CVE-2021-26720

Root file overwrite via predictable symlink paths

Published Feb 17, 2021 · Updated Aug 3, 2024

Symlink attacks in Debian's Avahi package through 0.8-4 allow local users to overwrite files or create empty files with root privileges. The root-run avahi-daemon-check-dns.sh follows attacker-controlled links when touch writes a disable tag or resolv.conf data is written to a predictable cache path. Control of the avahi service account is required; targeted overwrites can interrupt services, while the disable-tag path can create files or alter timestamps.

CVSS severity7.8
High
EPSS probability0.40%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Symlink attacks in Debian's Avahi package through 0.8-4 allow local users to overwrite files or create empty files with root privileges. The root-run avahi-daemon-check-dns.sh follows attacker-controlled links when touch writes a disable tag or resolv.conf data is written to a predictable cache path. Control of the avahi service account is required; targeted overwrites can interrupt services, while the disable-tag path can create files or alter timestamps.

The record

CVE
CVE-2021-26720
Published
Feb 17, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-59, T1068
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Feb 17, 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 vectorlocal
Required privilegesauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.8NISTCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
4.6NISTAV:L/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)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-59Improper Link Resolution Before File Access ('Link Following')
T1068Exploitation for Privilege Escalation

Public exploit references

No public exploit references are available in this record.

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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