CriticalCVE-2021-41773

    Path traversal and file disclosure vulnerability in Apache HTTP Server 2.4.49 (CVE-2021-41773)

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

    What is affected, what to do

    A flaw was found in a change made to path normalization in Apache HTTP Server 2.4.49. An attacker could use a path traversal attack to map URLs to files outside the directories configured by Alias-like directives.

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Apache HTTP Server eq Apache HTTP Server 2.4 2.4.49..Apache HTTP Server 2.4 2.4.49

    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

    99.99 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.99

    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
    253.98 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
    99.99 %
    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
    99.99
    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 CVE sits at the top of the queue because it is network-reachable without authentication, yields direct remote code execution whenever CGI is enabled, and is listed by CISA as actively used in ransomware campaigns — opportunistic mass scanning has been continuous since disclosure and exploitation likelihood is effectively saturated. Observed actors range from crypto-mining botnets through initial-access brokers to ransomware affiliates leveraging Apache DMZ hosts as the entry point. For NIS2-regulated organisations, treat every Apache 2.4.49 asset as a P1 incident when internet-exposed or running CGI; strictly internal, segmented instances may be scored P2 after risk review but must still be lifted to 2.4.51+ within the standard patch window — not 2.4.50, which does not fully close the issue.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Upgrade Apache HTTP Server 2.4.49 to 2.4.51 or later immediately. Do not stop at 2.4.50 — Apache and CISA confirm that fix is incomplete (tracked as CVE-2021-42013). Only 2.4.51+ closes both path-traversal issues.
      • Inventory affected hosts: run httpd -v / apachectl -v across all Linux and Fedora 34/35 systems, and check for embedded Apache instances inside NetApp Cloud Backup and Oracle Instantis EnterpriseTrack 17.1/17.2/17.3 — consult vendor advisories for their separate hotfix bundles.
      • If patching cannot happen within hours, enforce Require all denied in a top-level <Directory /> block in every httpd.conf / apache2.conf, and disable CGI via a2dismod cgi cgid or by commenting out the LoadModule cgi_module lines. Validate with apachectl configtest then systemctl reload httpd.
      • Pull internet-exposed Apache instances offline or front them with a reverse proxy / WAF that enforces path-traversal filtering until the patch rollout completes. Prioritise DMZ hosts, admin portals and legacy CGI applications.
      • Compromise sweep: search access logs for the last 60 days for %2e%2e/, .%2e/, /cgi-bin/.%2e/ and requests touching /etc/passwd, /etc/shadow, /bin/sh, /bin/bash. Any hit triggers full IR workflow — host isolation, forensic imaging, credential rotation for every service reachable from the host.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: egress filtering on all web-server VLANs so a compromised host cannot establish outbound reverse shells (typical ports 4444, 1337, 9001, plus 443 to untrusted IPs). Inbound restricted to 80/443; management ports (22, 8080, 8443) reachable only from admin jump hosts.
      • WAF/IPS rule: block inbound URIs containing encoded dot segments (.%2e, %2e., %2e%2e, ..%2f, ..%5c) and double-encoded variants (%%32%65) in both request line and query string. ET OPEN Suricata/Snort rulesets ship matching SIDs for Apache 2.4.49 path traversal — enable in drop mode.
      • Filesystem hardening: declare explicit <Directory> blocks for DocumentRoot and every Alias target, and set the root filesystem <Directory /> to Require all denied. Apply Options -ExecCGI to CGI directories unless CGI is business-critical.
      • Least privilege: ensure the Apache service account (www-data/apache) has /usr/sbin/nologin, no write access to the web root, and that the systemd unit enforces ProtectSystem=strict, NoNewPrivileges=true, PrivateTmp=true.
      • EDR/HIDS: alert on any child process spawned by httpd/apache2 that resolves to sh, bash, python, perl, curl, wget or nc — this chain is the near-universal post-exploitation signal.
      • Configuration drift: place httpd.conf and conf.d/* under file-integrity monitoring (AIDE, Tripwire, Wazuh FIM) so attacker-planted Alias directives surface immediately.

