CVE-2026-105241: CWE-176 Improper Handling of Unicode Encoding in Apache Software Foundation Apache log4net
Description
CVE-2026-105241 is a medium severity vulnerability in Apache log4net affecting versions from 1.2.9 up to but not including 3.5.0. It involves improper handling of Unicode encoding in the SmtpPickupDirAppender component. Specifically, when the mail file writer encounters content it cannot encode, such as an unpaired UTF-16 surrogate, it throws an error causing all buffered events in the batch to be discarded and potentially leaving a truncated mail file in the pickup directory. This can allow an attacker who controls log message content to suppress logging of other events. The issue is fixed in version 3.5.0.
CVSS v3.1
Score 5.3medium
Affected software
Apache Software Foundation
Apache log4net
Apache Software Foundation
Apache log4net
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Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
This vulnerability in Apache log4net's SmtpPickupDirAppender arises from improper handling of Unicode encoding errors. When the appender attempts to write mail files containing unencodable content like unpaired UTF-16 surrogates, it throws an exception that discards the entire batch of buffered events, not just the problematic one. This results in loss of log records and possibly truncated mail files in the pickup directory. Only applications using SmtpPickupDirAppender are affected. The flaw affects versions >=1.2.9 and <3.5.0. Upgrading to 3.5.0 resolves the issue.
Potential Impact
The vulnerability allows suppression of log records by causing the appender to discard all buffered events when encountering unencodable Unicode content. This can lead to loss of important logging information and truncated mail files in the pickup directory. There is no direct confidentiality or availability impact, but integrity of logging is affected.
Mitigation Recommendations
Users should upgrade Apache log4net to version 3.5.0 or later, where this issue is fixed. No other mitigations are indicated. Patch status is confirmed by the vendor advisory.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- apache
- Date Reserved
- 2026-10-04T16:20:44.036Z
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 6ac5544a2cdf04f656d9e12a
Added to database: 10/06/2026, 20:04:26 UTC
Last enriched: 10/06/2026, 20:19:14 UTC
Last updated: 10/06/2026, 20:19:15 UTC
Views: 7
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