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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 17 Nov 2021. Known ransomware use
HTTP ServerWeb server / proxy · Apache

CVE-2021-41773

CRITICAL · 9.8 CVSS v3.1 Published 2021-10-05
Patch available
A vendor patch is available. No clean upgrade release — apply the published patch.
See remediation →
100/100
Remediation priority · Urgent
In the wild Ransomware High EPSS Public exploit Remotely reachable No privileges Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A path traversal vulnerability in Apache HTTP Server 2.4.49's path normalization allows attackers to access files outside directories configured by Alias-like directives. If these directories lack 'require all denied' protection and CGI scripts are enabled, attackers can achieve remote code execution. This flaw was actively exploited in the wild.

MitigationImmediately upgrade to Apache HTTP Server 2.4.51 or later. Additionally, verify all directory configurations have 'require all denied' and disable CGI for any aliased paths not requiring it, as a defense-in-depth measure.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
HTTP ServerWeb server / proxy
Affected:= 2.4.49
FedoraOperating system
Affected:= 34= 35
Instantis EnterprisetrackApplication
Affected:= 17.1= 17.2= 17.3
Cloud BackupApplication
Affected:all versions

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify Apache HTTP Server version
    Run 'httpd -v' or 'apache2 -v' to determine the installed Apache version
    Affected if The version is exactly 2.4.49 (earlier versions are not affected by this specific CVE)
  2. Locate Apache configuration files
    Find httpd.conf, apache2.conf, or included configuration files - typically in /etc/httpd/, /etc/apache2/, or the server root
    Affected if Configuration files exist and the server is running (vulnerability applies to the HTTP server configuration)
  3. Check for Alias or ScriptAlias directives
    Search configuration files for 'Alias' or 'ScriptAlias' directives using 'grep -r "Alias" /etc/httpd/' or similar
    Affected if Alias directives are present - the vulnerability exploits these to access files outside the intended directories
  4. Verify CGI is enabled for aliased paths
    Look for 'Options +ExecCGI' in <Directory> blocks matching aliased paths, or check if ScriptAlias is used instead of Alias
    Affected if CGI scripts are enabled for paths defined by Alias directives - this creates the RCE possibility when combined with path traversal
  5. Inspect access controls on parent directories
    Review <Directory> blocks in configuration to check if directories outside Alias paths have 'Require all denied' or equivalent restrictions
    Affected if Directories outside Alias paths lack 'require all denied' protection - this allows the path traversal to reach sensitive files or execute code

If the Apache version is 2.4.49 and the server uses Alias directives with CGI enabled on paths lacking proper 'require all denied' access controls, the environment is vulnerable to CVE-2021-41773.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Patch available Apply the vendor patch
Vendor patch www.openwall.com →
Interim mitigation

Immediately upgrade to Apache HTTP Server 2.4.51 or later. Additionally, verify all directory configurations have 'require all denied' and disable CGI for any aliased paths not requiring it, as a defense-in-depth measure.

Recommended fix High confidence

Apache HTTP Server 2.4.51 or later

  1. Upgrade Apache HTTP Server from version 2.4.49 to version 2.4.51 or later (2.4.50 had an incomplete fix for this issue)
  2. Verify the upgrade was successful by checking the server version (e.g., apachectl -v or httpd -v)
  3. Test that path traversal requests are no longer successful
  4. If using CGI scripts with aliased paths, ensure they are properly protected by 'Require all denied' directives in the configuration
  5. Restart the Apache service after upgrade

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in HTTP Server Exploited in the wild — priority engagement
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing8.0 h
  • Review / QA3.0 h
20.0 hours of engineering $3,420
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Scan for this in your stack

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-41773 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data