters in HTTP start-line fields

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
4 mo ago

Exploit prediction

0.23% chance of exploitation in the next 30 days, higher than 13% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 4.8 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
GitHub 3.1 4.8 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

Affected packages

ecosystem package affected fixed in
packagist guzzlehttp/psr7
<2.12.1
2.12.1

Affected products

  • guzzle psr7

Classification

Weakness
  • CWE-93 Improper Neutralization of CRLF Sequences
  • CWE-113 Improper Neutralization of CRLF Sequences in HTTP Headers

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.