dbcve.org
HTTP ServerWeb server / proxy · Apache

CVE-2016-4975

MEDIUM · 6.1 CVSS v3.0 Published 2018-08-14
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges

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
Possible CRLF injection allowing HTTP response splitting attacks for sites which use mod_userdir. This issue was mitigated by changes made in 2.4.25 and 2.2.32 which prohibit CR or LF injection into the "Location" or other outbound header key or value. Fixed in Apache HTTP Server 2.4.25 (Affected 2.4.1-2.4.23). Fixed in Apache HTTP Server 2.2.32 (Affected 2.2.0-2.2.31).

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

Apache HTTP Server contains a CRLF injection vulnerability in mod_userdir that allows attackers to inject Carriage Return (CR) and Line Feed (LF) characters into outbound HTTP headers, enabling HTTP response splitting attacks. This occurs when user directory paths are processed and reflected in headers without proper sanitization.

MitigationUpgrade Apache HTTP Server to version 2.4.25 or later for the 2.4.x branch, or 2.2.32 or later for the 2.2.x branch. The fix prohibits CR or LF injection into Location and other outbound headers.

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:= 2.2.0= 2.2.2= 2.2.3= 2.2.4= 2.2.6= 2.2.8= 2.2.9= 2.2.10= 2.2.11= 2.2.12= 2.2.13= 2.2.14

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
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/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. Identify installed Apache HTTP Server version
    Run 'httpd -v' or 'apache2 -v' to get the server version number
    Affected if The version is 2.2.0, 2.2.2, 2.2.3, 2.2.4, 2.2.6, 2.2.8, 2.2.9, 2.2.10, 2.2.11, 2.2.12, 2.2.13, or 2.2.14 (versions before 2.2.32)
  2. Confirm mod_userdir module is enabled
    Check the Apache configuration files (httpd.conf, apache2.conf) for 'LoadModule userdir_module' or run 'httpd -M' / 'apache2ctl -M' to list enabled modules
    Affected if mod_userdir is loaded and available in the server
  3. Verify UserDir directive is configured
    Search configuration files for 'UserDir' directives (e.g., 'UserDir public_html') or check if any user directories are being served
    Affected if UserDir is actively configured and serving user directory paths
  4. Check for unescaped user input in requests
    Review access logs and test for CRLF injection by sending a request with encoded newlines (e.g., %0D%0A) in the username portion of a UserDir URL path
    Affected if The server reflects user-supplied input in HTTP headers without sanitizing CR/LF characters

You are affected if Apache HTTP Server 2.2.x versions 2.2.0 through 2.2.14 are running with mod_userdir enabled and UserDir configured, allowing CRLF injection into outbound headers.

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.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade Apache HTTP Server to version 2.4.25 or later for the 2.4.x branch, or 2.2.32 or later for the 2.2.x branch. The fix prohibits CR or LF injection into Location and other outbound headers.

Fix this in HTTP Server Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA2.0 h
10.0 hours of engineering $1,780
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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-2016-4975 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