Bug 2547131 (CVE-2026-105243)

Summary: CVE-2026-105243 log4net-windows: log4net: Event log suppression via oversized messages in EventLogAppender
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
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Priority: medium    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Apache log4net. When running on Windows, the EventLogAppender truncates long messages to a size that still exceeds the limit accepted by the Windows Event Log, causing the log service to silently discard the entire entry. An attacker who can influence logged data can submit an excessively long message to trigger this failure. This issue allows an attacker to suppress audit log records and prevent security events from being recorded.
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Description OSIDB Bzimport 2026-10-06 20:02:16 UTC 
Insufficient Logging vulnerability in the EventLogAppender of Apache log4net.

Long messages were truncated to a fixed size that exceeds what the Windows Event Log accepts once the log and source names are counted, and the event log then stored nothing and reported nothing. A party whose data reaches a log message could suppress the whole record by making it long enough. Only applications on Windows that use EventLogAppender are affected.

This issue affects Apache log4net: from 1.2.9 before 3.5.0.

Users are recommended to upgrade to version 3.5.0, which fixes the issue.