CVE-2026-27820

CRITICALNVD 9.89.8—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 9.8 under NVD CVSS v3. EPSS exploit probability: 0.0%, top 96% 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
9.8
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 1%CVSS: 9.8Exploit: NoneExposed: 0

A fix is available — apply it.

zlib is a Ruby interface for the zlib compression/decompression library. Versions 3.0.0 and below, 3.1.0, 3.1.1, 3.2.0 and 3.2.1 contain a buffer overflow vulnerability in the Zlib::GzipReader. The zstream_buffer_ungets function prepends caller-provided bytes ahead of previously produced output but fails to guarantee the backing Ruby string has enough capacity before the memmove shifts the existing data. This can lead to memory corruption when the buffer length exceeds capacity. This issue has been fixed in versions 3.0.1, 3.1.2 and 3.2.3.

CVSS v3
9.8
EG Score
9.8(medium)
EPSS
42.9%
KEV
Not listed

Published

April 16, 2026

Last Modified

May 21, 2026

Advisory Details (1)

Auto-updated Jun 20, 2026
Patch available. Sources: github.
github Patch Available

Buffer Overflow vulnerability in Zlib::GzipReader · Advisory · ruby/zlib · GitHub

https://github.com/ruby/zlib/security/advisories/GHSA-g857-hhfv-j68w

Vendor Advisories for CVE-2026-27820(5)

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

Patch Availability(3)

Vendor / EcosystemFixed in / PatchReleasedSource
redhatruby4-0-main-4.0.0-33.3.hum12026-04-17redhat
redhatruby3-3-main-3.3.10-23.1.hum12026-04-09redhat
redhatruby3-4-main-3.4.8-31.1.hum12026-04-09redhat

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

(1 across 1 ecosystem)
RubyGems(1)
PackageVulnerable rangeFixed inDependents
zlib0.0.1 ... 3.0.0 (8 versions)3.0.1—

Weakness Classification(2)

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

Data Freshness Timeline

(refreshed 11× in last 7d / 48× 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 140 total refreshes for this CVE.

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

Related CVEs(same vendor + same CWE)

Frequently asked(5)

What is CVE-2026-27820?
CVE-2026-27820 is a critical vulnerability published on April 16, 2026. zlib is a Ruby interface for the zlib compression/decompression library. Versions 3.0.0 and below, 3.1.0, 3.1.1, 3.2.0 and 3.2.1 contain a buffer overflow vulnerability in the Zlib::GzipReader. The zstreambufferungets function prepends caller-provided bytes ahead of previously produced output but…
When was CVE-2026-27820 disclosed?
CVE-2026-27820 was first published in the National Vulnerability Database on April 16, 2026, with the most recent update on May 21, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-27820 actively exploited?
CVE-2026-27820 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 42.9% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2026-27820?
CVE-2026-27820 has a CVSS v3 base score of 9.8 (NVD).
How do I remediate CVE-2026-27820?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-27820, 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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