CVE-2026-49844: log4j Improper Encoding — Fix

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.

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CVSS v3.1 Score

5.9

MEDIUM

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS — Exploit Prediction

0.0066

Probability of exploitation

0.48%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-116 CWE-116

Affected Products

Vendor	Product	Versions
apache	log4j	2.13.1 — 2.25.5
apache	log4j	All
apache	log4j	All
apache	log4j	All
apache	log4j	All
apache	log4j	All
apache	log4j	All

References

Advisories & Patches

https://github.com/apache/logging-log4j2/pull/4163 https://logging.apache.org/cyclonedx/vdr.xml https://logging.apache.org/security.html#CVE-2026-49844

Other References

https://logging.apache.org/log4j/2.x/manual/json-template-layout.html#event-template-resolver-message

Frequently Asked Questions

What is CVE-2026-49844? +

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. It has a CVSS v3.1 base score of 5.9 (MEDIUM).

How severe is CVE-2026-49844? +

CVE-2026-49844 has a CVSS v3.1 score of 5.9 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0066, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-49844? +

CVE-2026-49844 affects products from apache, specifically: log4j. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-49844? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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