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                            CERT-Renater

                Note d'Information No. 2026/VULN744
_____________________________________________________________________


DATE                : 13/07/2026

HARDWARE PLATFORM(S): /

OPERATING SYSTEM(S): Systems running Apache Log4j API versions prior
                                  to 2.25.5, 2.26.1.
 
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https://lists.apache.org/thread/l8cy6mdtgrv3m0vrdr77cdvzgpgkdtqq
_____________________________________________________________________



CVE-2026-49844: Apache Log4j API: Improper serialization of non-finite
floating-point values in MapMessage.asJson()
Severity: moderate 

Affected versions:

- Apache Log4j API (org.apache.logging.log4j:log4j-api) 2.13.1 before
2.25.5
- Apache Log4j API (org.apache.logging.log4j:log4j-api) 2.26.0 before
2.26.1
- Apache Log4j API (org.apache.logging.log4j:log4j-api) 3.0.0-alpha1
through 3.0.0-beta2

Description:

Improper encoding of non-finite floating-point values during MapMessage
JSON serialization in Apache Log4j API produces output that is not
valid JSON. This issue affects Apache Log4j API versions 2.13.1 through
2.25.4 and version 2.26.0.

The fix for CVE-2026-34481 did not cover all code paths: when a
MapMessage contains a non-finite IEEE 754 value (NaN, Infinity,
or -Infinity), MapMessage.asJson() emits the corresponding bare token.
RFC 8259 does not permit these tokens, so a conformant parser rejects
the resulting document.

The defect is reachable only when both of the following conditions hold:

  *  The application uses the  message resolver
https://logging.apache.org/log4j/2.x/manual/json-template-layout.html#event-template-resolver-message
  of JsonTemplateLayout or any other layout that relies on
MapMessage.asJson() or MapMessage.getFormattedMessage(new String[]{"JSON"}).
  *  The application logs a MapMessage that contains an
attacker-controlled floating-point value.


An attacker who can supply a non-finite value can cause the affected
layout to emit malformed JSON, which may corrupt the enclosing
log record or disrupt downstream log ingestion and parsing.

Users are advised to upgrade to Apache Log4j API 2.25.5 or 2.26.1,
both of which emit RFC 8259-compliant JSON for non-finite values.

Credit:

Himanshu Anand (finder)

References:

https://logging.apache.org/log4j/2.x/manual/json-template-layout.html#event-template-resolver-message
https://github.com/apache/logging-log4j2/pull/4163
https://logging.apache.org/cyclonedx/vdr.xml
https://logging.apache.org/security.html#CVE-2026-49844
https://logging.apache.org/
https://www.cve.org/CVERecord?id=CVE-2026-49844



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