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
5.3EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0.4%CVSS: 5.3Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

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
5.3MEDIUMmedium confidence
EG Risk
24
EG Risk 24/100

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
Severity53% × 45%
Exploitation0% × 40%
Automatability0% × 15%
EPSS PROB
0.4%
EPSS %ILE
35th
KEV
Not listed

Published

June 8, 2026

Last Modified

August 17, 2026

Advisory Details (4)

Auto-updated Jun 13, 2026
Patch available. 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

(8 across 5 ecosystems)
Hex(3)
PackageVulnerable rangeFix by version rangeDependents
cowboy1.0.0 ... 2.9.0 (31 versions)
  • every version up to 2.16.0: fixed in 2.16.0
—
cowlib2.10.0 ... 2.9.1 (17 versions)
  • 2.9.0 on: no fix on record
—
gun1.0.0 ... 2.6.0 (23 versions)
  • every version up to 2.16.0: fixed in 2.16.0
—
Debian:14(2)
PackageVulnerable rangeFix by version rangeDependents
erlang-cowboy0.10.0+dfsg1-1 ... 2.0.0~pre.1+dfsg1-4 (12 versions)
  • every version up to 2.16.1+dfsg-1: fixed in 2.16.1+dfsg-1
—
rabbitmq-server4.0.5-10 ... 4.3.2-5 (16 versions)
  • every version on: no fix on record
—
Debian:11(1)
PackageVulnerable rangeFix by version rangeDependents
rabbitmq-server3.10.8-1 ... 4.3.2-4 (38 versions)
  • every version on: no fix on record
—
Debian:12(1)
PackageVulnerable rangeFix by version rangeDependents
rabbitmq-server3.10.8-1.1 ... 4.3.2-5 (28 versions)
  • every version on: no fix on record
—
Debian:13(1)
PackageVulnerable rangeFix by version rangeDependents
rabbitmq-server4.0.5-10 ... 4.3.2-5 (18 versions)
  • every version on: no fix on record
—

