CVE-2026-43966

MEDIUMNVD 5.35.3—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 5.3 under NVD CVSS v3. EPSS exploit probability: 0.2%, top 84% 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, nvd
Trending — 3 sources updated this week
5.3EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 5.3Exploit: None knownExposed: 0

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

Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') vulnerability in ninenines cowlib allows HTTP response splitting via non-VCHAR bytes in structured-fields string values.

cow_http_struct_hd:escape_string/2 in cowlib only escapes \ and ", passing all other bytes through verbatim. This creates an encoder/decoder asymmetry: the matching parser accepts only printable ASCII (0x20–0x7E, excluding " and \), but the encoder emits any byte including CR and LF. An application that builds a structured HTTP header via cow_http_struct_hd:item/1 (or a higher-level wrapper such as cow_http_hd:wt_protocol/1) from attacker-controlled input can have \r\n injected into the serialized header value. Once on the wire, the injected CRLF terminates the current header and any following bytes are interpreted as a new header, enabling HTTP response splitting.

This issue affects cowlib from 2.9.0.

CVSS v3
5.3
EG Score
5.3(medium)
EG Risk
24(Track)
EG Risk 24/100SSVC: Track

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
Severity53% × 45%
Exploitation0% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
16%
KEV
Not listed

Published

June 8, 2026

Last Modified

August 17, 2026

Advisory Details (4)

Auto-updated Jun 13, 2026
Upstream fix merged — awaiting tagged release. Sources: github_commit.
generic

OSV - Open Source Vulnerabilities

https://osv.dev/vulnerability/EEF-CVE-2026-43966
github_commit

commit 4f35609eb371 (ninenines/gun)

Fix landed in ninenines/gun commit 4f35609eb371 — awaiting tagged release

https://github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42
github_commit

commit f77cb9b5e730 (ninenines/cowboy)

Fix landed in ninenines/cowboy commit f77cb9b5e730 — awaiting tagged release

https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef
generic

HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2 | Erlang Ecosystem Foundation CNA

https://cna.erlef.org/cves/CVE-2026-43966.html

Vendor Advisories for CVE-2026-43966(2)

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

Affected Packages

(7 across 5 ecosystems)
Hex(3)
PackageVulnerable rangeFixed inDependents
cowboy1.0.0 ... 2.9.0 (31 versions)2.16.0—
cowlib2.10.0 ... 2.9.1 (16 versions)——
gun1.0.0 ... 2.5.0 (22 versions)2.16.0—
Debian:11(1)
PackageVulnerable rangeFixed inDependents
rabbitmq-server3.10.8-1 ... 4.3.2-4 (38 versions)——
Debian:12(1)
PackageVulnerable rangeFixed inDependents
rabbitmq-server3.10.8-1.1 ... 4.3.2-4 (27 versions)——
Debian:13(1)
PackageVulnerable rangeFixed inDependents
rabbitmq-server4.0.5-10 ... 4.3.2-4 (17 versions)——
Debian:14(1)
PackageVulnerable rangeFixed inDependents
rabbitmq-server4.0.5-10 ... 4.3.2-4 (15 versions)——

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 13× in last 7d / 79× 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 172 total refreshes for this CVE.

