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HTTP ServerWeb server / proxy · Apache

CVE-2026-33523

MEDIUM · 6.5 CVSS v3.1 Published 2026-05-04
Fix available
A fix is available. Upgrade to 2.4.67 or later.
See remediation →
74/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

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
HTTP response splitting vulnerability in multiple Apache HTTP Server modules with untrusted or compromised backend servers. This issue affects Apache HTTP Server: from through 2.4.66. Users are recommended to upgrade to version 2.4.67, which fixes the issue.

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 · moderate confidence

HTTP response splitting vulnerability in multiple Apache HTTP Server modules allows attackers to inject malicious content into HTTP response headers when the server processes responses from untrusted or compromised backend servers. This occurs due to insufficient validation of header data, potentially enabling cache poisoning or cross-site scripting attacks.

MitigationUpgrade Apache HTTP Server to version 2.4.67 to resolve the vulnerability, and ensure backend servers are trusted and not compromised.

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:>= 2.4.0, < 2.4.67

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
Low
Integrity
Low
Availability
None

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

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. Check Apache HTTP Server version
    Run 'httpd -v' or 'apache2 -v' to display the server version, or check the ServerTokens directive in the configuration
    Affected if The version displayed is 2.4.0 or higher but below 2.4.67
  2. Identify if proxy modules are loaded
    Run 'httpd -M' or 'apache2ctl -M' to list loaded modules, looking for proxy-related modules such as mod_proxy, mod_proxy_http, mod_proxy_balancer, or mod_rewrite with proxy flags
    Affected if Any proxy module (mod_proxy, mod_proxy_http, etc.) is loaded and enabled
  3. Check proxy configuration for backend connections
    Inspect the Apache configuration files (httpd.conf, apache2.conf, or included conf files) for ProxyPass, ProxyPassReverse, or RewriteRule with [P] flag directives that point to backend servers
    Affected if Proxy directives connecting to untrusted or external backend servers are configured

You are affected if Apache HTTP Server version is 2.4.0 through 2.4.66 and proxy modules are enabled with configurations connecting to backend servers.

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
Upgrade available Upgrade to 2.4.67 or later
Fixed in 2.4.67
Interim mitigation

Upgrade Apache HTTP Server to version 2.4.67 to resolve the vulnerability, and ensure backend servers are trusted and not compromised.

Recommended fix High confidence

2.4.67

  1. 1. Identify the current Apache HTTP Server version by running 'httpd -v' or 'apache2 -v'
  2. 2. Stop the Apache HTTP Server service (e.g., 'systemctl stop apache2' or 'systemctl stop httpd')
  3. 3. Back up the existing Apache configuration directory (typically /etc/apache2 or /etc/httpd) and document any custom configurations
  4. 4. Upgrade Apache HTTP Server to version 2.4.67 using the system's package manager or by compiling from source (e.g., 'apt-get update && apt-get install apache2' or download from httpd.apache.org)
  5. 5. Verify the upgrade was successful by checking 'httpd -v' or 'apache2 -v' shows version 2.4.67
  6. 6. Review the Apache configuration files for any custom settings that may need reapplication after upgrade
  7. 7. Start the Apache HTTP Server service (e.g., 'systemctl start apache2' or 'systemctl start httpd')
  8. 8. Test that the server is functioning correctly and serving content as expected
Caveat Minor version upgrades typically have low risk but test in non-production first

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

Fix this in HTTP Server Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing3.0 h
  • Review / QA2.0 h
10.0 hours of engineering $1,750
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,800.

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-2026-33523 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