# CVE-2020-5207: Request smuggling is possible in Ktor when both chunked TE and content length specified

> Request smuggling allows bypassing proxy security controls in Ktor when specific header conditions are met.

- **Severity:** Medium 5.4 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 27 Jan 2020
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-xrr9-rh8p-433v

## What is CVE-2020-5207?

CVE-2020-5207 is a medium-severity vulnerability in Ktor.io Ktor, published on 27 Jan 2020. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

In Ktor before 1.3.0, request smuggling is possible when running behind a proxy that doesn't handle Content-Length and Transfer-Encoding properly or doesn't handle \n as a headers separator.

Weakness: CWE-444 (Inconsistent Interpretation of HTTP Requests)

## Which versions are affected by CVE-2020-5207?

4 open source packages ship an affected version, including io.ktor:ktor-client-cio and io.ktor:ktor-server-cio. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `io.ktor:ktor-client-cio` | Maven | All versions | 1.3.0 |
| `io.ktor:ktor-client-cio` | Maven | All versions | 1.3.0 |
| `io.ktor:ktor-server-cio` | Maven | All versions | 1.3.0 |
| `io.ktor:ktor-server-cio` | Maven | All versions | 1.3.0 |

Products: Jetbrains Ktor.

## How do I fix CVE-2020-5207?

Upgrade io.ktor:ktor-client-cio to 1.3.0 or later.

## Is CVE-2020-5207 being exploited?

There is no known exploitation of CVE-2020-5207 in the wild and no public exploit code.

## Other vulnerabilities in Ktor.io Ktor

- [CVE-2019-12736](https://insights.sbomify.com/CVE-2019-12736/): Command injection in n/a (critical 9.8)
- [CVE-2022-29930](https://insights.sbomify.com/CVE-2022-29930/): SHA1 implementation in JetBrains Ktor Native 2.0.0 was returning the same value. (high 8.7)
- [CVE-2023-45612](https://insights.sbomify.com/CVE-2023-45612/): XML external entity in Ktor (high 8.6)
- [CVE-2022-48476](https://insights.sbomify.com/CVE-2022-48476/): Path traversal in Ktor (high 7.5)
- [CVE-2021-43203](https://insights.sbomify.com/CVE-2021-43203/): Auth bypass in n/a (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-5207 and fix it if it is affected.

What it is: CVE-2020-5207 is a medium-severity vulnerability in Ktor.io Ktor, published on 27 Jan 2020. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2020-5207 in the wild and no public exploit code.

Affected packages and fixed versions:
- io.ktor:ktor-client-cio (Maven): affected all versions; fixed in 1.3.0
- io.ktor:ktor-client-cio (Maven): affected all versions; fixed in 1.3.0
- io.ktor:ktor-server-cio (Maven): affected all versions; fixed in 1.3.0
- io.ktor:ktor-server-cio (Maven): affected all versions; fixed in 1.3.0

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-2020-5207/ (Markdown: https://insights.sbomify.com/CVE-2020-5207/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-5207
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-5207
- OSV: https://osv.dev/vulnerability/CVE-2020-5207

---
From sbomify Insights: https://insights.sbomify.com/CVE-2020-5207/
Summaries are generated and may contain mistakes; the official description above is the record.