CVE-2021-43523: In uClibc and uClibc-ng before 1.0.39, incorrect handling of special characters in domain names returned by DN

Q: How do I fix CVE-2021-43523?

uclibc: upgrade to newer than 0.9.33.2; uclibc-ng: upgrade to 1.0.39. Exact commands for each OS are on this page.

Q: Is CVE-2021-43523 being exploited?

CVE-2021-43523 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-43523 affect?

CVE-2021-43523 affects uclibc, uclibc-ng. Severity is critical (CVSS 9.6). In uClibc and uClibc-ng before 1.0.39, incorrect handling of special characters in domain names returned by DNS servers via gethostbyname, getaddrinfo, gethostbyaddr, and getnameinfo can lead to output of wrong hostnames (leading to domain

Q: How urgent is CVE-2021-43523?

Priority 61/100. Critical severity; Public exploit reference

This week

CRITICAL 9.6UPGRADEUPGRADE

CVE-2021-43523 is a critical-severity flaw (CVSS 9.6) affecting uclibc, uclibc-ng. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In uClibc and uClibc-ng before 1.0.39, incorrect handling of special characters in domain names returned by DNS servers via gethostbyname, getaddrinfo, gethostbyaddr, and getnameinfo can lead to output of wrong hostnames (leading to domain hijacking) or injection into applications (leading to remote code execution, XSS, applications crashes, etc.). In other words, a validation step, which is expected in any stub resolver, does not occur.

Why this priority

Critical severity
Public exploit reference

A fixed release or distro patch exists.

Risk facts

CVSS score	9.6 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	3.1% (top 12%)
CISA exploited list	No
Weakness	CWE-79
Published	2021-11-10
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: uclibc · uclibc-ng project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-11-10 15:15 UTCPublished CVE record published
2026-06-17 04:11 UTCUpdated Record last modified

uclibc uclibc UPGRADE

Affected: <= 0.9.33.2

Fixed in: newer than 0.9.33.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy uclibc || rpm -q uclibc || uclibc --version

2. UPGRADE to newer than 0.9.33.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade uclibc

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade uclibc

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

uclibc-ng project uclibc-ng UPGRADE

Affected: < 1.0.39

Fixed in: 1.0.39

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy uclibc-ng || rpm -q uclibc-ng || uclibc-ng --version

2. UPGRADE to 1.0.39 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade uclibc-ng

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade uclibc-ng

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-43523?

uclibc: upgrade to newer than 0.9.33.2; uclibc-ng: upgrade to 1.0.39. Exact commands for each OS are on this page.

Is CVE-2021-43523 being exploited?

CVE-2021-43523 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-43523 affect?

CVE-2021-43523 affects uclibc, uclibc-ng. Severity is critical (CVSS 9.6). In uClibc and uClibc-ng before 1.0.39, incorrect handling of special characters in domain names returned by DNS servers via gethostbyname, getaddrinfo, gethostbyaddr, and getnameinfo can lead to output of wrong hostnames (leading to domain

How urgent is CVE-2021-43523?

Priority 61/100. Critical severity; Public exploit reference

Related CVEs

CVE-2022-29503 CRITICAL 9.8 DOWNGRADE DOWNGRADE A memory corruption vulnerability exists in the libpthread linuxthreads functionality of uClibC 0.9.33.2 and u

References

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