CVE-2026-40021 (GCVE-0-2026-40021)

Vulnerability from cvelistv5 – Published: 2026-04-10 15:44 – Updated: 2026-04-10 17:35
VLAI?
Title
Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters
Summary
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.
Severity ?
6.3 (Medium)
CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
CWE
  • CWE-116 - Improper Encoding or Escaping of Output
Assigner
References
Impacted products
Vendor Product Version
Apache Software Foundation Apache Log4net Affected: 0 , < 3.3.0 (nuget)
    cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:*
 Create a notification for this product.
Credits
f00dat
Show details on NVD website
To clipboard 

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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



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