# CVE-2016-0772: Security feature bypass in n/a

> Man-in-the-middle attackers can bypass TLS encryption in Python's smtplib by blocking StartTLS commands, exposing email traffic to interception.

- **Severity:** Medium 6.5 (`CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 2 Sep 2016
- **Updated:** 5 Aug 2024
- **Also known as:** AZL-109605, DEBIAN-CVE-2016-0772, DLA-871-1, GHSA-6m57-q338-h677, PSF-2016-3, UBUNTU-CVE-2016-0772

## What is CVE-2016-0772?

CVE-2016-0772 is a medium-severity vulnerability in Python Python, published on 2 Sep 2016. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The smtplib library in CPython (aka Python) before 2.7.12, 3.x before 3.4.5, and 3.5.x before 3.5.2 does not return an error when StartTLS fails, which might allow man-in-the-middle attackers to bypass the TLS protections by leveraging a network position between the client and the registry to block the StartTLS command, aka a "StartTLS stripping attack."

Weakness: CWE-693 (Protection Mechanism Failure)

## Which versions are affected by CVE-2016-0772?

It affects 1 product.


Products: Python Python.

## How do I fix CVE-2016-0772?

Upgrade to a fixed version.

## Is CVE-2016-0772 being exploited?

There is no known exploitation of CVE-2016-0772 in the wild and no public exploit code.

Public exploit code:

- Python smtplib 2.7.11 / 3.4.4 / 3.5.1 - Man In The Middle StartTLS Stripping (exploitdb)

## Other vulnerabilities in Python Python

- [CVE-2008-5031](https://insights.sbomify.com/CVE-2008-5031/): Integer overflow in n/a (critical 10.0)
- [CVE-2014-4650](https://insights.sbomify.com/CVE-2014-4650/): Path traversal in n/a (critical 9.8)
- [CVE-2016-5636](https://insights.sbomify.com/CVE-2016-5636/): Integer overflow in n/a (critical 9.8)
- [CVE-2007-4559](https://insights.sbomify.com/CVE-2007-4559/): Path traversal in n/a (critical 9.8)
- [CVE-2017-17522](https://insights.sbomify.com/CVE-2017-17522/): Command injection in n/a (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2016-0772 and fix it if it is affected.

What it is: CVE-2016-0772 is a medium-severity vulnerability in Python Python, published on 2 Sep 2016. 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-2016-0772 in the wild and no public exploit code.

Affected product: Python Python. 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-2016-0772/ (Markdown: https://insights.sbomify.com/CVE-2016-0772/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2016-0772
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2016-0772
- OSV: https://osv.dev/vulnerability/CVE-2016-0772

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From sbomify Insights: https://insights.sbomify.com/CVE-2016-0772/
Summaries are generated and may contain mistakes; the official description above is the record.