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 8× in last 7d / 25× 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 210 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
  4. 2026-10-06 15:18 UTCEPSS rescore
  5. 2026-10-05 17:43 UTCEPSS rescore
  6. 2026-10-04 23:22 UTCEPSS rescore
  7. 2026-10-03 14:25 UTCEPSS rescore
  8. 2026-10-03 10:09 UTCOSV refresh
  9. 2026-10-01 19:50 UTCEPSS rescore
  10. 2026-09-30 15:03 UTCEPSS rescore
  11. 2026-09-27 13:48 UTCEPSS rescore
  12. 2026-09-26 15:58 UTCEPSS rescore
  13. 2026-09-23 17:53 UTCEPSS rescore
  14. 2026-09-20 20:15 UTCEPSS rescore
  15. 2026-09-18 19:28 UTCEPSS rescore
  16. 2026-09-17 19:31 UTCEPSS rescore
  17. 2026-09-16 14:08 UTCEPSS rescore
  18. 2026-09-16 05:15 UTCEPSS rescore
  19. 2026-09-14 04:48 UTCEG score recompute
  20. 2026-09-14 04:48 UTCVendor advisory
  21. 2026-09-13 16:47 UTCEPSS rescore
  22. 2026-09-12 15:01 UTCEPSS rescore
  23. 2026-09-11 14:53 UTCEPSS rescore
  24. 2026-09-11 09:37 UTCEPSS rescore
  25. 2026-09-10 09:34 UTCEPSS rescore
Show 75 more
  1. 2026-09-08 22:00 UTCEPSS rescore
  2. 2026-09-07 16:01 UTCEPSS rescore
  3. 2026-09-06 13:47 UTCEPSS rescore
  4. 2026-09-04 05:06 UTCEPSS rescore
  5. 2026-09-02 14:12 UTCEPSS rescore
  6. 2026-09-01 13:54 UTCEPSS rescore
  7. 2026-08-31 01:50 UTCEG score recompute
  8. 2026-08-31 01:50 UTCVendor advisory
  9. 2026-08-30 19:17 UTCEPSS rescore
  10. 2026-08-30 01:22 UTCEPSS rescore
  11. 2026-08-28 21:41 UTCEPSS rescore
  12. 2026-08-27 14:25 UTCEPSS rescore
  13. 2026-08-26 14:46 UTCEPSS rescore
  14. 2026-08-25 13:49 UTCEPSS rescore
  15. 2026-08-25 01:21 UTCEG score recompute
  16. 2026-08-25 01:21 UTCVendor advisory
  17. 2026-08-23 11:45 UTCEG score recompute
  18. 2026-08-23 11:45 UTCVendor advisory
  19. 2026-08-23 00:19 UTCEPSS rescore
  20. 2026-08-21 23:49 UTCEPSS rescore
  21. 2026-08-21 08:21 UTCEG score recompute
  22. 2026-08-21 08:21 UTCVendor advisory
  23. 2026-08-20 22:55 UTCEPSS rescore
  24. 2026-08-20 09:28 UTCEG score recompute
  25. 2026-08-20 09:28 UTCVendor advisory
  26. 2026-08-19 17:04 UTCEPSS rescore
  27. 2026-08-18 13:48 UTCEPSS rescore
  28. 2026-08-17 18:53 UTCEG score recompute▼ 1.00
  29. 2026-08-17 18:53 UTCVendor advisory
  30. 2026-08-17 18:05 UTCNVD updateCVSS v3 → 5.3
  31. 2026-08-17 13:47 UTCEPSS rescore
  32. 2026-08-17 09:22 UTCEG score recompute
  33. 2026-08-17 09:22 UTCVendor advisory
  34. 2026-08-16 14:56 UTCEPSS rescore
  35. 2026-08-16 10:39 UTCEG score recompute
  36. 2026-08-16 10:39 UTCVendor advisory
  37. 2026-08-16 02:14 UTCEPSS rescore
  38. 2026-08-15 11:57 UTCEG score recompute
  39. 2026-08-15 11:57 UTCVendor advisory
  40. 2026-08-15 01:30 UTCEPSS rescore
  41. 2026-08-14 09:01 UTCEG score recompute
  42. 2026-08-13 22:00 UTCEPSS rescore
  43. 2026-08-13 10:18 UTCEG score recompute
  44. 2026-08-13 10:18 UTCVendor advisory
  45. 2026-08-12 13:51 UTCEPSS rescore
  46. 2026-08-12 11:35 UTCEG score recompute
  47. 2026-08-12 11:35 UTCVendor advisory
  48. 2026-08-11 13:43 UTCEPSS rescore
  49. 2026-08-11 12:51 UTCEG score recompute
  50. 2026-08-11 12:51 UTCVendor advisory
  51. 2026-08-11 00:00 UTCEPSS rescore
  52. 2026-08-10 13:54 UTCVendor advisory
  53. 2026-08-09 15:11 UTCEG score recompute
  54. 2026-08-09 15:11 UTCVendor advisory
  55. 2026-08-09 13:46 UTCEPSS rescore
  56. 2026-08-08 16:37 UTCEPSS rescore
  57. 2026-08-08 16:28 UTCVendor advisory
  58. 2026-08-07 17:41 UTCEG score recompute
  59. 2026-08-06 18:58 UTCEG score recompute
  60. 2026-08-06 18:58 UTCVendor advisory
  61. 2026-08-06 13:46 UTCEPSS rescore
  62. 2026-08-05 20:15 UTCEG score recompute
  63. 2026-08-05 20:15 UTCVendor advisory
  64. 2026-08-05 19:17 UTCEPSS rescore
  65. 2026-08-04 16:22 UTCEG score recompute
  66. 2026-08-04 16:22 UTCVendor advisory
  67. 2026-08-04 15:10 UTCEPSS rescore
  68. 2026-08-04 10:38 UTCEPSS rescore
  69. 2026-08-03 15:20 UTCEG score recompute
  70. 2026-08-03 15:20 UTCVendor advisory
  71. 2026-08-03 10:36 UTCEPSS rescore
  72. 2026-08-02 09:32 UTCEG score recompute
  73. 2026-08-02 09:32 UTCVendor advisory
  74. 2026-08-02 02:27 UTCEPSS rescore
  75. 2026-08-01 10:49 UTCEG score recompute

Related CVEs(same product + same CWE)

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 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 0.4% probability of exploitation in the next 30 days (35th percentile of EPSS-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?
A fix for CVE-2026-43966 is available: update to the fixed version the vendor names in its advisory. The vendor advisories EchelonGraph has for CVE-2026-43966 are linked in the Vendor Advisories panel on this page.

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