CVE-2026-40021: Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list

Q: How do I fix CVE-2026-40021?

log4net: upgrade to 3.3.0. Exact commands for each OS are on this page.

Q: Is CVE-2026-40021 being exploited?

CVE-2026-40021 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-2026-40021 affect?

CVE-2026-40021 affects log4net. Severity is medium (CVSS 5.3). Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list and XmlLayoutSchemaLog4J https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list , in versions before 3.

Q: How urgent is CVE-2026-40021?

Priority 42/100. Remotely exploitable, no login, no user action; On your watchlist

This month

MEDIUM 5.3REMOTEWATCHLISTUPGRADE

CVE-2026-40021 is a medium-severity flaw (CVSS 5.3) affecting log4net. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list and XmlLayoutSchemaLog4J https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list , in versions before 3.3.0, fail to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets in MDC property keys and values, as well as the identity field that may carry attacker-influenced da

Why this priority

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

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.8% (top 45%)
CISA exploited list	No
Weakness	CWE-116
Published	2026-04-10
Last modified	2026-10-26

Publisher (CNA): Apache Software Foundation · Vendor: apache

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-10 16:16 UTCPublished CVE record published
2026-10-26 21:10 UTCUpdated Record last modified

apache log4net UPGRADE

Affected: < 3.3.0

Fixed in: 3.3.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy log4net || rpm -q log4net || log4net --version

2. UPGRADE to 3.3.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade log4net

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade log4net

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-2026-40021?

log4net: upgrade to 3.3.0. Exact commands for each OS are on this page.

Is CVE-2026-40021 being exploited?

CVE-2026-40021 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-2026-40021 affect?

CVE-2026-40021 affects log4net. Severity is medium (CVSS 5.3). Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list and XmlLayoutSchemaLog4J https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list , in versions before 3.

How urgent is CVE-2026-40021?

Priority 42/100. Remotely exploitable, no login, no user action; On your watchlist

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References

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