CVE-2026-43966

MEDIUMPre-NVD 6.36.3—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 6.3 under a secondary CVSS source (NVD's own analysis pending). EPSS exploit probability: 0.3%, top 77% 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
Trending — 3 sources updated this week
6.3
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS: 0%CVSS: 6.3Exploit: NoneExposed: 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
6.3
EG Score
6.3(medium)
EPSS
23.1%
KEV
Not listed

Published

June 8, 2026

Last Modified

June 9, 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

(1 across 1 ecosystem)
Hex(1)
PackageVulnerable rangeFixed inDependents
cowlib2.10.0 ... 2.9.1 (13 versions)——

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 18× in last 7d / 89× 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 16:27 UTCEPSS rescore
  2. 2026-07-06 02:23 UTCEPSS rescore
  3. 2026-07-05 17:29 UTCEG score recompute
  4. 2026-07-05 17:29 UTCGHSA enrichment
  5. 2026-07-05 02:30 UTCEPSS rescore
  6. 2026-07-04 14:58 UTCEG score recompute
  7. 2026-07-04 14:58 UTCGHSA enrichment
  8. 2026-07-04 06:31 UTCEPSS rescore
  9. 2026-07-03 16:14 UTCEG score recompute
  10. 2026-07-03 16:14 UTCGHSA enrichment
  11. 2026-07-02 16:52 UTCEG score recompute
  12. 2026-07-02 16:52 UTCGHSA enrichment
  13. 2026-07-01 18:07 UTCEG score recompute
  14. 2026-07-01 18:07 UTCGHSA enrichment
  15. 2026-07-01 15:07 UTCEPSS rescore
  16. 2026-06-30 23:22 UTCEPSS rescore
  17. 2026-06-30 18:29 UTCEG score recompute
  18. 2026-06-30 18:29 UTCGHSA enrichment
  19. 2026-06-29 19:45 UTCEG score recompute
  20. 2026-06-29 19:45 UTCGHSA enrichment
  21. 2026-06-29 14:06 UTCEPSS rescore
  22. 2026-06-28 21:02 UTCEG score recompute
  23. 2026-06-28 21:02 UTCGHSA enrichment
  24. 2026-06-28 14:07 UTCEPSS rescore
  25. 2026-06-28 04:56 UTCEPSS rescore
Show 64 more
  1. 2026-06-27 22:19 UTCEG score recompute
  2. 2026-06-27 22:19 UTCGHSA enrichment
  3. 2026-06-27 03:08 UTCEPSS rescore
  4. 2026-06-26 23:36 UTCEG score recompute
  5. 2026-06-26 23:36 UTCGHSA enrichment
  6. 2026-06-26 00:52 UTCEG score recompute
  7. 2026-06-26 00:52 UTCGHSA enrichment
  8. 2026-06-25 13:49 UTCEPSS rescore
  9. 2026-06-25 13:49 UTCEPSS rescore
  10. 2026-06-25 02:08 UTCEG score recompute
  11. 2026-06-25 02:08 UTCGHSA enrichment
  12. 2026-06-24 14:05 UTCEPSS rescore
  13. 2026-06-24 03:24 UTCEG score recompute
  14. 2026-06-24 03:24 UTCGHSA enrichment
  15. 2026-06-23 21:33 UTCEPSS rescore
  16. 2026-06-23 04:24 UTCEG score recompute
  17. 2026-06-23 04:24 UTCGHSA enrichment
  18. 2026-06-22 14:25 UTCEPSS rescore
  19. 2026-06-22 14:25 UTCEPSS rescore
  20. 2026-06-22 05:41 UTCEG score recompute
  21. 2026-06-22 05:41 UTCGHSA enrichment
  22. 2026-06-21 14:56 UTCEPSS rescore
  23. 2026-06-21 14:56 UTCEPSS rescore
  24. 2026-06-21 02:25 UTCEG score recompute
  25. 2026-06-21 02:25 UTCGHSA enrichment
  26. 2026-06-21 01:59 UTCEPSS rescore
  27. 2026-06-20 02:46 UTCEG score recompute
  28. 2026-06-20 02:46 UTCGHSA enrichment
  29. 2026-06-19 19:25 UTCEPSS rescore
  30. 2026-06-19 03:07 UTCEG score recompute
  31. 2026-06-19 03:07 UTCGHSA enrichment
  32. 2026-06-18 17:52 UTCEPSS rescore
  33. 2026-06-18 17:52 UTCEPSS rescore
  34. 2026-06-18 04:14 UTCEG score recompute
  35. 2026-06-18 04:14 UTCGHSA enrichment
  36. 2026-06-17 17:53 UTCEPSS rescore
  37. 2026-06-17 05:28 UTCEG score recompute
  38. 2026-06-17 05:28 UTCGHSA enrichment
  39. 2026-06-16 17:52 UTCEPSS rescore
  40. 2026-06-16 06:45 UTCEG score recompute
  41. 2026-06-16 06:44 UTCGHSA enrichment
  42. 2026-06-15 17:49 UTCEPSS rescore
  43. 2026-06-15 08:02 UTCEG score recompute
  44. 2026-06-15 08:02 UTCGHSA enrichment
  45. 2026-06-14 09:18 UTCEG score recompute
  46. 2026-06-14 09:18 UTCGHSA enrichment
  47. 2026-06-13 23:00 UTCEPSS rescore
  48. 2026-06-13 10:35 UTCEG score recompute
  49. 2026-06-13 10:35 UTCGHSA enrichment
  50. 2026-06-12 23:12 UTCEPSS rescore
  51. 2026-06-12 23:12 UTCEPSS rescore
  52. 2026-06-12 11:49 UTCEG score recompute
  53. 2026-06-12 11:49 UTCGHSA enrichment
  54. 2026-06-11 14:00 UTCEPSS rescore
  55. 2026-06-11 13:06 UTCEG score recompute
  56. 2026-06-11 13:06 UTCGHSA enrichment
  57. 2026-06-10 22:18 UTCEPSS rescore
  58. 2026-06-10 14:23 UTCEG score recompute
  59. 2026-06-10 14:23 UTCGHSA enrichment
  60. 2026-06-10 13:22 UTCEPSS rescore
  61. 2026-06-10 13:22 UTCEPSS rescore
  62. 2026-06-09 15:40 UTCEG score recompute
  63. 2026-06-09 15:40 UTCGHSA enrichment
  64. 2026-06-08 16:57 UTCEG score recompute

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 June 9, 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 23.1% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2026-43966?
CVE-2026-43966 has a CVSS v3 base score of 6.3 (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.

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