CVE-2021-26937Known exploitationPublic exploitNetwork attack vector

Session crash via out-of-bounds combining-character write

Published Feb 9, 2021 · Updated May 9, 2025

Out-of-bounds write in GNU Screen 4.8.0 and earlier allows remote attackers to crash a session using crafted UTF-8 input. The combining-character handler uses an out-of-range index to access the combchars array, causing an invalid write in encoding.c. The path requires UTF-8 processing and untrusted characters to reach an active display; a crash can also terminate a process supervised inside that session.

CVSS severity9.8
Critical
EPSS probability9.15%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds write in GNU Screen 4.8.0 and earlier allows remote attackers to crash a session using crafted UTF-8 input. The combining-character handler uses an out-of-range index to access the combchars array, causing an invalid write in encoding.c. The path requires UTF-8 processing and untrusted characters to reach an active display; a crash can also terminate a process supervised inside that session.

The record

CVE
CVE-2021-26937
Published
Feb 9, 2021
Updated
May 9, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-787, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 9, 2021CVE publishedPublication date reported by the CVE source.
  2. May 9, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8UbuntuCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationReported
Public exploitPublished

Weakness, pattern, technique

CWE-787Out-of-bounds Write
T1499Endpoint Denial of Service

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Reported exploitation

Technologies

Your stack

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Your stack

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