Apache Log4j API up to 2.25.4/2.26.0 MapMessage MapMessage.asJson encoding error

Summaryinfo

A vulnerability, which was classified as critical, was found in Apache Log4j API up to 2.25.4/2.26.0. Affected by this vulnerability is the function MapMessage.asJson of the component MapMessage. The manipulation results in encoding error. This vulnerability is reported as CVE-2026-49844. The attack can be launched remotely. No exploit exists.

Detailsinfo

A vulnerability, which was classified as critical, was found in Apache Log4j API up to 2.25.4/2.26.0. Affected is the function MapMessage.asJson of the component MapMessage. The manipulation with an unknown input leads to a encoding error vulnerability. CWE is classifying the issue as CWE-172. The product does not properly encode or decode the data, resulting in unexpected values. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at logging.apache.org. This vulnerability is traded as CVE-2026-49844 since 06/01/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 07/11/2026).

Upgrading to version 2.25.5 or 2.26.1 eliminates this vulnerability.

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Productinfo

Type

• Automation Software

Vendor

• Apache

Name

• Log4j API

Version

• 2.0
• 2.1
• 2.2
• 2.3
• 2.4
• 2.5
• 2.6
• 2.7
• 2.8
• 2.9
• 2.10
• 2.11
• 2.12
• 2.13
• 2.14
• 2.15
• 2.16
• 2.17
• 2.18
• 2.19
• 2.20
• 2.21
• 2.22
• 2.23
• 2.24
• 2.25
• 2.25.0
• 2.25.1
• 2.25.2
• 2.25.3
• 2.25.4
• 2.26.0

License

• open-source

Website

• Vendor: https://www.apache.org/

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