What is CVE-2021-31535?

CVE-2021-31535 is a critical-severity vulnerability in X.org Libx11, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2021-05-27.

How severe is CVE-2021-31535?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Is CVE-2021-31535 known to be exploited?

19 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Buffer overflow in X.org Libx11

CVE-2021-31535

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.

Vulnerability class: Buffer Overflow

Published 2021-05-27 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.106 (95.8th percentile), scored 2026-10-31.

Moderate probability of exploitation in the next 30 days — watch for active exploitation reports. 96th percentile — 95.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.106 · newest: 0.106 · change: 0.000

Affected products

Weakness classification (CWE)

CWE-120 — Buffer Copy without Checking Size of Input (Classic Buffer Overflow)

Public proof-of-concept exploits

References

unparalleled.eu/publications/2021/advisory-unpar-2021-1.txt (Exploit, Third Party Advisory)
lists.freedesktop.org/archives/xorg/ (Third Party Advisory)
www.openwall.com/lists/oss-security/2021/05/18/2 (mailing-list, Mailing List, Third Party Advisory)
seclists.org/fulldisclosure/2021/May/52 (mailing-list, Mailing List, Third Party Advisory)
packetstormsecurity.com/files/162737/libX11-Insufficient-Length-Check-Injection… (Exploit, VDB Entry, Third Party Advisory)
lists.debian.org/debian-lts-announce/2021/05/msg00021.html (mailing-list, Third Party Advisory)
www.debian.org/security/2021/dsa-4920 (vendor-advisory, Third Party Advisory)
security.gentoo.org/glsa/202105-16 (vendor-advisory, Third Party Advisory)
www.openwall.com/lists/oss-security/2021/05/18/3 (Mailing List, Third Party Advisory)
www.openwall.com/lists/oss-security/2021/05/18/2 (Mailing List, Third Party Advisory)

Frequently asked questions

What is CVE-2021-31535?: CVE-2021-31535 is a critical-severity vulnerability in X.org Libx11, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2021-05-27.
How severe is CVE-2021-31535?: Critical severity. CVSS v3 base score is 9.8 out of 10.
Is CVE-2021-31535 known to be exploited?: 19 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.