# CVE-2009-3639: Security feature bypass in n/a

> Remote attackers can bypass client-hostname restrictions in ProFTPD by using a crafted X.509 certificate with a null byte in the domain name.

- **Severity:** Medium 5.8 (`AV:N/AC:M/Au:N/C:N/I:P/A:P`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 28 Oct 2009
- **Updated:** 7 Aug 2024
- **Also known as:** DEBIAN-CVE-2009-3639, DSA-1925-1, GHSA-4vq9-9g4j-pgg4

## What is CVE-2009-3639?

CVE-2009-3639 is a medium-severity vulnerability in n/a, published on 28 Oct 2009. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The mod_tls module in ProFTPD before 1.3.2b, and 1.3.3 before 1.3.3rc2, when the dNSNameRequired TLS option is enabled, does not properly handle a '\0' character in a domain name in the Subject Alternative Name field of an X.509 client certificate, which allows remote attackers to bypass intended client-hostname restrictions via a crafted certificate issued by a legitimate Certification Authority, a related issue to CVE-2009-2408.

Weakness: CWE-310

## Which versions are affected by CVE-2009-3639?

It affects 1 product.


Products: Proftpd Proftpd.

## How do I fix CVE-2009-3639?

Upgrade to a fixed version.

## Is CVE-2009-3639 being exploited?

There is no known exploitation of CVE-2009-3639 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2010-4221](https://insights.sbomify.com/CVE-2010-4221/): Memory corruption in n/a (critical 10.0)
- [CVE-2010-20103](https://insights.sbomify.com/CVE-2010-20103/): ProFTPD 1.3.3c Backdoor Command Execution (critical 9.3)
- [CVE-2011-4130](https://insights.sbomify.com/CVE-2011-4130/): Use after free in n/a (critical 9.0)
- [CVE-2026-63090](https://insights.sbomify.com/CVE-2026-63090/): ProFTPD mod_sftp Heap Buffer Overflow via SFTP Packet Reassembly (high 8.7)
- [CVE-2026-35025](https://insights.sbomify.com/CVE-2026-35025/): ProFTPD ACL Bypass via /proc/self/root Path Prefix in RNFR (high 8.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2009-3639 and fix it if it is affected.

What it is: CVE-2009-3639 is a medium-severity vulnerability in n/a, published on 28 Oct 2009. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2009-3639 in the wild and no public exploit code.

Affected product: n/a. 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-2009-3639/ (Markdown: https://insights.sbomify.com/CVE-2009-3639/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2009-3639
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2009-3639
- OSV: https://osv.dev/vulnerability/CVE-2009-3639

---
From sbomify Insights: https://insights.sbomify.com/CVE-2009-3639/
Summaries are generated and may contain mistakes; the official description above is the record.