CVE-2021-31535: LookupCol.c in X.Org X through X11R7.7 and libX11 before 1.7.1 might allow remote attackers to execute arbitra

Q: How do I fix CVE-2021-31535?

libx11: upgrade to 1.7.1; x window system: upgrade to newer than x11r7.7; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-31535 being exploited?

CVE-2021-31535 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-31535 affect?

CVE-2021-31535 affects libx11, x window system, fedora. Severity is critical (CVSS 9.8). 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-n

Q: How urgent is CVE-2021-31535?

Priority 68/100. Critical severity; EPSS top 4%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

This week

CRITICAL 9.8REMOTEUPGRADEUPGRADEMITIGATE

CVE-2021-31535 is a critical-severity flaw (CVSS 9.8) affecting libx11, x window system, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

Critical severity
EPSS top 4%: very likely to be exploited
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	10.6% (top 4%)
CISA exploited list	No
Weakness	CWE-120
Published	2021-05-27
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: fedoraproject · x.org

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-05-27 13:15 UTCPublished CVE record published
2026-06-17 03:51 UTCUpdated Record last modified

x.org libx11 UPGRADE

Affected: < 1.7.1

Fixed in: 1.7.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy libx11 || rpm -q libx11 || libx11 --version

2. UPGRADE to 1.7.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade libx11

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade libx11

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

x.org x window system UPGRADE

Affected: <= x11r7.7

Fixed in: newer than x11r7.7

Safe to downgrade to: no unaffected older release

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

fedoraproject fedora MITIGATE

Affected: 33

Fixed in: no fixed release known

Safe to downgrade to: a release before 33

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-31535?

libx11: upgrade to 1.7.1; x window system: upgrade to newer than x11r7.7; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-31535 being exploited?

CVE-2021-31535 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-31535 affect?

CVE-2021-31535 affects libx11, x window system, fedora. Severity is critical (CVSS 9.8). 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-n

How urgent is CVE-2021-31535?

Priority 68/100. Critical severity; EPSS top 4%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2023-43787 HIGH 7.8 UPGRADE MITIGATE A vulnerability was found in libX11 due to an integer overflow within the XCreateImage() function
CVE-2023-43786 MEDIUM 5.5 UPGRADE MITIGATE A vulnerability was found in libX11 due to an infinite loop within the PutSubImage() function
CVE-2023-43785 MEDIUM 6.5 UPGRADE MITIGATE A vulnerability was found in libX11 due to a boundary condition within the _XkbReadKeySyms() function
CVE-2023-3138 HIGH 7.5 UPGRADE MITIGATE A vulnerability was found in libX11
CVE-2020-14363 HIGH 7.8 UPGRADE MITIGATE An integer overflow vulnerability leading to a double-free was found in libX11

References

← Back to latest CVEs