CriticalCVE-2021-42013

    Apache HTTP Server Path Traversal Vulnerability (CVE-2021-42013)

    Published In CISA KEV since Sources: SingCERT, CVE.org, OSV

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    What is affected, what to do

    This vulnerability allows for remote code execution (RCE) via a buffer overflow in Apache HTTP Server 2.4.49 and 2.4.50 if Alias-like directives are improperly configured. An attacker can access files outside configured directories and, if CGI scripts are enabled, gain full control over the instance. The vulnerability is actively exploited in ransomware campaigns. Affected products include Apache 2.4.49 and 2.4.50, particularly in Bitnami installations. Immediate patching to the latest version is crucial to mitigate this risk.

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update jd_edwards_enterpriseone_tools to version 9.2.6.0, secure_backup to version 18.1.0.1.0. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Apache HTTP Server eq Apache HTTP Server 2.4.49..Apache HTTP Server 2.4.49
    • Apache HTTP Server eq Apache HTTP Server 2.4.50..Apache HTTP Server 2.4.50

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

    Metrics

    9.8

    Priority

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

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: SingCERT, CVE.org, OSV

    99.96 %

    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
    253.97 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 exploited
    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
    9.8 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
    9.8
    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
    99.96 %
    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 · 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-22

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This vulnerability is on the KEV list and CISA flags it as actively used in ransomware campaigns — exploitation likelihood is among the highest in the entire KEV catalogue. The CVSS vector is network-reachable, requires no authentication, and no user interaction; working public PoCs circulated within hours of disclosure and are hammered at scale by mass scanners (Mirai variants, cryptominer botnets). Treat every externally reachable Apache 2.4.49/2.4.50 as P1; internal-only instances P2 but must be patched in the next change window. Particularly critical for KRITIS and NIS2-scope organisations: public-facing portals, reverse proxies in front of OT systems, and legacy web applications with CGI still enabled.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Upgrade every Apache HTTP Server 2.4.49 and 2.4.50 instance to 2.4.51 or later (the Apache fix release). On Fedora 34/35 pull the current httpd packages via dnf; patch NetApp Cloud Backup and Oracle Instantis EnterpriseTrack 17.3 per the respective vendor advisory.
      • If the patch cannot ship immediately, set <Directory /> with Require all denied as the global default in httpd.conf and all vhost includes, then explicitly allow only documented DocumentRoots.
      • Disable mod_cgi and mod_cgid (a2dismod cgi cgid or comment out the LoadModule lines) wherever CGI is not strictly required — this removes the RCE path entirely and downgrades the exploit to information disclosure only.
      • Place a reverse proxy / WAF (ModSecurity, F5, Cloudflare) in front of Apache and block requests containing %2e, %%32%65, .%2e, %2f sequences in the URI until patching is complete.
      • Build an exposure inventory: run curl -sI or an nmap scan --script http-server-header against all externally reachable port 80/443 endpoints; any Server: Apache/2.4.49 or Apache/2.4.50 goes straight into the top of the patch queue.
      • Run compromise triage on affected hosts: sweep the last 60 days of access logs for exploit patterns, isolate and image any host with a hit.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: keep Apache hosts in dedicated DMZ VLANs; restrict outbound traffic to needed backends (DB, API) via allowlist, and monitor for reverse-shell ports (4444, 1337, outbound 8080) and DNS exfiltration.
      • WAF rule: URI filter against double-encoded dots and slashes — regex (?:\.|%2e|%%32%65){2}(?:\/|%2f|%%32%66) denies; ModSecurity CRS 3.3+ ships matching rules (ID range 930100-930130, path traversal).
      • Set ServerSignature to Off and ServerTokens to Prod to frustrate fingerprinting; enforce version discovery in your asset-monitoring tool.
      • Filesystem hardening: Apache user (www-data, apache) must have no shell (/usr/sbin/nologin); restrict /bin, /usr/bin, /etc/passwd read-only via AppArmor or SELinux profile so RCE payloads cannot spawn /bin/sh or /bin/bash.
      • Audit alias directives: every Alias, ScriptAlias, AliasMatch must map to a concrete <Directory> block with Require all denied plus an explicit allow; never alias to /, /usr, /etc.
      • Enable IDS/IPS signatures: Suricata/Snort ET rules SID 2034226, 2034227, 2034228 (CVE-2021-42013) and 2034196 (CVE-2021-41773) in inline blocking mode.

