# CVE-2016-3739: Security feature bypass in n/a

> Attackers can spoof servers using valid certificates when cURL connects to IP addresses via SSLv3 or TLS, bypassing certificate validation.

- **Severity:** Medium 5.3 (`CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/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:** 20 May 2016
- **Updated:** 6 Aug 2024
- **Also known as:** CURL-CVE-2016-3739, DEBIAN-CVE-2016-3739, GHSA-4rcg-2r46-4hcc

## What is CVE-2016-3739?

CVE-2016-3739 is a medium-severity vulnerability in Haxx Curl, published on 20 May 2016. 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

The (1) mbed_connect_step1 function in lib/vtls/mbedtls.c and (2) polarssl_connect_step1 function in lib/vtls/polarssl.c in cURL and libcurl before 7.49.0, when using SSLv3 or making a TLS connection to a URL that uses a numerical IP address, allow remote attackers to spoof servers via an arbitrary valid certificate.

Weakness: CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2016-3739?

It affects 1 product.


Products: Haxx Curl.

## How do I fix CVE-2016-3739?

Upgrade to a fixed version.

## Is CVE-2016-3739 being exploited?

There is no known exploitation of CVE-2016-3739 in the wild and no public exploit code.

## Other vulnerabilities in Haxx Curl

- [CVE-2026-19931](https://insights.sbomify.com/CVE-2026-19931/): Negotiate ambient user conn reuse (critical 9.8)
- [CVE-2026-10536](https://insights.sbomify.com/CVE-2026-10536/): HTTP/2 stream-dependency tree UAF (critical 9.8)
- [CVE-2026-11856](https://insights.sbomify.com/CVE-2026-11856/): cross-origin Digest auth state leak (critical 9.8)
- [CVE-2026-8925](https://insights.sbomify.com/CVE-2026-8925/): SASL double-free (critical 9.8)
- [CVE-2026-9079](https://insights.sbomify.com/CVE-2026-9079/): stale proxy password leak (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2016-3739 and fix it if it is affected.

What it is: CVE-2016-3739 is a medium-severity vulnerability in Haxx Curl, published on 20 May 2016. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2016-3739 in the wild and no public exploit code.

Affected product: Haxx Curl. 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-2016-3739/ (Markdown: https://insights.sbomify.com/CVE-2016-3739/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2016-3739
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2016-3739
- OSV: https://osv.dev/vulnerability/CVE-2016-3739

---
From sbomify Insights: https://insights.sbomify.com/CVE-2016-3739/
Summaries are generated and may contain mistakes; the official description above is the record.