  1. 2026-08-25 13:49 UTCEPSS rescore
  2. 2026-08-25 01:21 UTCEG score recompute
  3. 2026-08-25 01:21 UTCVendor advisory
  4. 2026-08-23 11:45 UTCEG score recompute
  5. 2026-08-23 11:45 UTCVendor advisory
  6. 2026-08-23 00:19 UTCEPSS rescore
  7. 2026-08-21 23:49 UTCEPSS rescore
  8. 2026-08-21 08:21 UTCEG score recompute
  9. 2026-08-21 08:21 UTCVendor advisory
  10. 2026-08-20 22:55 UTCEPSS rescore
  11. 2026-08-20 09:28 UTCEG score recompute
  12. 2026-08-20 09:28 UTCVendor advisory
  13. 2026-08-19 17:04 UTCEPSS rescore
  14. 2026-08-18 13:48 UTCEPSS rescore
  15. 2026-08-17 18:53 UTCEG score recompute▼ 1.00
  16. 2026-08-17 18:53 UTCVendor advisory
  17. 2026-08-17 18:05 UTCNVD updateCVSS v3 → 5.3
  18. 2026-08-17 13:47 UTCEPSS rescore
  19. 2026-08-17 09:22 UTCEG score recompute
  20. 2026-08-17 09:22 UTCVendor advisory
  21. 2026-08-16 14:56 UTCEPSS rescore
  22. 2026-08-16 10:39 UTCEG score recompute
  23. 2026-08-16 10:39 UTCVendor advisory
  24. 2026-08-16 02:14 UTCEPSS rescore
  25. 2026-08-15 11:57 UTCEG score recompute
Show 75 more
  1. 2026-08-15 11:57 UTCVendor advisory
  2. 2026-08-15 01:30 UTCEPSS rescore
  3. 2026-08-14 09:01 UTCEG score recompute
  4. 2026-08-13 22:00 UTCEPSS rescore
  5. 2026-08-13 10:18 UTCEG score recompute
  6. 2026-08-13 10:18 UTCVendor advisory
  7. 2026-08-12 13:51 UTCEPSS rescore
  8. 2026-08-12 11:35 UTCEG score recompute
  9. 2026-08-12 11:35 UTCVendor advisory
  10. 2026-08-11 13:43 UTCEPSS rescore
  11. 2026-08-11 12:51 UTCEG score recompute
  12. 2026-08-11 12:51 UTCVendor advisory
  13. 2026-08-11 00:00 UTCEPSS rescore
  14. 2026-08-10 13:54 UTCVendor advisory
  15. 2026-08-09 15:11 UTCEG score recompute
  16. 2026-08-09 15:11 UTCVendor advisory
  17. 2026-08-09 13:46 UTCEPSS rescore
  18. 2026-08-08 16:37 UTCEPSS rescore
  19. 2026-08-08 16:28 UTCVendor advisory
  20. 2026-08-07 17:41 UTCEG score recompute
  21. 2026-08-06 18:58 UTCEG score recompute
  22. 2026-08-06 18:58 UTCVendor advisory
  23. 2026-08-06 13:46 UTCEPSS rescore
  24. 2026-08-05 20:15 UTCEG score recompute
  25. 2026-08-05 20:15 UTCVendor advisory
  26. 2026-08-05 19:17 UTCEPSS rescore
  27. 2026-08-04 16:22 UTCEG score recompute
  28. 2026-08-04 16:22 UTCVendor advisory
  29. 2026-08-04 15:10 UTCEPSS rescore
  30. 2026-08-04 10:38 UTCEPSS rescore
  31. 2026-08-03 15:20 UTCEG score recompute
  32. 2026-08-03 15:20 UTCVendor advisory
  33. 2026-08-03 10:36 UTCEPSS rescore
  34. 2026-08-02 09:32 UTCEG score recompute
  35. 2026-08-02 09:32 UTCVendor advisory
  36. 2026-08-02 02:27 UTCEPSS rescore
  37. 2026-08-01 10:49 UTCEG score recompute
  38. 2026-08-01 10:49 UTCVendor advisory
  39. 2026-08-01 04:16 UTCEPSS rescore
  40. 2026-07-31 04:52 UTCEG score recompute
  41. 2026-07-31 04:52 UTCVendor advisory
  42. 2026-07-30 16:28 UTCEPSS rescore
  43. 2026-07-30 01:30 UTCEPSS rescore
  44. 2026-07-29 09:14 UTCEG score recompute
  45. 2026-07-29 09:14 UTCVendor advisory
  46. 2026-07-29 09:14 UTCGHSA enrichment
  47. 2026-07-28 15:36 UTCEPSS rescore
  48. 2026-07-28 08:12 UTCEG score recompute
  49. 2026-07-28 08:12 UTCVendor advisory
  50. 2026-07-28 08:12 UTCGHSA enrichment
  51. 2026-07-27 14:13 UTCEPSS rescore
  52. 2026-07-27 08:32 UTCEG score recompute
  53. 2026-07-27 08:32 UTCVendor advisory
  54. 2026-07-27 08:32 UTCGHSA enrichment
  55. 2026-07-26 14:54 UTCEPSS rescore
  56. 2026-07-26 14:54 UTCEPSS rescore
  57. 2026-07-26 07:09 UTCEG score recompute
  58. 2026-07-26 07:09 UTCVendor advisory
  59. 2026-07-26 07:09 UTCGHSA enrichment
  60. 2026-07-25 14:18 UTCEPSS rescore
  61. 2026-07-24 14:17 UTCEPSS rescore
  62. 2026-07-23 14:18 UTCEPSS rescore
  63. 2026-07-23 03:14 UTCEG score recompute
  64. 2026-07-22 14:08 UTCEPSS rescore
  65. 2026-07-21 15:24 UTCEPSS rescore
  66. 2026-07-20 17:08 UTCEPSS rescore
  67. 2026-07-19 14:31 UTCEPSS rescore
  68. 2026-07-19 02:29 UTCEPSS rescore
  69. 2026-07-18 10:04 UTCEPSS rescore
  70. 2026-07-16 17:03 UTCEPSS rescore
  71. 2026-07-15 22:21 UTCVendor advisory
  72. 2026-07-15 22:21 UTCGHSA enrichment
  73. 2026-07-15 16:57 UTCEPSS rescore
  74. 2026-07-15 02:00 UTCEPSS rescore
  75. 2026-07-14 02:45 UTCGHSA enrichment

Frequently asked(5)

What is CVE-2026-43966?
CVE-2026-43966 is a medium vulnerability published on June 8, 2026. Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') vulnerability in ninenines cowlib allows HTTP response splitting via non-VCHAR bytes in structured-fields string values. cowhttpstructhd:escapestring/2 in cowlib only escapes \ and ", passing all other…
When was CVE-2026-43966 disclosed?
CVE-2026-43966 was first published in the National Vulnerability Database on June 8, 2026, with the most recent update on August 17, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-43966 actively exploited?
CVE-2026-43966 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 83.9% of all scored CVEs.
What is the CVSS score of CVE-2026-43966?
CVE-2026-43966 has a CVSS v3 base score of 5.3 (NVD).
How do I remediate CVE-2026-43966?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-43966, 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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