CVE-2026-55991

MEDIUMPre-NVD 5.95.9—
EchelonGraph scoreMEDIUM confidence

Score 5.9 from GitHub Security Advisory published 2026-07-22. a secondary CVSS source baseline 5.9; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, secondary
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

July 24, 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 22× in last 7d / 99× 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 199 total refreshes for this CVE.

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

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 July 24, 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 85.1% of all scored CVEs.
What is the CVSS score of CVE-2026-55991?
CVE-2026-55991 has a CVSS v3 base score of 5.9 (NVD).
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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