# CVE-2021-29433: Denial of service (via resource exhaustion) due to improper input validation

> Attackers can cause resource exhaustion and denial of service in Sydent by sending improperly validated parameters to third-party identifier confirmation endpoints.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L`)
- **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-pw4v-gr34-2553, PYSEC-2021-24

## What is CVE-2021-29433?

CVE-2021-29433 is a medium-severity denial of service 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. In Sydent versions 2.2.0 and prior, sissing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. A patch for the vulnerability is in version 2.3.0. No workarounds are known to exist.

Weakness: CWE-400 (Uncontrolled Resource Consumption), CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2021-29433?

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-2021-29433?

Upgrade matrix-sydent to 2.3.0 or later.

## Is CVE-2021-29433 being exploited?

There is no known exploitation of CVE-2021-29433 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-2021-29431](https://insights.sbomify.com/CVE-2021-29431/): SSRF in Sydent due to missing validation of hostnames (high 7.7)
- [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)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-29433 and fix it if it is affected.

What it is: CVE-2021-29433 is a medium-severity denial of service 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-2021-29433 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-2021-29433/ (Markdown: https://insights.sbomify.com/CVE-2021-29433/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-29433
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-29433
- OSV: https://osv.dev/vulnerability/CVE-2021-29433

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-29433/
Summaries are generated and may contain mistakes; the official description above is the record.