CVE-2026-40021Network attack vector

Silent log loss via unsanitized XML fields

Published Apr 10, 2026 · Updated Apr 10, 2026

Improper output encoding in Apache Log4net before 3.3.0 allows remote attackers to suppress log events through crafted logged data. XmlLayout and XmlLayoutSchemaLog4J pass XML 1.0-forbidden characters from MDC keys, MDC values, or the identity field into serialization without sanitization, causing an exception that silently discards the event. The application must use an affected XML layout and place attacker-influenced data in one of those fields; each trigger removes a record from audit and detection pipelines.

CVSS severity6.3
Medium
EPSS probability0.75%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper output encoding in Apache Log4net before 3.3.0 allows remote attackers to suppress log events through crafted logged data. XmlLayout and XmlLayoutSchemaLog4J pass XML 1.0-forbidden characters from MDC keys, MDC values, or the identity field into serialization without sanitization, causing an exception that silently discards the event. The application must use an affected XML layout and place attacker-influenced data in one of those fields; each trigger removes a record from audit and detection pipelines.

The record

CVE
CVE-2026-40021
Published
Apr 10, 2026
Updated
Apr 10, 2026
Vendor
The Apache Software Foundation
Product
Apache log4net
Classifications
CWE-116, T1070
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 10, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 10, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <3.3.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Apache Software Foundation · Apache log4netversion=0 <3.3.0

Published CVSS scores

6.3Apache Software FoundationCVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N
5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-116Improper Encoding or Escaping of Output
T1070Indicator Removal

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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