CriticalCVE-2021-45046

    Apache Log4j2 Deserialization of Untrusted Data Vulnerability (CVE-2021-45046)

    Published In CISA KEV since Sources: Siemens ProductCERT, Red Hat, CVE.org, Debian, OSV, SingCERT

    What is affected, what to do

    It was found that the fix to address CVE-2021-44228 in Apache Log4j 2.15.0 was incomplete in certain non-default configurations. This could allows attackers with control over Thread Context Map (MDC) input data when the logging configuration uses a non-default Pattern Layout with either a Context Lookup (for example, $${ctx:loginId}) or a Thread Context Map pattern (%X, %mdc, or %MDC) to craft malicious input data using a JNDI Lookup pattern resulting in an information leak and remote code execution in some environments and local code execution in all environments.

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update Apache Log4j to version 2.16.0. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Apache Log4j lt Apache Log4j2..2.16.0
    Fixed in
    • Apache Log4j 2.16.0

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    9.0

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.0

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: Siemens ProductCERT, Red Hat, CVE.org, Debian, OSV, SingCERT

    99.98 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.98

    Show all metrics
    Severity
    Critical
    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
    250.15 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
    actively exploitedSource: ANSSI / CERT-FR How we classify
    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
    9.0 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
    9.0
    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
    99.98 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    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
    99.98
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    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
    Actively exploited
    Published
    In CISA KEV since

    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
    CISA assessment (SSVC)
    Exploitation: active · not automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 4 February 2025
    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-917

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This is the incomplete fix for Log4Shell and, in common albeit non-default logging configurations, yields remote code execution over the network without authentication or user interaction; the higher attack complexity matters little in practice because automated mass scanners spray obfuscated payloads indiscriminately, and its exploitation likelihood sits among the very highest of any CVE. CISA flags known use in ransomware campaigns, and typical targets are internet-facing Java applications, middleware and vendor appliances where Log4j is deeply embedded and hard to inventory. Treat any system running log4j-core below 2.16.0/2.12.2 — especially those "fixed" only with formatMsgNoLookups — as critical: remediate internet-facing instances immediately and internal systems that place user input into the MDC within days.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory every log4j-core instance: scan file systems, container images and fat/shaded JARs (including nested WAR/EAR/JAR) with find / -name "log4j-core-*.jar" plus SBOM/SCA tooling (Syft/Grype, Trivy). Include embedded copies inside appliances and third-party products (e.g. CVAT).
      • Patch: upgrade to at least Log4j 2.16.0 (Java 8) or 2.12.2 (Java 7). Because follow-on issues exist (CVE-2021-45105, CVE-2021-44832), deploy the current 2.x release listed on the Apache Log4j security page. 2.15.0 is not a safe version.
      • Kill a false sense of safety: log4j2.formatMsgNoLookups=true / LOG4J_FORMAT_MSG_NO_LOOKUPS=true does not mitigate this CVE — treat systems "hardened" only this way as vulnerable.
      • Prioritise internet-facing Java services: web apps, VPN/remote-access portals, middleware and appliances bundling Log4j get patched first, or are placed behind a reverse proxy/WAF or pulled offline until fixed.
      • If compromise is suspected: isolate the host, preserve memory and logs, and rotate any credentials reachable from it (service accounts, DB credentials, cloud keys).

    2. Runbook step 2

      Mitigation layers

      • Application configuration: in every log4j2.xml/log4j2.properties, remove Context Lookups (${ctx:...}) and Thread Context Map patterns (%X, %mdc, %MDC) from Pattern Layouts wherever MDC values derive from user input (headers, login names, session attributes).
      • Library hardening: where upgrading is impossible, strip the JNDI lookup class: zip -q -d log4j-core-*.jar org/apache/logging/log4j/core/lookup/JndiLookup.class, then restart and verify functionality.
      • Network segmentation / egress filtering: default-deny outbound traffic from application servers to LDAP (389/636), RMI (1099) and arbitrary high ports on the internet; allowlist required destinations only. Force DNS through internal, logged resolvers.
      • WAF/IPS: enable signatures for ${jndi: and obfuscated variants (${${lower:j}ndi:, ${${::-j}${::-n}di:, ${env:...}, ${ctx:...}) in URI, headers (User-Agent, X-Forwarded-For, Referer, X-Api-Version) and body — a supplement to patching, since obfuscation routinely bypasses WAF rules.
      • IAM / least privilege: run Java services under dedicated unprivileged accounts; restrict write access to application directories and mount /tmp noexec to hinder second-stage payloads.

    3. Runbook step 3

      Detection rules

      • Web/proxy logs: hunt for lookup strings in any field, e.g. SPL: index=web ("${jndi:" OR "${ctx:" OR "${${lower:" OR "${${::-") | stats count by src_ip, uri, http_user_agent. Include URL-encoded forms (%24%7Bjndi).
      • Network telemetry (Zeek/Suricata): outbound LDAP/RMI from app servers to external hosts (conn.log with id.resp_p in (389,636,1099) and non-RFC1918 destination) and DNS queries with unusual callback/exfil subdomains; keep ET Open Log4j rules enabled.
      • EDR / Sysmon EID 1: process chains with java/java.exe spawning shells or downloaders. Sigma shape: ParentImage|endswith: '\java.exe' AND Image|endswith: ['\cmd.exe','\powershell.exe']; on Linux (auditd execve): parent java → sh, bash, curl, wget.
      • Sysmon EID 3 / EDR network events: a Java process connecting to an external IP on 389/1099, followed by an HTTP class fetch — strong signal of a successful JNDI callback.
      • Application logs: grep your own Log4j output for ${jndi: or ${ctx: — hits prove attacker input reached the logger; correlate with MDC fields such as login names.

    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
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    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
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    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

    It was found that the fix to address CVE-2021-44228 in Apache Log4j 2.15.0 was incomplete in certain non-default configurations. This could allows attackers with control over Thread Context Map (MDC) input data when the logging configuration uses a non-default Pattern Layout with either a Context Lookup (for example, $${ctx:loginId}) or a Thread Context Map pattern (%X, %mdc, or %MDC) to craft malicious input data using a JNDI Lookup pattern resulting in an information leak and remote code execution in some environments and local code execution in all environments. Log4j 2.16.0 (Java 8) and 2.12.2 (Java 7) fix this issue by removing support for message lookup patterns and disabling JNDI functionality by default.

    Source: siemens_advisory, redhat_csaf, NVD (NIST), CISA KEV Catalog, CVE.org (MITRE), Debian Security Advisories, OSV (Open Source Vulnerabilities), cvelistv5, CSA/SingCERT

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    Apache Software FoundationApache Log4j
    • Apache Log4j2 – < 2.16.0
    • 2.16.0
    apachelog4j
    • 2.0
    • 2.0.1 – < 2.12.2
    • 2.13.0 – < 2.16.0
    • —
    • 2.12.2
    • 2.16.0
    cvatcomputer_vision_annotation_tool
    • —
    • —
    intelaudio_development_kit
    • —
    • —
    inteldatacenter_manager
    • —
    • —
    intelgenomics_kernel_library
    • —
    • —
    inteloneapi
    • —
    • —
    intelsecure_device_onboard
    • —
    • —
    intelsensor_solution_firmware_development_kit
    • —
    • —
    intelsystem_debugger
    • —
    • —
    intelsystem_studio
    • —
    • —
    siemenscaptial
    • 2019.1
    • < 2019.1
    • —
    • 2019.1
    siemenscomos
    • —
    • —
    siemensdesigo_cc_advanced_reports
    • 5.1
    • 4.2
    • 5.0
    • 4.0
    • 4.1
    • —
    • —
    • —
    • —
    • —
    siemensdesigo_cc_info_center
    • 5.1
    • 5.0
    • —
    • —
    siemense-car_operation_center
    • < 2021-12-13
    • 2021-12-13
    siemensenergy_engage
    • 3.1
    • —
    siemensenergyip
    • 9.0
    • 8.5
    • 8.7
    • 8.6
    • —
    • —
    • —
    • —
    siemensenergyip_prepay
    • 3.8
    • 3.7
    • —
    • —
    siemensgma-manager
    • < 8.6.2j-398
    • 8.6.2j-398
    siemenshead-end_system_universal_device_integration_system
    • —
    • —
    siemensindustrial_edge_management
    • —
    • —
    siemensindustrial_edge_management_hub
    • < 2021-12-13
    • 2021-12-13
    siemenslogo\!_soft_comfort
    • —
    • —
    siemensmendix
    • —
    • —
    siemensmindsphere
    • < 2021-12-11
    • 2021-12-11
    siemensnavigator
    • < 2021-12-13
    • 2021-12-13
    siemensnx
    • —
    • —
    siemensopcenter_intelligence
    • ≤ 3.2
    • —
    siemensoperation_scheduler
    • ≤ 1.1.3
    • —
    siemenssentron_powermanager
    • 4.2
    • 4.1
    • —
    • —
    siemenssiguard_dsa
    • 4.3
    • 4.2
    • 4.4
    • —
    • —
    • —
    siemenssipass_integrated
    • 2.85
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxdebianapache-log4j2bullseye
    Linuxdebianapache-log4j2buster
    Linuxdebiandebian_linux10.0
    Linuxdebiandebian_linux11.0
    Othersiemens6bk1602-0aa12-0tp0_firmware—
    Othersiemens6bk1602-0aa22-0tp0_firmware—
    Othersiemens6bk1602-0aa32-0tp0_firmware—
    Othersiemens6bk1602-0aa42-0tp0_firmware—
    Othersiemens6bk1602-0aa52-0tp0_firmware—
    Otherfedoraprojectfedora34
    Otherfedoraprojectfedora35
    Othersiemenssppa-t3000_ses3000_firmware—

    Weakness classes (CWE)

    • CWE-917Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')

      Base

      The product constructs all or part of an expression language (EL) statement in a framework such as a Java Server Page (JSP) using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended EL statement before it is executed.

      Entry at MITRE CWE-917

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2021-ALE-022 – [MaJ] Vulnérabilité dans Apache Log4j (clôturée 2022-05-04) (opens in a new tab)cert.cyber.gouv.fractively exploited

    References & sources

    • NVD (NIST)vuln/detail/CVE-2021-45046advisory
    • oracle.comsecurity-alerts/cpujul2022.htmlweb, x_refsource_MISC, Third-party advisory
    • oracle.comsecurity-alerts/cpujan2022.htmlweb, x_refsource_MISC, Patch, Third-party advisory
    • oracle.comsecurity-alerts/cpuapr2022.htmlweb, x_refsource_MISC, Third-party advisory
    • oracle.comsecurity-alerts/alert-cve-2021-44228.htmlweb, x_refsource_CONFIRM, Third-party advisory
    • openwall.comlists/oss-security/2021/12/14/4web
    • kb.cert.orgVU#930724web, Third-party advisory, x_refsource_CERT-VN, Third-party advisory, US government resource
    • intel.comcontent/www/us/en/security-center/advisory/intel-sa-00646.htmlweb, x_refsource_CONFIRM, Third-party advisory
    • debian.orgDSA-5022web, Vendor advisory, x_refsource_DEBIAN, Third-party advisory
    • CVE.org (MITRE)CVERecord?id=CVE-2021-44228web, x_refsource_MISC, Not applicable
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-45046web, government-resource, US government resource
    • tools.cisco.com20211210 Vulnerabilities in Apache Log4j Library Affecting Cisco Products: December 2021web, Vendor advisory, x_refsource_CISCO, Third-party advisory
    • security.gentoo.orgglsa/202310-16web, Third-party advisory
    • sec.cloudapps.cisco.comsecurity/center/content/CiscoSecurityAdvisory/cisco-sa-apache-l…web
    • psirt.global.sonicwall.comvuln-detail/SNWLID-2021-0032web, x_refsource_CONFIRM, Third-party advisory
    • logging.apache.orglog4j/2.x/security.htmlweb, x_refsource_CONFIRM, Mitigation, Release notes, Vendor advisory
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web
    • lists.fedoraproject.orgarchives/list/package-announce@lists.fedoraproject.org/message/…web

    Linked advisories

    Embed badge

    Show the severity and KEV status of this vulnerability on your site – with a link back here. The image is updated hourly.

    Badge preview for CVE-2021-45046
    <a href="https://intel.neosec.eu/en/cve/CVE-2021-45046"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-45046.svg?lang=en" alt="CVE-2021-45046 – NEOSEC Intel" height="20"></a>