CVE Tools
Sign in

CVE-2026-40021

Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters

Published: Apr 10, 2026Updated: Apr 22, 2026 Sources: CVE List NVDCWE-116
5.3MEDIUM
NVD MITRE
EPSS Score
0.8%
Top 49.9%
CISA KEV
Not in KEV
Exploits
No Known Exploits
Remediation
Patch Available

Description

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.

No summary for this CVE yet.

CVSS Vector Breakdown

AV:NAC:LPR:NUI:NS:UC:NI:LA:N
Exploitability
AV:NAttack Vector
Network
AC:LAttack Complexity
Low
PR:NPrivileges Required
None
UI:NUser Interaction
None
Scope
S:UScope
Unchanged
Impact
C:NConfidentiality
None
I:LIntegrity
Low
A:NAvailability
None
Open in CVSS calculator →

Weaknesses

CWE-116

Affected Products

Apache Log4netApache Software Foundation
Mixed
Web & CMS Plugins / cms-core
log4netapache
On-premLibrary
OSS Libraries / generic-library
apache software foundationoss-projectUSWeb & CMS Pluginsaka apache foundation
apacheoss-projectUSWeb & CMS Pluginsaka apache http server, apache httpd

Exploitability

Official Patch Available

References

https://github.com/apache/logging-log4net/pull/280
github.com
https://logging.apache.org/security.html#CVE-2026-40021
logging.apache.org
https://logging.apache.org/cyclonedx/vdr.xml
logging.apache.org
and 3 more references View all →

Timeline

Published
Apr 10, 2026
Last Updated
Apr 22, 2026

Unlock Complete Vulnerability Intelligence

Get the full picture for CVE-2026-40021 and every CVE in our database. Create a free account — no credit card required.

Create Free Account
Plain-language analysis
Impact assessment and exploitation scenario in plain English
Attack graph visualization
Interactive attack path and kill chain mapping
Exploit details & PoC links
ExploitDB, Metasploit, GitHub PoCs with direct links
Nuclei scanner templates
Ready-to-use vulnerability scanner templates
Full remediation guide
Patch instructions, workarounds, and compliance impact
Interactive AI chat
Ask questions about this vulnerability in natural language
Related vulnerabilities
Semantically similar CVEs and attack patterns
REST API & MCP access
Integrate vulnerability data into your workflows