CVE-2026-55991

MEDIUMPre-NVD 5.95.9—
EchelonGraph scoreMEDIUM confidence

Score 5.9 from GitHub Security Advisory published 2026-07-22. the CNA's CVSS baseline 5.9; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: cna:nlnet labs, epss, ghsa
Trending — 4 sources updated this week
5.9EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 5.9Exploit: None knownExposed: 0

A fix is available — apply it.

In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can trigger a libngtcp2 assertion (if compiled with assertions on) and terminate the entire Unbound process using a single DNS-over-QUIC (DoQ) connection and one normal DNS query. This is caused by an erroneous error value passed to libngtcp2. When 'ngtcp2_conn_writev_stream()' returns 'NGTCP2_ERR_STREAM_DATA_BLOCKED', Unbound continues to call 'ngtcp2_ccerr_set_application_error()' with a '-1' error value. The 'int' literal '-1' is implicitly converted to the function's 'uint64_t error_code' parameter as '0xFFFFFFFFFFFFFFFF'. The follow-on 'ngtcp2_conn_write_connection_close()' serialises that value as a QUIC variable-length integer; because '2^64-1' exceeds the 62-bit varint ceiling, 'ngtcp2_put_uvarintlen()' fails 'assert(n < 4611686018427387904ULL)' and the whole resolver process aborts. A remote, unauthenticated DoQ client can trigger this deterministically with a single QUIC connection by advertising 'initial_max_stream_data_bidi_local = 1' in its transport parameters and sending one DoQ query without ever reading the stream.

CVSS v3
5.9
EG Score
5.9(medium)
EG Risk
27(Track)
EG Risk 27/100SSVC: 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
Severity59% × 45%
Exploitation0% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
15%
KEV
Not listed

Published

July 22, 2026

Last Modified

September 16, 2026

Advisory Details (1)

Auto-updated Jul 22, 2026
No patch confirmed yet.
generic
https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt

Vendor Advisories for CVE-2026-55991(3)

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

Patch Availability(1)

Vendor / EcosystemFixed in / PatchReleasedSource
redhatunbound-main-1.25.2-0.1.hum12026-07-22redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Affected Packages

(4 across 4 ecosystems)
Debian:11(1)
PackageVulnerable rangeFixed inDependents
unbound1.13.1-1 ... 1.25.2-1 (47 versions)——
Debian:12(1)
PackageVulnerable rangeFixed inDependents
unbound1.17.1-2 ... 1.26.0-2 (25 versions)——
Debian:13(1)
PackageVulnerable rangeFixed inDependents
unbound1.22.0-2 ... 1.26.0-2 (15 versions)——
Debian:14(1)
PackageVulnerable rangeFixed inDependents
unbound1.22.0-2 ... 1.25.1-1 (9 versions)1.25.2-1—

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 23× in last 7d / 97× 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 208 total refreshes for this CVE.

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

Frequently asked(5)

What is CVE-2026-55991?
CVE-2026-55991 is a medium vulnerability published on July 22, 2026. In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can trigger a libngtcp2 assertion (if compiled with assertions on) and terminate the entire Unbound process using a single DNS-over-QUIC (DoQ) connection and one normal DNS query. This is caused by an erroneous…
When was CVE-2026-55991 disclosed?
CVE-2026-55991 was first published in the National Vulnerability Database on July 22, 2026, with the most recent update on September 16, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-55991 actively exploited?
CVE-2026-55991 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 84.9% of all scored CVEs.
What is the CVSS score of CVE-2026-55991?
CVE-2026-55991 has a CVSS v4.0 base score of 5.9 (CNA self-assessment; NVD's own analysis pending).
How do I remediate CVE-2026-55991?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-55991, 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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