# CVE-2016-4975: mod_userdir CRLF injection

> Attackers can inject CRLF characters to split HTTP responses in Apache sites using mod_userdir, requiring an upgrade to mitigate.

- **Severity:** Medium 6.1 (`CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 14 Aug 2018
- **Updated:** 16 Sep 2024
- **Also known as:** DEBIAN-CVE-2016-4975, GHSA-crcg-r773-w4rv, UBUNTU-CVE-2016-4975

## What is CVE-2016-4975?

CVE-2016-4975 is a medium-severity vulnerability in Apache Http Server, published on 14 Aug 2018. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

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).

Weakness: CWE-93 (Improper Neutralization of CRLF Sequences)

## Which versions are affected by CVE-2016-4975?

It affects 1 product.


Products: Apache Http Server.

## How do I fix CVE-2016-4975?

Upgrade to a fixed version.

## Is CVE-2016-4975 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2016-4975 exists.

Public exploit code:

- Apache mod_userdir CRLF injection (nuclei): http/cves/2016/CVE-2016-4975.yaml

## Other vulnerabilities in Apache Http Server

- [CVE-2021-42013](https://insights.sbomify.com/CVE-2021-42013/): Apache HTTP Server Path Traversal (critical 9.8, exploited in the wild)
- [CVE-2024-38475](https://insights.sbomify.com/CVE-2024-38475/): Apache HTTP Server Improper Escaping of Output (critical 9.1, exploited in the wild)
- [CVE-2021-40438](https://insights.sbomify.com/CVE-2021-40438/): Apache HTTP Server-Side Request Forgery (SSRF) (critical 9.0, exploited in the wild)
- [CVE-2019-0211](https://insights.sbomify.com/CVE-2019-0211/): Apache HTTP Server Privilege Escalation (high 7.8, exploited in the wild)
- [CVE-2021-41773](https://insights.sbomify.com/CVE-2021-41773/): Apache HTTP Server Path Traversal (high 7.5, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2016-4975 and fix it if it is affected.

What it is: CVE-2016-4975 is a medium-severity vulnerability in Apache Http Server, published on 14 Aug 2018. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2016-4975 exists.

Affected product: Apache Http Server. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2016-4975/ (Markdown: https://insights.sbomify.com/CVE-2016-4975/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2016-4975
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2016-4975
- OSV: https://osv.dev/vulnerability/CVE-2016-4975

---
From sbomify Insights: https://insights.sbomify.com/CVE-2016-4975/
Summaries are generated and may contain mistakes; the official description above is the record.