CVE-2026-105243: Insufficient Logging vulnerability in the EventLogAppender of Apache log4net. Long messages were truncated to

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MEDIUM 5.3REMOTEWATCHLISTUPGRADE

CVE-2026-105243 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.

Insufficient Logging vulnerability in the EventLogAppender of Apache log4net. Long messages were truncated to a fixed size that exceeds what the Windows Event Log accepts once the log and source names are counted, and the event log then stored nothing and reported nothing. A party whose data reaches a log message could suppress the whole record by making it long enough. Only applications on Windows that use EventLogAppender are affected. This i

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-778
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 15:26 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-2026-105243?

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

Is CVE-2026-105243 being exploited?

CVE-2026-105243 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-2026-105243 affect?

CVE-2026-105243 affects log4net. Severity is medium (CVSS 5.3). Insufficient Logging vulnerability in the EventLogAppender of Apache log4net. Long messages were truncated to a fixed size that exceeds what the Windows Event Log accepts once the log and source names are counted, and the event log then st

How urgent is CVE-2026-105243?

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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