CVE-2026-105241: Apache log4net: Unencodable content discards a whole SmtpPickupDirAppender batch
Apache log4net CVE-2026-105241 lets unencodable Unicode discard an entire SmtpPickupDirAppender email batch.
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Vulnerability4 · 1 read
CVE-2026-105241
Unicode encoding flaw drops log4net SMTP pickup batches
- CVSS 3.1
- 5.3 medium
- EPSS
- —
- Published
- ·
- Modified
AI analysisSummaries by Vani AI
Apache log4net mishandles Unicode that its mail file writer cannot encode, such as an unpaired UTF-16 surrogate, in SmtpPickupDirAppender. When that content is written, the write throws, every buffered event in the batch is discarded rather than only the bad event, and a truncated mail file can be left in the pickup directory. Anyone whose data reaches a log message can suppress records of other events that were batched with it, which is an integrity loss (CVSS 3.1 base score 5.3) and not code execution or data theft. Only applications that use SmtpPickupDirAppender on log4net 1.2.9 before 3.5.0 are affected. There is no known public proof of concept and it is not listed in CISA KEV.
What to do: Upgrade Apache log4net to 3.5.0. Confirm whether the application uses SmtpPickupDirAppender; if it does, treat untrusted data that can reach log messages as able to drop a whole buffered mail batch, and review the SMTP pickup directory for truncated messages.
| Product | Affected |
|---|---|
| Apache log4net | 1.2.9 before 3.5.0 |
Estimated exposure
moderatethousands to low tens of thousands of applications using SmtpPickupDirAppender — log4net is a widely used .NET logging library with a very large NuGet download history, but the flaw applies only to the SmtpPickupDirAppender configuration, which is a minority of deployments, so the exposed set is far smaller than all…
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
Description · NVD
Improper Handling of Unicode Encoding vulnerability in the SmtpPickupDirAppender of Apache log4net. Content that the mail file writer cannot encode, such as an unpaired UTF-16 surrogate, made the write throw. Every buffered event in the batch was discarded, not only the one carrying the content, and a truncated mail could be left in the pickup directory. A party whose data reaches a log message could suppress the records of other events. Only applications that use SmtpPickupDirAppender 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.