CVE-2026-40021
Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters
Description
Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list and XmlLayoutSchemaLog4J https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list , in versions before 3.3.0, fail to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets in MDC property keys and values, as well as the identity field that may carry attacker-influenced data. This causes an exception during serialization and the silent loss of the affected log event. An attacker who can influence any of these fields can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity. Users are advised to upgrade to Apache Log4net 3.3.0, which fixes this issue.
INFO
Published Date :
April 10, 2026, 4:16 p.m.
Last Modified :
April 22, 2026, 2:13 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | MEDIUM | [email protected] | ||||
| CVSS 4.0 | MEDIUM | f0158376-9dc2-43b6-827c-5f631a4d8d09 | ||||
| CVSS 4.0 | MEDIUM | [email protected] |
Solution
- Upgrade to Apache Log4net version 3.3.0.
- Sanitize characters forbidden by XML specification.
- Ensure MDC property keys and values are safe.
- Validate identity field data.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-40021.
| URL | Resource |
|---|---|
| https://github.com/apache/logging-log4net/pull/280 | Issue Tracking |
| https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7 | Mailing List Vendor Advisory |
| https://logging.apache.org/cyclonedx/vdr.xml | Product |
| https://logging.apache.org/log4net/manual/configuration/layouts.html | Product |
| https://logging.apache.org/security.html#CVE-2026-40021 | Vendor Advisory |
| http://www.openwall.com/lists/oss-security/2026/04/10/11 | Mailing List Third Party Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-40021 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-40021
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-40021 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-40021 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Apr. 22, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N Added CPE Configuration OR *cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:* versions up to (excluding) 3.3.0 Added Reference Type Apache Software Foundation: https://github.com/apache/logging-log4net/pull/280 Types: Issue Tracking Added Reference Type Apache Software Foundation: https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7 Types: Mailing List, Vendor Advisory Added Reference Type Apache Software Foundation: https://logging.apache.org/cyclonedx/vdr.xml Types: Product Added Reference Type Apache Software Foundation: https://logging.apache.org/log4net/manual/configuration/layouts.html Types: Product Added Reference Type Apache Software Foundation: https://logging.apache.org/security.html#CVE-2026-40021 Types: Vendor Advisory Added Reference Type CVE: http://www.openwall.com/lists/oss-security/2026/04/10/11 Types: Mailing List, Third Party Advisory -
CVE Modified by af854a3a-2127-422b-91ae-364da2661108
Apr. 10, 2026
Action Type Old Value New Value Added Reference http://www.openwall.com/lists/oss-security/2026/04/10/11 -
New CVE Received by [email protected]
Apr. 10, 2026
Action Type Old Value New Value Added Description Apache Log4net's XmlLayout https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list and XmlLayoutSchemaLog4J https://logging.apache.org/log4net/manual/configuration/layouts.html#layout-list , in versions before 3.3.0, fail to sanitize characters forbidden by the XML 1.0 specification https://www.w3.org/TR/xml/#charsets in MDC property keys and values, as well as the identity field that may carry attacker-influenced data. This causes an exception during serialization and the silent loss of the affected log event. An attacker who can influence any of these fields can exploit this to suppress individual log records, impairing audit trails and detection of malicious activity. Users are advised to upgrade to Apache Log4net 3.3.0, which fixes this issue. Added CVSS V4.0 AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Added CWE CWE-116 Added Reference https://github.com/apache/logging-log4net/pull/280 Added Reference https://lists.apache.org/thread/q8otftjswhk69n3kxslqg7cobr0x4st7 Added Reference https://logging.apache.org/cyclonedx/vdr.xml Added Reference https://logging.apache.org/log4net/manual/configuration/layouts.html Added Reference https://logging.apache.org/security.html#CVE-2026-40021