# What is CVE-2026-105240?

> CVE-2026-105240 is a medium-severity flaw (CVSS 5.3) in Log4net from Apache, published on 6 October 2026. It is not on the list of flaws known to be exploited today. The latest forecast puts its chance of use in the next 30 days at 0.8%. If you run an affected version, plan in the vendor’s fix.

Canonical page: https://www.precursorintelligence.com/what-is/cve-2026-105240

## At a glance

- **Verdict:** Plan it in. No sign of exploitation and a low forecast. Patch it in your normal update cycle.
- **Precursor Intelligence risk score:** 23 out of 100 (Low); action: Monitor. Kept up to date as the evidence changes.
- **Severity:** Medium (CVSS 5.3 out of 10)
- **Known to be exploited:** No, not on the list of flaws known to be exploited today
- **Chance of use in the next 30 days:** 0.8% (latest exploit-likelihood forecast)
- **Published:** 6 October 2026
- **Record last updated:** 9 October 2026
- **Main product:** Log4net from Apache
- **Affected products:** 1

## What is CVE-2026-105240? In plain terms

It can be attacked from anywhere on the internet without logging in, and nobody has to click anything. If it works, the attacker can change some data.

### The official description

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 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 kind of flaw is it?

- CWE-158: Improper Neutralization of Null Byte or NUL Character

### How an attack on it works

- Where an attacker can be: Anywhere on the internet
- How hard it is to pull off: Easy: no special conditions
- Login needed first: No login needed
- Does someone have to click: No, nobody has to do anything
- What an attacker gets: Can change some data

Severity vector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`

## Is CVE-2026-105240 being exploited? Not that we know of

It is not on the list of flaws known to be exploited today. The latest forecast puts the chance of it being used in the next 30 days at 0.8%. That can change at any time, which is why we keep watching.

Exploit likelihood over the last 7 days: 0.8%, 0.8%, 0.8%, 0.8%, 0.8%, 0.8%, 0.8%.

### Timeline

- 6 October 2026: Published
- 9 October 2026: Record last updated

### How Precursor Intelligence ranks it

CVE-2026-105240 scores **23 out of 100 (Low)**, with the action "Monitor".

Every published flaw gets a risk score from 0 to 100, kept up to date as the evidence changes. Four things go into the score, from the one that counts most to the ones that count less:

- Evidence it is being exploited: biggest factor
- How severe it is: strong factor
- How widely exposed it is: smaller factor
- Threat activity around it: smaller factor

85 and above is Critical. Anything confirmed as exploited is always marked for an emergency patch, whatever its score. When this flaw matches software you track, it lands in your list with its score and the reasons behind it.

## Who is affected by CVE-2026-105240?

The published record lists 1 product. Versions are as published: "1.2.9 to before 3.5.0" means from 1.2.9 up to, but not including, 3.5.0.

- [Log4net](https://www.precursorintelligence.com/product/apache/log4net) by [Apache](https://www.precursorintelligence.com/vendors/apache) (Application): 1.2.9 to before 3.5.0

## How do I fix CVE-2026-105240?

1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? Precursor Intelligence can work it out from your public website, or you can pick it from a list.
2. **Apply the vendor’s fix.** Install the update named in the vendor’s advisory. We’ve put the advisories and patch notes first in the list.
3. **If you can’t patch today.** Follow any mitigation the vendor publishes, and limit who can reach the affected system from the internet until the update is in.
4. **Keep an eye on it.** Track the product and we tell you as soon as we spot this flaw on the list of exploited flaws, or a jump in its exploit likelihood.

### References

- [github.com](https://github.com/apache/logging-log4net/commit/6046fe9d7f353b49fe995361c071ec2eea8f7ef6) (Patch)
- [github.com](https://github.com/apache/logging-log4net/pull/315) (Patch)
- [lists.apache.org](https://lists.apache.org/thread.html/gyw2d4ppy82t1vlpdknkfd1nmzp21z17) (Mailing list, Vendor advisory)
- [openwall.com](http://www.openwall.com/lists/oss-security/2026/10/07/18) (Mailing list, Third-party advisory)

## Questions

### What is CVE-2026-105240?

CVE-2026-105240 is a medium-severity flaw in Log4net from Apache, published on 6 October 2026. It can be attacked from anywhere on the internet without logging in, and nobody has to click anything. If it works, the attacker can change some data.

### How serious is CVE-2026-105240?

It scores 5.3 out of 10 on the CVSS severity scale, which is medium. Our risk score for CVE-2026-105240 is 23 out of 100, which is Low. Its action is "Monitor". No sign of exploitation and a low forecast. Patch it in your normal update cycle.

### Is CVE-2026-105240 being exploited?

It is not on the list of flaws known to be exploited today. The latest forecast puts the chance of it being used in the next 30 days at 0.8%. That can change at any time, which is why we keep watching.

### Which versions does CVE-2026-105240 affect?

For Log4net from Apache: 1.2.9 to before 3.5.0.

### How do I fix CVE-2026-105240?

Install the update the vendor names in its advisory, linked first under "How do I fix" on this page. If you can't patch today, follow the vendor's mitigation and limit who can reach the affected system from the internet.

## Does CVE-2026-105240 affect you?

Precursor Intelligence keeps checking for you:

- **Tell us what you run.** Pick your software from a list, or let us work it out from your website.
- **We match it to every flaw.** Over 400,000 published flaws, checked against your list.
- **Ranked by real-world risk.** Every flaw gets a 0 to 100 risk score, kept up to date, led by evidence criminals are using it.
- **Told as soon as we know.** An alert by email, in the app or by webhook to your own tools.

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