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Fencer Vulns
critical 9.8

CVE-2021-26937 json

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…

published 2021-02-09 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-13719

cvss
9.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
9.3%
EPSS 9.3% chance of exploitation in 30 days, percentile 95%
public exploits
0
none known

Summary

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.

Patch / fix

  • screen Debian 14, Debian 13, Debian 12 4.8.0-5

debian advisories DEBIAN-CVE-2021-26937

  • screen Ubuntu 16.04 LTS 4.3.1-2ubuntu0.1
  • screen Ubuntu 18.04 LTS 4.6.2-1ubuntu1.1
  • screen Ubuntu 20.04 LTS 4.8.0-1ubuntu0.1
  • screen Ubuntu Pro 14.04 LTS 4.1.0~20120320gitdb59704-9ubuntu0.1~esm2

ubuntu advisories UBUNTU-CVE-2021-26937

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • screen RHEL 7 4.1.0-0.27.20120314git3c2946.el7_9
  • screen RHEL 7.7 TUS, RHEL 7.7 E4S, RHEL 7.7 AUS 4.1.0-0.25.20120314git3c2946.el7_7.1

red hat advisories RHSA-2022:1074RHSA-2021:0742

  • screen SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +11 more 4.6.2-5.3.1
  • screen SLES for SAP 12 SP5, SLES for SAP 12 SP4, SLES for SAP 12 SP3 +13 more 4.0.4-23.6.1

suse advisories SUSE-SU-2021:0492-1SUSE-SU-2021:0491-1

  • screen openSUSE Tumbleweed 4.9.0-1.1
  • screen openSUSE Leap 15.2 4.6.2-lp152.6.3.1

opensuse advisories openSUSE-SU-2024:11884-1openSUSE-SU-2021:0304-1

  • screen Alpine 3.10 4.6.2-r2
  • screen Alpine 3.11 4.7.0-r2
  • screen Alpine 3.12 4.8.0-r1
  • screen Alpine 3.24, Alpine 3.23, Alpine 3.22 +9 more 4.8.0-r4

alpine advisories ALPINE-CVE-2021-26937

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • screen CBL-Mariner 2.0 4.9.1-1

azure linux advisories AZL-7356

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

  • screen Arch Linux 4.8.0-3

arch linux advisories AVG-1553

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-26937 updated cvss 9.6

screen: crash when processing combining chars

  • Red Hat Enterprise Linux 6 screen
    affected: workaround, no fix planned “This flaw is in utf8 processing; if your screen configuration does not enable utf8 (through configuration such as "defencoding utf-8" in .screenrc), you are not vulnerable.”
  • Red Hat Enterprise Linux Client Optional (v. 7) screen 4.1.0-0.27.20120314git3c2946.el7_9 Red Hat Enterprise Linux Server (v. 7) screen 4.1.0-0.27.20120314git3c2946.el7_9
    fixed: vendor fix “This flaw is in utf8 processing; if your screen configuration does not enable utf8 (through configuration such as "defencoding utf-8" in .screenrc), you are not vulnerable.” RHSA-2021:0742
  • Red Hat Enterprise Linux Server AUS (v. 7.7) screen 4.1.0-0.25.20120314git3c2946.el7_7.1 Red Hat Enterprise Linux Server E4S (v. 7.7) screen 4.1.0-0.25.20120314git3c2946.el7_7.1 Red Hat Enterprise Linux Server TUS (v. 7.7) screen 4.1.0-0.25.20120314git3c2946.el7_7.1
    fixed: vendor fix “This flaw is in utf8 processing; if your screen configuration does not enable utf8 (through configuration such as "defencoding utf-8" in .screenrc), you are not vulnerable.” RHSA-2022:1074
  • Red Hat Enterprise Linux Workstation (v. 7) screen 4.1.0-0.27.20120314git3c2946.el7_9
    fixed: vendor fix “This flaw is in utf8 processing; if your screen configuration does not enable utf8 (through configuration such as "defencoding utf-8" in .screenrc), you are not vulnerable.” RHSA-2021:0742

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

9.26% chance of exploitation in the next 30 days, higher than 95% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • lists.gnu.org https://lists.gnu.org/archive/html/screen-devel/2021-02/msg00000.html referenced PoC, tagged "exploit" by a source, unverified
  • openwall.com https://www.openwall.com/lists/oss-security/2021/02/09/3 referenced PoC, tagged "exploit" by a source, unverified
  • openwall.com http://www.openwall.com/lists/oss-security/2021/02/09/8 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 7.5 AV:N/AC:L/Au:N/C:P/I:P/A:P

Classification

Weakness
  • CWE-88 Improper Neutralization of Argument Delimiters in a Command

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.