CVE-2021-31873: An issue was discovered in klibc before 2.0.9

Q: How do I fix CVE-2021-31873?

klibc: upgrade to 2.0.9; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-31873 being exploited?

CVE-2021-31873 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-2021-31873 affect?

CVE-2021-31873 affects klibc, debian linux. Severity is critical (CVSS 9.8). An issue was discovered in klibc before 2.0.9. Additions in the malloc() function may result in an integer overflow and a subsequent heap buffer overflow.

Q: How urgent is CVE-2021-31873?

Priority 65/100. Critical severity; Remotely exploitable, no login, no user action; On your watchlist

This week

CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATE

CVE-2021-31873 is a critical-severity flaw (CVSS 9.8) affecting klibc, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An issue was discovered in klibc before 2.0.9. Additions in the malloc() function may result in an integer overflow and a subsequent heap buffer overflow.

Why this priority

Critical severity
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	2.2% (top 18%)
CISA exploited list	No
Weakness	CWE-190
Published	2021-04-30
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · klibc project

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

Am I affected?

Timeline

2021-04-30 06:15 UTCPublished CVE record published
2026-06-17 03:52 UTCUpdated Record last modified

klibc project klibc UPGRADE

Affected: < 2.0.9

Fixed in: 2.0.9

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 2.0.9 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade klibc

Note

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

debian debian linux MITIGATE

Affected: 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

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-2021-31873?

klibc: upgrade to 2.0.9; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-31873 being exploited?

CVE-2021-31873 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-2021-31873 affect?

CVE-2021-31873 affects klibc, debian linux. Severity is critical (CVSS 9.8). An issue was discovered in klibc before 2.0.9. Additions in the malloc() function may result in an integer overflow and a subsequent heap buffer overflow.

How urgent is CVE-2021-31873?

Priority 65/100. Critical severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

Related CVEs

CVE-2021-31872 CRITICAL 9.8 UPGRADE MITIGATE An issue was discovered in klibc before 2.0.9
CVE-2021-31871 HIGH 7.5 UPGRADE MITIGATE An issue was discovered in klibc before 2.0.9
CVE-2021-31870 CRITICAL 9.8 UPGRADE MITIGATE An issue was discovered in klibc before 2.0.9
CVE-2011-1930 CRITICAL 9.8 UPGRADE MITIGATE In klibc 1.5.20 and 1.5.21, the DHCP options written by ipconfig to /tmp/net-$DEVICE.conf are not properly esc

References

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