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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 15 Dec 2021. Known ransomware use
Enterprise LinuxOperating system · Redhat

CVE-2021-40438

CRITICAL · 9.0 CVSS v3.1 Published 2021-09-16
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 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 crafted request uri-path can cause mod_proxy to forward the request to an origin server choosen by the remote user. This issue affects Apache HTTP Server 2.4.48 and earlier.

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

This is a Server-Side Request Forgery (SSRF) vulnerability in Apache HTTP Server's mod_proxy module. Attackers can craft a malicious request URI-path to cause the proxy to forward requests to an origin server of their choosing, potentially allowing access to internal services or exfiltration of data.

MitigationUpgrade Apache HTTP Server to version 2.4.49 or later. If upgrade is not immediately possible, review and restrict mod_proxy configuration, or temporarily disable the module if not required.

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
Enterprise LinuxOperating system
Affected:= 8.0
Enterprise Linux EusOperating system
Affected:= 8.1= 8.2= 8.4= 8.6= 8.8
Enterprise Linux For Arm 64Operating system
Affected:= 8.0
Enterprise Linux For Arm 64 EusOperating system
Affected:= 8.6= 8.8
Enterprise Linux For Ibm Z SystemsOperating system
Affected:= 7.0_s390x= 8.0
Enterprise Linux For Ibm Z Systems EusOperating system
Affected:= 8.1= 8.4= 8.8
Enterprise Linux For Ibm Z Systems Eus S390xOperating system
Affected:= 8.2
Enterprise Linux For Power Big EndianOperating system
Affected:= 7.0

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
High
Privileges
None
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/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. Verify mod_proxy module is loaded
    Run 'httpd -M 2>/dev/null | grep proxy' or 'apachectl -M 2>/dev/null | grep proxy' to list loaded proxy modules. Look for mod_proxy, mod_proxy_http, mod_proxy_connect, or mod_proxy_balancer.
    Affected if mod_proxy or any proxy sub-module is loaded and active
  2. Check Apache HTTP Server version
    Run 'httpd -v' or 'apachectl -v' to obtain the server version number (e.g., 2.4.37).
    Affected if Version is 2.4.48 or earlier (vulnerable); versions 2.4.49 and later contain the fix
  3. Identify active proxy configuration
    Search configuration files (httpd.conf, included .conf files) for 'ProxyPass', 'ProxyPassMatch', or RewriteRule with the [P] flag (e.g., 'RewriteRule ^/(.*)$ http://backend/$1 [P]').
    Affected if Any ProxyPass, ProxyPassMatch, or [P] rewrite rule points to a backend server, making the server a potential SSRF target
  4. Check for exposed proxy endpoints
    Review Listen directives and virtual host configurations to identify IP addresses and ports the Apache server accepts connections on.
    Affected if The proxy module is accessible over the network (not localhost-only) and has a writable proxy configuration
  5. Verify version against affected Red Hat releases
    Run 'cat /etc/redhat-release' to identify the RHEL version. Compare against the listed affected versions: 8.0, 8.1, 8.2, 8.4, 8.6, 8.8 for various architectures.
    Affected if Running RHEL 8.0, 8.1, 8.2, 8.4, 8.6, 8.8 or the specific architecture versions listed (7.0, 8.0 for s390x, 7.0 for Power BE)

You are affected if mod_proxy is enabled AND the Apache version is 2.4.48 or earlier (or an unpatched RHEL version from the affected list), AND a proxy configuration exists that could accept malicious URI-path requests.

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.oracle.com →
Interim mitigation

Upgrade Apache HTTP Server to version 2.4.49 or later. If upgrade is not immediately possible, review and restrict mod_proxy configuration, or temporarily disable the module if not required.

Recommended fix High confidence

Apache HTTP Server 2.4.49 or later (via RHEL httpd packages)

  1. 1. Identify all systems running Apache HTTP Server 2.4.48 or earlier on the affected RHEL versions
  2. 2. Run 'dnf check-update' or 'yum check-update' to check for available httpd updates
  3. 3. For RHEL 8.x systems: Execute 'dnf update httpd' or 'yum update httpd' to apply the security fix
  4. 4. For RHEL 7.x systems: Execute 'yum update httpd' to apply the security fix
  5. 5. After updating, restart the Apache HTTP Server service using 'systemctl restart httpd'
  6. 6. Verify the installed version is 2.4.49 or later using 'httpd -v' or 'apachectl -v'
  7. 7. Test that proxy functionality still works correctly for legitimate configurations
Caveat Ensure any custom mod_proxy configurations are tested after upgrade, as minor behavior changes in proxy handling may occur

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

Fix this in Enterprise Linux Exploited in the wild — priority engagement
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing4.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,080
Get it fixed fast

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,328.

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-2021-40438 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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