CVE-2021-27135Public exploitNetwork attack vector

Code execution via undersized selection buffer

Published Feb 10, 2021 · Updated Aug 3, 2024

Out-of-bounds write in xterm before Patch #366 allows remote attackers to execute arbitrary code or crash the terminal via crafted UTF-8 text. SaltTextAway in button.c underallocates the selection buffer because its size estimate fails to account for combining characters, causing selection conversion to write beyond the allocation. The crafted line must reach an xterm display and be selected by a user; successful exploitation runs code in the xterm process or terminates that process.

CVSS severity9.8
Critical
EPSS probability7.84%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds write in xterm before Patch #366 allows remote attackers to execute arbitrary code or crash the terminal via crafted UTF-8 text. SaltTextAway in button.c underallocates the selection buffer because its size estimate fails to account for combining characters, causing selection conversion to write beyond the allocation. The crafted line must reach an xterm display and be selected by a user; successful exploitation runs code in the xterm process or terminates that process.

The record

CVE
CVE-2021-27135
Published
Feb 10, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
T1204
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 10, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record 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

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
9.6Amazon LinuxCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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 exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

T1204User Execution

Public exploit references

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

Technologies

Your stack

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Your stack

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