TI-2026-28864 Aliases CVE-2026-40021 GHSA-4f7c-pmjv-c25w

Improper output encoding in Apache Log4net

LOW 29CWE-116Published 2026-04-10 · Updated 2026-04-10

Key facts

Attack vector: Network
Privileges required: None
User interaction: None
Attack complexity: High
Attack requirements: None
Impact type: —
Confidentiality · integrity · availability: None · None · None
Fix status: Fixed

Attack profile from CVSS 4.0 · CNA. Fix status from CNA and GitHub.

Summary

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 data. This causes an exception during serialization and the silent loss of the affected log event. An attacker who can influence any of these fields can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity. Users are advised to upgrade to Apache Log4net 3.3.0, which fixes this issue.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV

EPSS 0.00802 56th percentileScored 2026-11-07

Fix Fixed Apache Log4net (Apache Software Foundation): 3.3.0log4net (NuGet): 3.3.0

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: None
Automatable: No
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
Apache	Apache Log4net	before 3.3.0	3.3.0	Patch available	CNA
—	log4net (NuGet)	before 3.3.0	3.3.0	Patch available	GHSA-4f7c-pmjv-c25w

Apache Log4net VendorApache Affected before 3.3.0 Fixed in 3.3.0 Patch available CNA
log4net (NuGet) Vendor— Affected before 3.3.0 Fixed in 3.3.0 Patch available GHSA-4f7c-pmjv-c25w

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:apache:log4net::::::::	before 3.3.0	Vulnerable	Apache

cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:* Versions before 3.3.0 Vulnerable ·Apache

Remediation

Fix · CNA Apache Log4net (Apache Software Foundation) Fixed in 3.3.0
Fix · GHSA-4f7c-pmjv-c25w log4net (NuGet) Fixed in 3.3.0 Advisory →

Vendor assessments

Red Hat

Red Hat's page for this CVE →

Not affected: 1 product

Red Hat

Timeline

2026-10-02 Threatist record created from CVE-2026-40021
2026-10-03 Alias added: GHSA-4f7c-pmjv-c25w

References

Patch 1

https://github.com/apache/logging-log4net/pull/280

Advisory 5

Other 2

TI-2018-01190 XML external entity injection in Apache log4net CVSS 3.1CRITICAL 9.8
TI-2026-68924 RemoteSyslogAppender in Apache log4net silently drops non-ASCII text CVSS 3.1MEDIUM 5.3
TI-2026-68923 Apache log4net EventLogAppender silently discards oversize records CVSS 3.1MEDIUM 5.3
TI-2026-68922 Log4net drops log events when ASP.NET request validation fails CVSS 3.1MEDIUM 5.3
TI-2026-68921 Unencodable content drops a whole batch in Apache log4net appender CVSS 3.1MEDIUM 5.3
TI-2026-68920 NUL character truncates log records in Apache log4net debug output CVSS 3.1MEDIUM 5.3

See all 8 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences