Apache Log4j JSON Template Layout: Improper Encoding or Escaping of Output (CVE-2026-34481)
Published Last modified Sources: BSI
What is affected, what to do
Apache Log4j's JsonTemplateLayout https://logging.apache.org/log4j/2.x/manual/json-template-layout.html , in versions up to and including 2.25.3, produces invalid JSON output when log events contain non-finite floating-point values (NaN, Infinity, or -Infinity), which are prohibited by RFC 8259. This may cause downstream log processing systems to reject or fail to index affected records. An attacker can exploit this issue only if both of the following conditions are met: * The application uses JsonTemplateLayout. * The application logs a MapMessage, or logs an object directly (e.g., via Logger.info(Object), which wraps it in an ObjectMessage), where the message contains an attacker-controlled floating-point value.
Update Apache Log4j JSON Template Layout to version 2.25.4.
- Affected
- Apache Log4j JSON Template Layout
lt 2.14.0..2.25.4 - Apache Log4j JSON Template Layout
lte 3.0.0-alpha1..3.0.0-beta3
- Apache Log4j JSON Template Layout
- Fixed in
- Apache Log4j JSON Template Layout
2.25.4
- Apache Log4j JSON Template Layout
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Metrics
7.5
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
7.5
CVSS v3
Severity High on the scale from 0 to 10.
Source: BSI
0.7 %
EPSS score
Low — model estimates < 1% exploitation likelihood.
Source: FIRST EPSS, percentile 52.7
Show all metrics
- Severity
- Medium
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 84.59 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- no public PoC known
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 7.5 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 7.5
CVSS v4
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 6.3
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 0.7 %
- Low — model estimates < 1% exploitation likelihood.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 52.7
- High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- —
- Published
CWE
CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.
Read in glossary- CWE-116
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
Apache Log4j's JsonTemplateLayout https://logging.apache.org/log4j/2.x/manual/json-template-layout.html , in versions up to and including 2.25.3, produces invalid JSON output when log events contain non-finite floating-point values (NaN, Infinity, or -Infinity), which are prohibited by RFC 8259. This may cause downstream log processing systems to reject or fail to index affected records. An attacker can exploit this issue only if both of the following conditions are met: * The application uses JsonTemplateLayout. * The application logs a MapMessage, or logs an object directly (e.g., via Logger.info(Object), which wraps it in an ObjectMessage), where the message contains an attacker-controlled floating-point value. Users are advised to upgrade to Apache Log4j JSON Template Layout 2.25.4, which corrects this issue. Note: The fix released in version 2.25.4 did not cover all affected code paths. CVE-2026-49844 was assigned to the remaining issue, which concerns the MapMessage.asJson() serialization in Apache Log4j API and is fixed in versions 2.25.5 and 2.26.1.
Source: BSI WID Portal
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| Apache Software Foundation | Apache Log4j JSON Template Layout |
|
|
| Apache Software Foundation | Apache Tomcat |
|
|
| netty | netty |
|
|
| Spring | Spring Framework |
|
|
| apache | log4j |
|
|
| apache | tomcat |
|
|
| vmware | spring_framework |
|
|
| Apache | Tomcat |
|
|
| IBM | Operational Decision Manager |
|
|
| IBM | QRadar SIEM |
|
|
| Open Source | Netty |
|
|
| Oracle | Communications |
|
|
From bundled advisories, not CVE-specific (33)
Product list of a bundled advisory that does not state which product this CVE affects.
| Affected versions | Fixed in | ||
|---|---|---|---|
| Apache | log4j |
|
|
| Atlassian | Bamboo Data Center LTS |
|
|
| Atlassian | Bitbucket Data Center |
|
|
| Atlassian | Bitbucket Data Center LTS |
|
|
| Atlassian | Confluence Data Center LTS |
|
|
| Atlassian | Crucible |
|
|
| Atlassian | Fisheye |
|
|
| Atlassian | Jira Data Center LTS |
|
|
| Atlassian | Jira Service Management Data Center LTS |
|
|
| IBM | SPSS |
|
|
| IBM | Tivoli Netcool/OMNIbus |
|
|
| Microsoft | Azure Linux |
|
|
| RealObjects | PDFreactor |
|
|
| VMware Tanzu | Spring Framework |
|
|
Vendors and products
- Apache Software Foundation Apache Log4j Core
- Apache Software Foundation
- IBM Operational Decision Manager
- Oracle Communications
- Oracle Construction And Engineering
- Oracle Enterprise Manager
- Oracle Financial Services Applications
- Oracle Fusion Middleware
- Oracle Oracle Hyperion Essbase Administration Services
- Oracle Retail Applications
- Oracle Spatial Studio
- Oracle Supply Chain
Weakness classes (CWE)
CWE-116Improper Encoding or Escaping of Output
Class
The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.
Entry at MITRE CWE-116
References & sources
- spring.iosecurity/cve-2026-41855
- NVD (NIST)vuln/detail/CVE-2026-41855advisory
- github.comspring-projects/spring-framework/commit/9bec52b1ec741976658e916…web
- github.comspring-projects/spring-frameworkpackage
- openwall.comlists/oss-security/2026/06/29/25web, Third-party advisory
- lists.apache.orgthread/dcjdcnnnww9hhdm016hr0l7hpw1bzjfpweb, Vendor advisory, Vendor advisory, Mailing list
- NVD (NIST)vuln/detail/CVE-2026-55956web
- github.comnetty/netty/security/advisories/GHSA-558v-64gr-wgg4x_refsource_CONFIRM
- github.comnetty/nettypackage
- github.comnetty/netty/releases/tag/netty-4.1.136.Finalweb
- github.comnetty/netty/releases/tag/netty-4.2.16.Finalweb
- NVD (NIST)vuln/detail/CVE-2026-34481advisory
- github.comapache/logging-log4j2/pull/4080web, Patch, Issue tracking
- github.comapache/logging-log4j2package
- lists.apache.orgthread/n34zdv00gbkdbzt2rx9rf5mqz6lhopcvweb, Vendor advisory, Mailing list, Vendor advisory
- logging.apache.orgcyclonedx/vdr.xmlweb, Vendor advisory, Product
- logging.apache.orglog4j/2.x/manual/json-template-layout.htmlweb, related, Technical description
- logging.apache.orgsecurity.htmlweb, Vendor advisory, Vendor advisory
- openwall.comlists/oss-security/2026/04/10/10web, Mailing list, Third-party advisory
Changes to this CVE
Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).
One NVD translation into another language is hidden.
Linked CVEs
| High | CVE-2026-59901 | Netty Bzip2Decoder: Infinite Loop in RLE State Machine Leads to Event-Loop Thread Hang | 7.5 | — |
| Medium | CVE-2026-55956 | Apache Tomcat: Security constraints for default servlet ignored method | 6.5 | — |
| Critical | CVE-2026-41855 | Spring Framework Unsafe Deserialization via Jackson JMS Converters | 9.8 | — |
| High | CVE-2026-34481 | Apache Log4j JSON Template Layout: Improper Encoding or Escaping of Output | 7.5 | — |
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<a href="https://intel.neosec.eu/en/cve/CVE-2026-34481"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-34481.svg?lang=en" alt="CVE-2026-34481 – NEOSEC Intel" height="20"></a>