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CVE-2026-40021— Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters

Quick assessment

Affected
Apache Software Foundation Apache Log4net
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Apache log4net是美国阿帕奇（Apache）基金会的一款日志输出工具。 Apache Log4net 3.3.0之前版本存在安全漏洞，该漏洞源于XmlLayout和XmlLayoutSchemaLog4J未清理XML 1.0规范禁止的字符，可能导致日志记录丢失。

AI Predicted 5.3 Difficulty: Easy EPSS 0.98% · P61

Possible ATT&CK Techniques 2 AI

T1078 · Valid Accounts T1562
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I. Basic Information for CVE-2026-40021

Vulnerability Information
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Vulnerability Title
Apache Log4net: Silent log event loss in XmlLayout and XmlLayoutSchemaLog4J due to unescaped XML 1.0 forbidden characters
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
对输出编码和转义不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
Apache log4net 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Apache log4net是美国阿帕奇（Apache）基金会的一款日志输出工具。 Apache Log4net 3.3.0之前版本存在安全漏洞，该漏洞源于XmlLayout和XmlLayoutSchemaLog4J未清理XML 1.0规范禁止的字符，可能导致日志记录丢失。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Apache Software Foundation Apache Log4net 0 ~ 3.3.0 cpe:2.3:a:apache:log4net:*:*:*:*:*:*:*:*

II. Public POCs for CVE-2026-40021

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III. Intelligence Information for CVE-2026-40021

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Patches & Fixes for CVE-2026-40021 (1)

Mailing List Discussions for CVE-2026-40021 (1)

Vendor Pages for CVE-2026-40021 (1)

Same Patch Batch · Apache Software Foundation · 2026-04-10 · 8 CVEs total

CVE-2026-40023 Apache Log4cxx, Apache Log4cxx (Conan), Apache Log4cxx (Brew): Silent log event loss in XM
CVE-2026-34481 Apache Log4j JSON Template Layout: Improper serialization of non-finite floating-point val
CVE-2026-34480 Apache Log4j Core: Silent log event loss in XmlLayout due to unescaped XML 1.0 forbidden c
CVE-2026-34479 Apache Log4j 1 to Log4j 2 bridge: Silent log event loss in Log4j1XmlLayout due to unescape
CVE-2026-34478 Apache Log4j Core: Log injection in Rfc5424Layout due to silent configuration incompatibil
CVE-2026-34477 Apache Log4j Core: verifyHostName attribute silently ignored in TLS configuration, allowin
CVE-2026-39304 Apache ActiveMQ Client, Apache ActiveMQ Broker, Apache ActiveMQ All, Apache ActiveMQ: Inco

IV. Related Vulnerabilities

Same product: Apache Log4net
Same vendor: Apache Software Foundation
Same weakness: CWE-116

V. Comments for CVE-2026-40021

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