GHSA-vm85-hxw5-5432 · Severity: medium · Ecosystem: composer — guzzlehttp/psr7: CRLF Injection in HTTP Start-Line Serialization
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.
Conclusion & alert: CVE-2026-55766 is rated Low Risk (21/100): CVSS Medium severity, with low exploitation likelihood (EPSS 0.16%). Mandatory action: Monitor for updates and reassess as exploit intelligence or EPSS changes.
Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.
EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).
| # | Date | Old EPSS score | New EPSS score | Delta (New - Old) |
|---|---|---|---|---|
| 1 | 2026-06-24 | — | 0.16% | — |
Full EPSS history (1 record total)
CVSS metrics for this CVE.
| Base score | Version | Severity | Vector | Exploitability | Impact | Score source |
|---|---|---|---|---|---|---|
| 4.8 | 3.1 | MEDIUM |
|
2.2 | 2.5 | [email protected] |
GHSA-vm85-hxw5-5432 · Severity: medium · Ecosystem: composer — guzzlehttp/psr7: CRLF Injection in HTTP Start-Line Serialization
| vendor | priority | summary | link |
|---|---|---|---|
debian
|
not yet assigned | CVE-2026-55766 not yet assigned priority: Debian including 1 source packages (php-guzzlehttp-psr7), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): open 3, resolved 2. | https://security-tracker.debian.org/tracker/CVE-2026-55766 |
ubuntu
|
medium | CVE-2026-55766 medium priority: Ubuntu including 1 source packages (php-guzzlehttp-psr7), 6 status rows across 6 suites (focal, jammy, noble, questing, resolute, upstream): needs-triage 5, released 1. | https://ubuntu.com/security/CVE-2026-55766 |
| URL | Tags |
|---|---|
| https://github.com/guzzle/psr7/security/advisories/GHSA-vm85-hxw5-5432 | Mitigation Vendor Advisory |