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 86% 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
4.8EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0.2%CVSS: 4.8Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

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
4.8MEDIUMmedium confidence
EG Risk
22
EG Risk 22/100

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity48% × 45%
Exploitation0% × 40%
Automatability0% × 15%
EPSS PROB
0.2%
EPSS %ILE
12th
KEV
Not listed

Published

June 19, 2026

Last Modified

June 30, 2026

Advisory Details (1)

Auto-updated Jun 23, 2026
Patch available. Sources: github.
githubMentions a patch (unverified)

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

(5 across 5 ecosystems)
Debian:11(1)
PackageVulnerable rangeFix by version rangeDependents
php-guzzlehttp-psr71.7.0-1 ... 3.0.0-1 (42 versions)
  • every version on: no fix on record
—
Debian:12(1)
PackageVulnerable rangeFix by version rangeDependents
php-guzzlehttp-psr72.10.3-1 ... 3.1.0-1 (28 versions)
  • every version on: no fix on record
—
Debian:13(1)
PackageVulnerable rangeFix by version rangeDependents
php-guzzlehttp-psr72.7.1-1, 2.7.1-1+deb13u1
  • every version up to 2.7.1-1+deb13u2: fixed in 2.7.1-1+deb13u2
—
Debian:14(1)
PackageVulnerable rangeFix by version rangeDependents
php-guzzlehttp-psr72.10.3-1 ... 2.9.0-2 (9 versions)
  • every version up to 2.12.1-1: fixed in 2.12.1-1
—
Packagist(1)
PackageVulnerable rangeFix by version rangeDependents
guzzlehttp/psr71.0.0 ... 2.9.1 (61 versions)
  • every version up to 2.12.1: fixed in 2.12.1
—

Weakness Classification(2)

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

Data Freshness Timeline

(refreshed 7× in last 7d / 27× 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.

Showing the most recent 100 of 137 total refreshes for this CVE.

  1. 2026-10-09 20:07 UTCEPSS rescore
  2. 2026-10-08 17:12 UTCEPSS rescore
  3. 2026-10-07 23:48 UTCEPSS rescore
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  9. 2026-10-02 15:11 UTCOSV refresh
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  24. 2026-09-13 16:47 UTCEPSS rescore
  25. 2026-09-13 13:16 UTCEG score recompute
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  1. 2026-09-11 14:53 UTCEPSS rescore
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  3. 2026-09-08 22:01 UTCEPSS rescore
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  8. 2026-09-01 13:54 UTCEPSS rescore
  9. 2026-09-01 04:40 UTCEPSS rescore
  10. 2026-08-30 20:52 UTCEG score recompute
  11. 2026-08-30 19:17 UTCEPSS rescore
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  17. 2026-08-24 22:18 UTCEG score recompute
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  19. 2026-08-23 00:19 UTCEPSS rescore
  20. 2026-08-22 23:56 UTCEG score recompute
  21. 2026-08-21 23:49 UTCEPSS rescore
  22. 2026-08-21 07:22 UTCEG score recompute
  23. 2026-08-20 22:55 UTCEPSS rescore
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  25. 2026-08-19 17:04 UTCEPSS rescore
  26. 2026-08-19 07:19 UTCEG score recompute
  27. 2026-08-18 13:48 UTCEPSS rescore
  28. 2026-08-17 13:47 UTCEPSS rescore
  29. 2026-08-17 08:47 UTCEG score recompute
  30. 2026-08-16 14:56 UTCEPSS rescore
  31. 2026-08-16 10:02 UTCEG score recompute
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  35. 2026-08-14 12:32 UTCEG score recompute
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  37. 2026-08-12 14:57 UTCEG score recompute
  38. 2026-08-12 13:51 UTCEPSS rescore
  39. 2026-08-11 16:13 UTCEG score recompute
  40. 2026-08-11 00:00 UTCEPSS rescore
  41. 2026-08-09 17:45 UTCEG score recompute
  42. 2026-08-09 13:46 UTCEPSS rescore
  43. 2026-08-08 19:01 UTCEG score recompute
  44. 2026-08-08 16:37 UTCEPSS rescore
  45. 2026-08-06 21:32 UTCEG score recompute
  46. 2026-08-06 13:47 UTCEPSS rescore
  47. 2026-08-05 22:48 UTCEG score recompute
  48. 2026-08-05 19:17 UTCEPSS rescore
  49. 2026-08-05 00:03 UTCEG score recompute
  50. 2026-08-04 15:10 UTCEPSS rescore
  51. 2026-08-04 10:39 UTCEPSS rescore
  52. 2026-08-04 01:19 UTCEG score recompute
  53. 2026-08-03 10:36 UTCEPSS rescore
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  57. 2026-08-01 04:16 UTCEPSS rescore
  58. 2026-07-31 03:48 UTCEG score recompute
  59. 2026-07-30 16:28 UTCEPSS rescore
  60. 2026-07-30 03:00 UTCEG score recompute
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  62. 2026-07-29 01:39 UTCEG score recompute
  63. 2026-07-28 15:37 UTCEPSS rescore
  64. 2026-07-27 18:04 UTCEG score recompute
  65. 2026-07-26 16:57 UTCEG score recompute
  66. 2026-07-26 14:54 UTCEPSS rescore
  67. 2026-07-25 14:18 UTCEPSS rescore
  68. 2026-07-24 14:18 UTCEPSS rescore
  69. 2026-07-23 14:18 UTCEPSS rescore
  70. 2026-07-23 03:20 UTCEG score recompute
  71. 2026-07-22 14:08 UTCEPSS rescore
  72. 2026-07-21 15:25 UTCEPSS rescore
  73. 2026-07-20 17:08 UTCEPSS rescore
  74. 2026-07-19 14:31 UTCEPSS rescore
  75. 2026-07-19 14:31 UTCEPSS rescore

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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 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 0.2% probability of exploitation in the next 30 days (12th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2026-55766?
CVE-2026-55766 has a CVSS base score of 4.8 (a secondary CVSS source that NVD displays; NVD's own analysis pending).
How do I remediate CVE-2026-55766?
A fix for CVE-2026-55766 is available: update to the fixed version the vendor names in its advisory. The vendor advisories EchelonGraph has for CVE-2026-55766 are linked in the Vendor Advisories panel on this page.

Dependency Blast Radius

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