    3. Runbook step 3

      Detection rules

      • Web access log: grep for /cgi-bin/.%2e/, /icons/.%%32%65/, %%32%65%%32%65/, /.%%32%65/ in access_log; example grep -E "%%32%65|\.%2e|%2e\." /var/log/apache2/access.log. Any hit returning HTTP 200 or 500 is an alert.
      • Sigma shape: selection: fieldname: cs-uri-stem contains: ['/cgi-bin/', '%2e%2e', '%%32%65'] AND sc-status in [200, 500] — ship as high severity to the SIEM.
      • Sysmon EID 1 / auditd execve on Apache hosts: process ancestry httpd or apache2 → sh, bash, python, perl, curl, wget, nc. KQL: DeviceProcessEvents | where InitiatingProcessFileName in ('httpd','apache2') and FileName in ('sh','bash','python','perl','curl','wget','nc','id','uname').
      • Zeek http.log: uri contains %2e%2e, %%32%65, or four consecutive path segments with encoded dots; correlate against status_code 200.
      • EDR/Netflow: alert on outbound connections from Apache processes to non-whitelisted destinations (especially TCP/4444, TCP/1337, or dynamic high ports to external IPs) as reverse-shell indicator.

    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 for CVE-2021-41773 in Apache HTTP Server 2.4.50 was insufficient. An attacker could use a path traversal attack to map URLs to files outside the directories configured by Alias-like directives. If files outside of these directories are not protected by the usual default configuration "require all denied", these requests can succeed. If CGI scripts are also enabled for these aliased pathes, this could allow for remote code execution. This issue only affects Apache 2.4.49 and Apache 2.4.50 and not earlier versions.

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

    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 HTTP Server
    • Apache HTTP Server 2.4.49
    • Apache HTTP Server 2.4.50
    • —
    • —
    apachehttp_server
    • 2.4.50
    • 2.4.49
    • —
    • —
    netappcloud_backup
    • —
    • —
    oracleinstantis_enterprisetrack
    • 17.1
    • 17.3
    • 17.2
    • —
    • —
    • —
    oraclejd_edwards_enterpriseone_tools
    • < 9.2.6.0
    • 9.2.6.0
    oraclesecure_backup
    • < 18.1.0.1.0
    • 18.1.0.1.0
    Distribution and container packages (2)

    Downstream packages (e.g. Bitnami, Debian, Ubuntu, Red Hat). Versions refer to the distribution, not the vendor.

    Distribution and container packages
    Affected versionsFixed in
    bitnamiapache
    • 2.4.49
    • 2.4.50
    • —
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Otherfedoraprojectfedora34
    Otherfedoraprojectfedora35

    Weakness classes (CWE)

    • CWE-22Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

      Base

      The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.

      Entry at MITRE CWE-22

    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
    Packet StormApache HTTP Server 2.4.50 Path Traversal Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormApache HTTP Server 2.4.50 Remote Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormApache 2.4.49 2.4.50 Traversal Remote Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormApache HTTP Server 2.4.50 Remote Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormApache HTTP Server 2.4.50 CVE 2021 42013 Exploitation (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormApache 2.4.50 Remote Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    PoC-in-GitHub (nomi-sec)vulhub-httpd-cve-2021-42013 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)cve-2021-42013 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)apache-cve-2021-42013-lab (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    • jvn.jpJVN#51106450web, Third-party advisory, x_refsource_JVN, Third-party advisory
    • packetstormsecurity.comfiles/164501/Apache-HTTP-Server-2.4.50-Path-Traversal-Code-Exec…web, x_refsource_MISC, Exploit, Third-party advisory, Vulnerability database
    • packetstormsecurity.comfiles/164609/Apache-HTTP-Server-2.4.50-Remote-Code-Execution.ht…web, x_refsource_MISC, Exploit, Third-party advisory, Vulnerability database
    • packetstormsecurity.comfiles/164629/Apache-2.4.49-2.4.50-Traversal-Remote-Code-Executi…web, x_refsource_MISC, Exploit, Third-party advisory, Vulnerability database
    • packetstormsecurity.comfiles/164941/Apache-HTTP-Server-2.4.50-Remote-Code-Execution.ht…web, x_refsource_MISC, Exploit, Third-party advisory, Vulnerability database
    • packetstormsecurity.comfiles/165089/Apache-HTTP-Server-2.4.50-CVE-2021-42013-Exploitat…web, x_refsource_MISC, Third-party advisory, Vulnerability database
    • packetstormsecurity.comfiles/167397/Apache-2.4.50-Remote-Code-Execution.htmlweb, x_refsource_MISC, Exploit, Third-party advisory, Vulnerability database
    • openwall.com[oss-security] 20211007 CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211008 Re: CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211008 Re: CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211008 Re: CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211008 Re: CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211008 Re: CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211008 Re: CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211009 Re: CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211011 Re: CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211015 Re: CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211016 Re: CVE-2021-42013: Path Traversal and Remote Code Execution in Apache HTTP Server 2.4.49 and 2.4.50 (incomplete fix of CVE-2021-41773)web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • httpd.apache.orgsecurity/vulnerabilities_24.htmlweb, x_refsource_MISC, Release notes, Vendor advisory
    • lists.apache.org[httpd-cvs] 20211008 [httpd-site] branch main updated: * Align with CVE-2021-42013 based on the latest findingsweb, Mailing list, x_refsource_MLIST, Mailing list, Patch
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