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CVE-2021-26937: encoding.c in GNU Screen through 4.8.0 allows remote attackers to cause a denial of service (invalid write acc

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CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2021-26937 is a critical-severity flaw (CVSS 9.8) affecting screen, debian linux, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

encoding.c in GNU Screen through 4.8.0 allows remote attackers to cause a denial of service (invalid write access and application crash) or possibly have unspecified other impact via a crafted UTF-8 character sequence.

Why this priority

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

A fixed release or distro patch exists.

Risk facts

CVSS score9.8 (Critical)
Attack vectorNetwork (remote)
Attack complexityLow
Privileges neededNone
User interactionNone
EPSS exploit chance9.1% (top 5%)
CISA exploited listNo
WeaknessCWE-88
Published2021-02-09
Last modified2026-06-17
Publisher (CNA): MITRE · Vendor: debian · fedoraproject · gnu

Share: LinkedIn · X · Reddit

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

Am I affected?

Timeline

gnu screen UPGRADE
Affected: <= 4.8.0
Fixed in: newer than 4.8.0
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy screen || rpm -q screen || screen --version
2. UPGRADE to newer than 4.8.0 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade screen
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade screen
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
debian debian linux MITIGATE
Affected: 9.0 · 10.0
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: 32 · 33
Fixed in: no fixed release known
Safe to downgrade to: a release before 32
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.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.
Fedora 46 · shotcut · FIXED · 26.10.30-1.fc46 · FEDORA-2026-7787c7fc2d · source
sudo dnf upgrade --advisory=FEDORA-2026-7787c7fc2d
Ubuntu 20.04 · screen · FIXED · 4.8.0-1ubuntu0.1
sudo apt update && sudo apt install --only-upgrade screen dpkg --compare-versions "$(dpkg-query -W -f='${Version}' screen)" ge "4.8.0-1ubuntu0.1" && echo PATCHED || echo STILL-VULNERABLE
Debian 12 · screen · FIXED · 4.8.0-5
sudo apt update && sudo apt install --only-upgrade screen dpkg --compare-versions "$(dpkg-query -W -f='${Version}' screen)" ge "4.8.0-5" && echo PATCHED || echo STILL-VULNERABLE
Debian 13 · screen · FIXED · 4.8.0-5
sudo apt update && sudo apt install --only-upgrade screen dpkg --compare-versions "$(dpkg-query -W -f='${Version}' screen)" ge "4.8.0-5" && echo PATCHED || echo STILL-VULNERABLE
RHEL 7 · screen · FIXED · screen-4.1.0-0.27.20120314git3c2946.el7_9
sudo dnf upgrade --advisory=RHSA-2021:0742 rpm -q screen
RHEL 7.7 Advanced Update Support · screen · FIXED · screen-4.1.0-0.25.20120314git3c2946.el7_7.1
sudo dnf upgrade --advisory=RHSA-2022:1074 rpm -q screen
RHEL 7.7 Telco Extended Update Support · screen · FIXED · screen-4.1.0-0.25.20120314git3c2946.el7_7.1
sudo dnf upgrade --advisory=RHSA-2022:1074 rpm -q screen
RHEL 7.7 Update Services for SAP Solutions · screen · FIXED · screen-4.1.0-0.25.20120314git3c2946.el7_7.1
sudo dnf upgrade --advisory=RHSA-2022:1074 rpm -q screen
RHEL 6 · screen · DEFERRED
Alpine 3.20 · screen · FIXED · screen-4.8.0-r4
apk add --upgrade screen apk info -v screen
Alpine 3.21 · screen · FIXED · screen-4.8.0-r4
apk add --upgrade screen apk info -v screen
Alpine 3.22 · screen · FIXED · screen-4.8.0-r4
apk add --upgrade screen apk info -v screen
Alpine 3.23 · screen · FIXED · screen-4.8.0-r4
apk add --upgrade screen apk info -v screen
Amazon Linux 2 · see advisory · FIXED · ALAS2-2021-1623 · source
sudo yum update --advisory=ALAS2-2021-1623
Amazon Linux 1 · see advisory · FIXED · ALAS-2021-1492 · source
sudo yum update --advisory=ALAS-2021-1492
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-2021-26937?

screen: upgrade to newer than 4.8.0; 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-2021-26937 being exploited?

CVE-2021-26937 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-2021-26937 affect?

CVE-2021-26937 affects screen, debian linux, fedora. Severity is critical (CVSS 9.8). encoding.c in GNU Screen through 4.8.0 allows remote attackers to cause a denial of service (invalid write access and application crash) or possibly have unspecified other impact via a crafted UTF-8 character sequence.

How urgent is CVE-2021-26937?

Priority 73/100. Critical severity; EPSS top 5%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

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References

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