CVE-2026-40021: apache software foundation apache log4net Improper Encoding or Escaping of Output

Vulnerability summary

CVE-2026-40021 is a medium (CVSS 6.3) Improper Encoding or Escaping of Output vulnerability in apache software foundation apache log4net. EPSS estimates a 0.98% chance of exploitation in the next 30 days, higher than 61.6% of scored vulnerabilities.

Timeline

1. 10-04-2026
   PublishedVulnerability first disclosed.
2. 10-04-2026
   Last modifiedVulnerability information updated.

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.

• CWE-116Improper Encoding or Escaping of OutputThe product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.