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

> Attackers can suppress logging records by injecting unencodable content into SmtpPickupDirAppender, causing batch discards in Apache log4net.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 6 Oct 2026
- **Updated:** 7 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-105241, GHSA-r5vq-477m-wcfc, UBUNTU-CVE-2026-105241

## What is CVE-2026-105241?

CVE-2026-105241 is a medium-severity denial of service vulnerability in Apache log4net, published on 6 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-176 (Improper Handling of Unicode Encoding)

## Which versions are affected by CVE-2026-105241?

It affects 1 product.


Products: Apache Log4net.

## How do I fix CVE-2026-105241?

No fix has been published for CVE-2026-105241 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-105241 being exploited?

There is no known exploitation of CVE-2026-105241 in the wild and no public exploit code.

## Other vulnerabilities in Apache log4net

- [CVE-2018-1285](https://insights.sbomify.com/CVE-2018-1285/): XML External Entity attack in log4net (critical 9.8)
- [CVE-2026-40021](https://insights.sbomify.com/CVE-2026-40021/): Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters (medium 6.3)
- [CVE-2026-105239](https://insights.sbomify.com/CVE-2026-105239/): Apache log4net: NUL character truncates EventLogAppender records (medium 5.3)
- [CVE-2026-105240](https://insights.sbomify.com/CVE-2026-105240/): Apache log4net: NUL character truncates OutputDebugStringAppender records (medium 5.3)
- [CVE-2026-105242](https://insights.sbomify.com/CVE-2026-105242/): Apache log4net: Request validation failure drops the event in the aspnet-request converter (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-105241 and fix it if it is affected.

What it is: CVE-2026-105241 is a medium-severity denial of service vulnerability in Apache log4net, published on 6 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-105241 in the wild and no public exploit code.

Affected product: Apache log4net. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-105241/ (Markdown: https://insights.sbomify.com/CVE-2026-105241/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-105241
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-105241
- OSV: https://osv.dev/vulnerability/CVE-2026-105241

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-105241/
Summaries are generated and may contain mistakes; the official description above is the record.