    3. Runbook step 3

      Detection rules

      • Web access logs (CLF/Combined): regex (\.|%2e|%%32%65){2,}[\/\\%] on the request URI. KQL example: W3CIISLog | where csUriStem matches regex @"(\.|%2e){2}[/\\]" — translate accordingly for Apache logs ingested into Elastic or Splunk.
      • Sysmon EID 1 / auditd execve: parent httpd/apache2 with child /bin/sh, /bin/bash, id, uname, whoami, curl, wget, nc. Sigma shape: parent_image|endswith: '/httpd' AND image|endswith: ('/sh','/bash','/curl','/wget','/nc').
      • Suricata/Zeek: HTTP requests with URI length over 200 characters and three or more %2e occurrences; add a secondary alert on HTTP 200 responses whose body contains root:x:0:0: (indicates /etc/passwd exfiltration).
      • Network telemetry: outbound connections from the web-server segment to non-whitelisted destinations on ports 4444, 1337, 9001, 8888 — standard reverse-shell listeners after successful RCE.
      • FIM alert on new executable files under /var/www/, /usr/lib/cgi-bin/ or /tmp/ created by the Apache UID — strong indicator of a dropped webshell or stager.

    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

    A flaw was found in a change made to path normalization in Apache HTTP Server 2.4.49. 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 is known to be exploited in the wild. This issue only affects Apache 2.4.49 and not earlier versions. The fix in Apache HTTP Server 2.4.50 was found to be incomplete, see CVE-2021-42013.

    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 2.4.49
    • —
    apachehttp_server
    • 2.4.49
    • —
    netappcloud_backup
    • —
    • —
    oracleinstantis_enterprisetrack
    • 17.1
    • 17.3
    • 17.2
    • —
    • —
    • —
    Distribution and container packages (1)

    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
    • —

    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
    Reference tagged as exploitopenwall.com (opens in a new tab)PoC (publicly reported)reported, not verified—
    Reference tagged as exploitopenwall.com (opens in a new tab)PoC (publicly reported)reported, not verified—
    Packet StormApache HTTP Server 2.4.49 Path Traversal (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormApache HTTP Server 2.4.49 Path Traversal 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—
    PoC-in-GitHub (nomi-sec)vulhub-httpd-cve-2021-41773 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773-Apache-Path-Traversal-Lab (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773-Exploit (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773-Exploit-Lab (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)cybersecurity_portfolio (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773-Apache-Lab (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)cve-2021-41773-source-code-analysis (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773-apache (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)mass_cve-2021-41773 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)vulhub-apache (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)Apache-CVE-2021-41773 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773---Apache-Path-Traversal---RCE (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773 (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)APACHE-PATH-TRAVERSAL-RCE-CVE-2021-41773- (opens in a new tab)github.comPoC (publicly reported)reported, not verified
    PoC-in-GitHub (nomi-sec)CVE-2021-41773-POC (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    • packetstormsecurity.comfiles/164418/Apache-HTTP-Server-2.4.49-Path-Traversal-Remote-Co…web, x_refsource_MISC, Exploit, Third-party advisory, Vulnerability database
    • packetstormsecurity.comfiles/164418/Apache-HTTP-Server-2.4.49-Path-Traversal.htmlweb, 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
    • openwall.com[oss-security] 20211005 CVE-2021-41773: Path traversal and file disclosure vulnerability in Apache HTTP Server 2.4.49web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • openwall.com[oss-security] 20211007 RE: CVE-2021-41773: Path traversal and file disclosure vulnerability in Apache HTTP Server 2.4.49web, Mailing list, x_refsource_MLIST, Mailing list, Third-party advisory
    • 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, Exploit, 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, Exploit, 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, Patch, 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, Patch, 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
    • lists.apache.org[httpd-users] 20211005 [users@httpd] CVE-2021-41773: Path traversal and file disclosure vulnerability in Apache HTTP Server 2.4.49web, Mailing list, x_refsource_MLIST, Mailing list
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