CVE-2026-55766

MEDIUMPre-NVD 4.84.8—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 4.8 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.2%, top 95% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, secondary
4.8
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS: 0%CVSS: 4.8Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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.

CVSS v3
4.8
EG Score
4.8(medium)
EPSS
5.3%
KEV
Not listed

Published

June 19, 2026

Last Modified

June 30, 2026

Advisory Details (1)

Auto-updated Jun 23, 2026
Patch available. Sources: github.
github Patch Available

CRLF Injection in HTTP Start-Line Serialization in guzzlehttp/psr7 · Advisory · guzzle/psr7 · GitHub

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

Vendor Advisories for CVE-2026-55766(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Affected Packages

(1 across 1 ecosystem)
Packagist(1)
PackageVulnerable rangeFixed inDependents
guzzlehttp/psr71.0.0 ... 2.9.1 (61 versions)2.12.1—

Weakness Classification(2)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 16× in last 7d / 36× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-07-06 21:38 UTCEG score recompute
  2. 2026-07-06 16:27 UTCEPSS rescore
  3. 2026-07-06 16:27 UTCEPSS rescore
  4. 2026-07-06 02:23 UTCEPSS rescore
  5. 2026-07-06 02:23 UTCEPSS rescore
  6. 2026-07-05 22:53 UTCEG score recompute
  7. 2026-07-05 02:31 UTCEPSS rescore
  8. 2026-07-05 02:30 UTCEPSS rescore
  9. 2026-07-05 00:08 UTCEG score recompute
  10. 2026-07-04 06:31 UTCEPSS rescore
  11. 2026-07-04 06:31 UTCEPSS rescore
  12. 2026-07-04 01:19 UTCEG score recompute
  13. 2026-07-03 02:34 UTCEG score recompute
  14. 2026-07-02 03:50 UTCEG score recompute
  15. 2026-07-01 15:07 UTCEPSS rescore
  16. 2026-07-01 05:05 UTCEG score recompute
  17. 2026-06-30 06:20 UTCEG score recompute
  18. 2026-06-29 14:06 UTCEPSS rescore
  19. 2026-06-29 14:06 UTCEPSS rescore
  20. 2026-06-29 07:35 UTCEG score recompute
  21. 2026-06-28 14:07 UTCEPSS rescore
  22. 2026-06-28 08:50 UTCEG score recompute
  23. 2026-06-28 04:56 UTCEPSS rescore
  24. 2026-06-28 04:56 UTCEPSS rescore
  25. 2026-06-27 10:05 UTCEG score recompute
Show 11 more
  1. 2026-06-27 03:08 UTCEPSS rescore
  2. 2026-06-26 11:21 UTCEG score recompute
  3. 2026-06-25 13:49 UTCEPSS rescore
  4. 2026-06-25 07:41 UTCEG score recompute
  5. 2026-06-24 14:05 UTCEPSS rescore
  6. 2026-06-24 08:56 UTCEG score recompute
  7. 2026-06-23 10:10 UTCEG score recompute
  8. 2026-06-22 11:25 UTCEG score recompute
  9. 2026-06-21 12:29 UTCEG score recompute
  10. 2026-06-20 13:42 UTCEG score recompute
  11. 2026-06-19 14:57 UTCEG score recompute

Frequently asked(5)

What is CVE-2026-55766?
CVE-2026-55766 is a medium vulnerability published on June 19, 2026. 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…
When was CVE-2026-55766 disclosed?
CVE-2026-55766 was first published in the National Vulnerability Database on June 19, 2026, with the most recent update on June 30, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-55766 actively exploited?
CVE-2026-55766 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 5.3% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2026-55766?
CVE-2026-55766 has a CVSS v3 base score of 4.8 (NVD).
How do I remediate CVE-2026-55766?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-55766, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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