CVE-2026-105240: Improper Neutralization of Null Byte or NUL Character vulnerability in the OutputDebugStringAppender of Apache

This month

MEDIUM 5.3REMOTEWATCHLISTUPGRADE

CVE-2026-105240 is a medium-severity flaw (CVSS 5.3) affecting log4net. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Improper Neutralization of Null Byte or NUL Character vulnerability in the OutputDebugStringAppender of Apache log4net. A NUL character in logged content ended the debug output record at that point, so everything the layout rendered after it, including exception text and trailing fields, was silently lost. A party whose data reaches a log message could hide the rest of that record. Only applications on Windows that use OutputDebugStringAppender

Why this priority

Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.8% (top 43%)
CISA exploited list	No
Weakness	CWE-158
Published	2026-10-06
Last modified	2026-10-09

Publisher (CNA): Apache Software Foundation · Vendor: apache

Last updated 2026-10-09 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-10-06 20:17 UTCPublished CVE record published
2026-10-09 15:41 UTCUpdated Record last modified

apache log4net UPGRADE

Affected: >= 1.2.9 and < 3.5.0

Fixed in: 3.5.0

Safe to downgrade to: a release before 1.2.9

1. Check installed version

apt-cache policy log4net || rpm -q log4net || log4net --version

2. UPGRADE to 3.5.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade log4net

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade log4net

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 1.2.9 (Debian / Ubuntu)

apt-cache madison log4net sudo apt install log4net=<VERSION_BEFORE_1.2.9> sudo apt-mark hold log4net

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list log4net sudo dnf downgrade log4net-<VERSION_BEFORE_1.2.9> sudo dnf versionlock add log4net

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-105240?

log4net: upgrade to 3.5.0. Exact commands for each OS are on this page.

Is CVE-2026-105240 being exploited?

CVE-2026-105240 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-105240 affect?

CVE-2026-105240 affects log4net. Severity is medium (CVSS 5.3). Improper Neutralization of Null Byte or NUL Character vulnerability in the OutputDebugStringAppender of Apache log4net. A NUL character in logged content ended the debug output record at that point, so everything the layout rendered after

How urgent is CVE-2026-105240?

Priority 43/100. Remotely exploitable, no login, no user action; On your watchlist

Software pages

Windows vulnerabilities

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References

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