CVE-2021-43523Public exploitNetwork attack vector

Application injection via unvalidated DNS resolver output

Published Nov 10, 2021 · Updated May 5, 2025

Improper hostname validation in uClibc and uClibc-ng before 1.0.39 allows context-dependent attackers to inject resolver output. The DNS resolver functions accept decoded answer and reverse-lookup names without rejecting nonprintable or disallowed characters, so crafted names can be misread or passed unchanged to consuming applications. Exploitation requires an attacker-controlled lookup or DNS response; the resulting impact depends on the consumer and includes cache poisoning, XSS, SQL injection, code execution, or a crash.

CVSS severity9.6
Critical
EPSS probability3.07%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper hostname validation in uClibc and uClibc-ng before 1.0.39 allows context-dependent attackers to inject resolver output. The DNS resolver functions accept decoded answer and reverse-lookup names without rejecting nonprintable or disallowed characters, so crafted names can be misread or passed unchanged to consuming applications. Exploitation requires an attacker-controlled lookup or DNS response; the resulting impact depends on the consumer and includes cache poisoning, XSS, SQL injection, code execution, or a crash.

The record

CVE
CVE-2021-43523
Published
Nov 10, 2021
Updated
May 5, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-79
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 10, 2021CVE publishedPublication date reported by the CVE source.
  2. May 5, 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

6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P
9.6NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/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 exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

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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