Improper Neutralization of Null Byte or NUL Character vulnerability in the OutputDebugStringAppender of Apache log4net. (CVE-2026-105240)
Description
CVE-2026-105240 is a vulnerability in Apache log4net's OutputDebugStringAppender on Windows platforms. It involves improper neutralization of the null byte (NUL) character in logged content, causing the debug output record to terminate prematurely and omit subsequent data such as exception details. This allows a party whose data is logged to hide parts of the log message. The issue affects log4net versions from 1.2.9 up to but not including 3.5.0. Upgrading to version 3.5.0 resolves the vulnerability.
CVSS v3.1
Score 5.3medium
Affected software
pkg:deb/ubuntu/log4net?arch=source&distro=xenialpkg:deb/ubuntu/log4net?arch=source&distro=bionicpkg:deb/ubuntu/log4net?arch=source&distro=focalpkg:deb/ubuntu/log4net?arch=source&distro=jammypkg:deb/ubuntu/log4net?arch=source&distro=noblepkg:deb/ubuntu/log4net?arch=source&distro=stonkingpkg:deb/ubuntu/log4net?arch=source&distro=resoluteRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability in Apache log4net's OutputDebugStringAppender arises because a NUL character in the logged content prematurely ends the debug output record, causing loss of any layout-rendered content after the NUL, including exception text and trailing fields. This behavior can be exploited by an attacker who can influence log message content to hide parts of the log record. This affects only Windows applications using OutputDebugStringAppender. The affected versions are from 1.2.9 before 3.5.0. The issue is fixed in version 3.5.0.
Potential Impact
The vulnerability allows an attacker who can inject data into log messages to truncate the debug output, potentially hiding important log information such as exceptions or trailing fields. This can impair forensic analysis or troubleshooting but does not directly impact confidentiality or availability. The CVSS score is 5.3 (medium severity) reflecting limited impact on integrity without confidentiality or availability loss.
Mitigation Recommendations
Users should upgrade Apache log4net to version 3.5.0 or later, where this vulnerability is fixed. No other mitigations are specified or required.
Technical Details
- Gcve Source
- db.gcve.eu
- Osv Id
- UBUNTU-CVE-2026-105240
- Osv Schema Version
- 1.7.0
- Ecosystems
- ["Ubuntu:16.04:LTS","Ubuntu:18.04:LTS","Ubuntu:20.04:LTS","Ubuntu:22.04:LTS","Ubuntu:24.04:LTS","Ubuntu:26.10","Ubuntu:26.04:LTS"]
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 6ad3f4f22cdf04f6566575e4
Added to database: 10/17/2026, 22:21:38 UTC
Last enriched: 10/17/2026, 22:40:28 UTC
Last updated: 10/17/2026, 22:40:28 UTC
Views: 2
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