Bug 2457315 (CVE-2026-40021)

Summary: CVE-2026-40021 log4net: Apache Log4net: Denial of Service for logging via unsanitized XML characters
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in Apache Log4net. An attacker who can influence specific data fields within log messages can exploit this vulnerability. By injecting characters forbidden by the XML 1.0 specification, the attacker can cause an exception during log serialization, leading to the silent loss of log events. This can impair audit trails and hinder the detection of malicious activity, effectively causing a denial of service for logging.
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Bug Depends On: 2457916, 2457917    
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Description OSIDB Bzimport 2026-04-10 16:01:43 UTC 
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.