Improper Encoding or Escaping of Output Affecting log4net package, versions [,3.3.0)
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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- published14 Apr 2026
- disclosed10 Apr 2026
- creditf00dat
Introduced: 10 Apr 2026
NewCVE-2026-40021 (opens in a new tab)How to fix?
Upgrade log4net to version 3.3.0 or higher.
Overview
Affected versions of this package are vulnerable to Improper Encoding or Escaping of Output in the XmlLayout and XmlLayoutSchemaLog4J layouts due to improper sanitisation of unescaped XML 1.0 forbidden characters in MDC property keys, values, or the identity field. An attacker can cause silent loss of log events by injecting forbidden characters into these fields, resulting in exceptions that prevent affected log records from being written.