CVE-2026-105240 in log4net
Summary
by MITRE • 10/06/2026
Improper Neutralization of Null Byte or NUL Character vulnerability in the OutputDebugStringAppender of Apache log4net.
A NUL character in logged content ended the debug output record at that point, so everything the layout rendered after it, including exception text and trailing fields, was silently lost. A party whose data reaches a log message could hide the rest of that record. Only applications on Windows that use OutputDebugStringAppender 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.
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Analysis
by VulDB Data Team • 10/07/2026
The vulnerability identified as Improper Neutralization of Null Byte or NUL Character within Apache log4net specifically impacts the OutputDebugStringAppender component on Windows operating systems. This security flaw arises from a fundamental failure in input validation and data sanitization when handling string content destined for debug output streams. In many programming environments, particularly those interfacing with native Windows APIs like OutputDebugStringA or similar mechanisms used by log4net's implementation, the null character serves as a standard string terminator. When an attacker or user-supplied data contains a NUL byte within a logged message, the underlying system interprets this character not merely as part of the payload but as the end-of-string marker. Consequently, any subsequent characters in that specific log entry are truncated and discarded before they can be written to the debug output buffer. This behavior creates a significant integrity gap where the completeness of audit trails is compromised without raising immediate alarms within standard logging frameworks.
The operational impact of this vulnerability extends beyond simple data loss; it introduces potential security blind spots for application monitoring and incident response teams. Because the truncation occurs silently, developers and system administrators reviewing logs may perceive that an event was logged completely when in fact critical details were omitted. This is particularly dangerous if the truncated portion contains exception stack traces, error codes, or contextual metadata essential for debugging production issues or forensic analysis of security incidents. An adversary who can influence log content could exploit this behavior to hide malicious activities or obfuscate attack signatures by injecting NUL characters at strategic points in their input data. This effectively allows them to sanitize the visible portion of logs while leaving destructive actions unrecorded, thereby evading detection mechanisms that rely on comprehensive log analysis.
From a classification perspective, this vulnerability aligns with CWE-628, which defines Improper Neutralization of NUL Byte or Null Character in a Context where it can result in the interpretation of incomplete data. It also relates to CWE-134, specifically regarding the use of externally-controlled format strings if the logging framework processes inputs without proper escaping before passing them to native APIs that expect null-terminated strings. In terms of adversary tactics, this flaw supports ATT&CK technique T1078, Valid Accounts or System Information Discovery, by allowing attackers to manipulate system logs to avoid detection during reconnaissance or persistence phases. It also touches upon T1562, Impair Defenses, as the ability to truncate log entries directly undermines the integrity of defensive logging mechanisms that are critical for maintaining situational awareness within an IT environment.
The scope of this vulnerability is limited to applications running on Windows platforms that utilize the OutputDebugStringAppender in Apache log4net versions ranging from 1.2.9 up to, but not including, version 3.5.0. Other logging appenders or non-Windows environments are generally unaffected because they do not rely on the same native API behaviors regarding null termination. The root cause lies in the framework's failure to escape or replace NUL characters before passing them to the underlying Windows debugging functions. This represents a classic case of trusting input data without adequate sanitization for the specific constraints of the target system interface.
To mitigate this risk, organizations must upgrade Apache log4net to version 3.5.0 or later, where the issue has been resolved by implementing proper handling of NUL characters within the OutputDebugStringAppender logic. For environments that cannot immediately patch due to dependency conflicts, alternative mitigation strategies include restricting input sources that write directly to debug logs and ensuring that any user-controllable data is validated before being passed to logging methods. Additionally, organizations should consider supplementing native Windows debug logging with other log storage mechanisms such as file-based appenders or remote syslog servers which do not suffer from the same null-byte truncation vulnerabilities. Regular auditing of log integrity and implementing checksums for critical log entries can also help detect instances where data has been silently truncated due to this flaw.