What’s Vulnerable › Vulnerabilities

CVE-2021-31535

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 10.6% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2021-05-27 · updated 2026-06-17
• Weakness: CWE-120

Description

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.

Affected products

• Fedoraproject Fedora: 33
• x.org libX11: before 1.7.1 — fixed in 1.7.1
• X.Org X Window System: through x11r7.7

How to fix

Upgrade to a fixed version: x.org libX11 1.7.1. Check the vendor advisory for mitigations.

References

• http://packetstormsecurity.com/files/162737/libX11-Insufficient-Length-Check-Injection.html
• http://seclists.org/fulldisclosure/2021/May/52
• http://www.openwall.com/lists/oss-security/2021/05/18/2
• https://gitlab.freedesktop.org/xorg/lib/libx11/-/commit/8d2e02ae650f00c4a53deb625211a0527126c605
• https://lists.apache.org/thread.html/r61db8e7dcb56dc000a5387a88f7a473bacec5ee01b9ff3f55308aacc%40%3Cdev.kafka.apache.org%3E
• https://lists.apache.org/thread.html/r61db8e7dcb56dc000a5387a88f7a473bacec5ee01b9ff3f55308aacc%40%3Cusers.kafka.apache.org%3E
• https://lists.apache.org/thread.html/rbf4ce74b0d1fa9810dec50ba3ace0caeea677af7c27a97111c06ccb7%40%3Cdev.kafka.apache.org%3E
• https://lists.apache.org/thread.html/rbf4ce74b0d1fa9810dec50ba3ace0caeea677af7c27a97111c06ccb7%40%3Cusers.kafka.apache.org%3E
• https://lists.debian.org/debian-lts-announce/2021/05/msg00021.html
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TEOT4RLB76RVPJQKGGTIKTBIOLHX2NR6/
• https://lists.freedesktop.org/archives/xorg/
• https://lists.x.org/archives/xorg-announce/2021-May/003088.html

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