CVE-2021-27135: xterm before Patch #366 allows remote attackers to execute arbitrary code or cause a denial of service (segmen

Q: How do I fix CVE-2021-27135?

xterm: upgrade to 366; debian linux: mitigate or apply the listed mitigation; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-27135 being exploited?

CVE-2021-27135 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-2021-27135 affect?

CVE-2021-27135 affects xterm, debian linux, fedora. Severity is critical (CVSS 9.8). xterm before Patch #366 allows remote attackers to execute arbitrary code or cause a denial of service (segmentation fault) via a crafted UTF-8 combining character sequence.

Q: How urgent is CVE-2021-27135?

Priority 68/100. Critical severity; EPSS top 5%: very likely to be exploited; Remotely exploitable, no login, no user action; On your watchlist

This week

CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2021-27135 is a critical-severity flaw (CVSS 9.8) affecting xterm, debian linux, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

xterm before Patch #366 allows remote attackers to execute arbitrary code or cause a denial of service (segmentation fault) via a crafted UTF-8 combining character sequence.

Why this priority

Critical severity
EPSS top 5%: very likely to be exploited
Remotely exploitable, no login, no user action
On your watchlist

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	7.8% (top 5%)
CISA exploited list	No
Published	2021-02-10
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · fedoraproject · invisible-island

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

Am I affected?

Timeline

2021-02-10 16:15 UTCPublished CVE record published
2026-06-17 03:44 UTCUpdated Record last modified

invisible-island xterm UPGRADE

Affected: < 366

Fixed in: 366

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 366 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade xterm

Note

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

debian debian linux MITIGATE

Affected: 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

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-2021-27135?

xterm: upgrade to 366; debian linux: mitigate or apply the listed mitigation; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-27135 being exploited?

CVE-2021-27135 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-2021-27135 affect?

CVE-2021-27135 affects xterm, debian linux, fedora. Severity is critical (CVSS 9.8). xterm before Patch #366 allows remote attackers to execute arbitrary code or cause a denial of service (segmentation fault) via a crafted UTF-8 combining character sequence.

How urgent is CVE-2021-27135?

Priority 68/100. Critical severity; EPSS top 5%: very likely to be exploited; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

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CVE-2008-2383 HIGH 9.3 DOWNGRADE CRLF injection vulnerability in xterm allows user-assisted attackers to execute arbitrary commands via LF (aka
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References

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