# CVE-2021-29431: SSRF in Sydent due to missing validation of hostnames

> Attackers can induce Sydent to send HTTP requests to internal systems, enabling port enumeration and potential internal network access.

- **Severity:** High 7.7 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Apr 2021
- **Updated:** 3 Aug 2024
- **Also known as:** GHSA-9jhm-8m8c-c3f4, PYSEC-2021-22

## What is CVE-2021-29431?

CVE-2021-29431 is a high-severity server-side request forgery vulnerability in matrix-org sydent, published on 15 Apr 2021. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Sydent is a reference Matrix identity server. Sydent can be induced to send HTTP GET requests to internal systems, due to lack of parameter validation or IP address blacklisting. It is not possible to exfiltrate data or control request headers, but it might be possible to use the attack to perform an internal port enumeration. This issue has been addressed in in 9e57334, 8936925, 3d531ed, 0f00412. A potential workaround would be to use a firewall to ensure that Sydent cannot reach internal HTTP resources.

Weakness: CWE-918 (Server-Side Request Forgery), CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2021-29431?

3 open source packages ship an affected version, including matrix-sydent. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `matrix-sydent` | PyPI | All versions | 2.3.0 |
| `matrix-sydent` | PyPI | All versions | 2.3.0 |
| `matrix-sydent` | PyPI | All versions | 2.3.0 |

Products: Matrix Sydent.

## How do I fix CVE-2021-29431?

Upgrade matrix-sydent to 2.3.0 or later.

## Is CVE-2021-29431 being exploited?

There is no known exploitation of CVE-2021-29431 in the wild and no public exploit code.

## Other vulnerabilities in matrix-org sydent

- [CVE-2023-38686](https://insights.sbomify.com/CVE-2023-38686/): Sydent does not verify email server certificates (critical 9.3)
- [CVE-2021-29430](https://insights.sbomify.com/CVE-2021-29430/): Denial of service attack via memory exhaustion (high 7.5)
- [CVE-2019-11842](https://insights.sbomify.com/CVE-2019-11842/): matrix-sydent and matrix-synapse Use Cryptographically Weak PRNG (high 7.5)
- [CVE-2019-11340](https://insights.sbomify.com/CVE-2019-11340/): Matrix Sydent mishandles emails (medium 5.9)
- [CVE-2021-29432](https://insights.sbomify.com/CVE-2021-29432/): Malicious users could control the content of invitation emails (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-29431 and fix it if it is affected.

What it is: CVE-2021-29431 is a high-severity server-side request forgery vulnerability in matrix-org sydent, published on 15 Apr 2021. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-29431 in the wild and no public exploit code.

Affected packages and fixed versions:
- matrix-sydent (PyPI): affected all versions; fixed in 2.3.0
- matrix-sydent (PyPI): affected all versions; fixed in 2.3.0
- matrix-sydent (PyPI): affected all versions; fixed in 2.3.0

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-2021-29431/ (Markdown: https://insights.sbomify.com/CVE-2021-29431/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-29431
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-29431
- OSV: https://osv.dev/vulnerability/CVE-2021-29431

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-29431/
Summaries are generated and may contain mistakes; the official description above is the record.