CVE-2021-31535

CRITICALNVD 9.89.8—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 9.8 under NVD CVSS v3. EPSS exploit probability: 10.6%, top 5% 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
Elevated
9.8
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 11%CVSS: 9.8Exploit: NoneExposed: 0

A fix is available — apply it.

LookupCol.c in X.Org X through X11R7.7 and libX11 before 1.7.1 might allow remote attackers to execute arbitrary code. The libX11 XLookupColor request (intended for server-side color lookup) contains a flaw allowing a client to send color-name requests with a name longer than the maximum size allowed by the protocol (and also longer than the maximum packet size for normal-sized packets). The user-controlled data exceeding the maximum size is then interpreted by the server as additional X protocol requests and executed, e.g., to disable X server authorization completely. For example, if the victim encounters malicious terminal control sequences for color codes, then the attacker may be able to take full control of the running graphical session.

CVSS v3
9.8
EG Score
9.8(medium)
EG Risk
65(Track*)
EG Risk 65/100SSVC: 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
Severity98% × 45%
Exploitation40% × 40%
Automatability30% × 15%
Action: Watch closely — could escalate to Attend.
EPSS
95.3%
KEV
Not listed

Published

May 27, 2021

Last Modified

November 21, 2024

Advisory Details (10)

Auto-updated Jun 5, 2026
Patch available.
generic

[SECURITY] Fedora 33 Update: libX11-1.7.2-3.fc33 - package-announce - Fedora mailing-lists

https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TEOT4RLB76RVPJQKGGTIKTBIOLHX2NR6/
generic Patch Available

[SECURITY] [DLA 2666-1] libx11 security update

https://lists.debian.org/debian-lts-announce/2021/05/msg00021.html
generic
https://lists.apache.org/thread.html/rbf4ce74b0d1fa9810dec50ba3ace0caeea677af7c27a97111c06ccb7%40%3Cusers.kafka.apache.org%3E
generic
https://lists.apache.org/thread.html/rbf4ce74b0d1fa9810dec50ba3ace0caeea677af7c27a97111c06ccb7%40%3Cdev.kafka.apache.org%3E
generic
https://lists.apache.org/thread.html/r61db8e7dcb56dc000a5387a88f7a473bacec5ee01b9ff3f55308aacc%40%3Cusers.kafka.apache.org%3E
generic
https://lists.apache.org/thread.html/r61db8e7dcb56dc000a5387a88f7a473bacec5ee01b9ff3f55308aacc%40%3Cdev.kafka.apache.org%3E
generic

Reject string longer than USHRT_MAX before sending them on the wire (8d2e02ae) · Commits · xorg / lib / libX11 · GitLab

https://gitlab.freedesktop.org/xorg/lib/libx11/-/commit/8d2e02ae650f00c4a53deb625211a0527126c605
generic

oss-security - libX11 security advisory: May 18, 2021

http://www.openwall.com/lists/oss-security/2021/05/18/2
generic

Full Disclosure: CVE-2021-31535 libX11 Insufficient Length Checks PoC and Archeology

http://seclists.org/fulldisclosure/2021/May/52
generic

Packet Storm

http://packetstormsecurity.com/files/162737/libX11-Insufficient-Length-Check-Injection.html

Vendor Advisories for CVE-2021-31535(2)

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

Patch Availability(6)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntulibx11-dev (2:1.7.0-2ubuntu0.1) @ hirsute2026-05-23ubuntu
ubuntulibx11-xcb1 (2:1.6.3-1ubuntu2.2+esm1) @ xenial2026-05-23ubuntu
redhatlibX11-0:1.6.8-5.el82021-11-09redhat
redhatredhat-virtualization-host-0:4.3.18-20210903.0.el7_92021-09-09redhat
redhatansible-tower-38/ansible-tower-rhel7:3.8.4-12021-09-08redhat
redhatlibX11-0:1.6.7-4.el7_92021-08-30redhat

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.

Weakness Classification(1)

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

Additional Vendor Advisories

(4)

Vendors that published advisories for this CVE beyond the curated set above. Broader coverage but minimal per-row detail — click through for the original advisory.

Data Freshness Timeline

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

Frequently asked(5)

What is CVE-2021-31535?
CVE-2021-31535 is a critical vulnerability published on May 27, 2021. LookupCol.c in X.Org X through X11R7.7 and libX11 before 1.7.1 might allow remote attackers to execute arbitrary code. The libX11 XLookupColor request (intended for server-side color lookup) contains a flaw allowing a client to send color-name requests with a name longer than the maximum size…
When was CVE-2021-31535 disclosed?
CVE-2021-31535 was first published in the National Vulnerability Database on May 27, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-31535 actively exploited?
CVE-2021-31535 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 95.3% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-31535?
CVE-2021-31535 has a CVSS v3 base score of 9.8 (NVD).
How do I remediate CVE-2021-31535?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-31535, 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.

Dependency Blast Radius

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