# CVE-2021-34549: Denial of service in n/a

> An attacker can cause algorithm inefficiency and denial of service in Tor by triggering the use of an attacker-chosen circuit ID.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 29 Jun 2021
- **Updated:** 4 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-34549, GHSA-qx32-4xrf-2j53, UBUNTU-CVE-2021-34549

## What is CVE-2021-34549?

CVE-2021-34549 is a high-severity denial of service vulnerability in n/a, published on 29 Jun 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An issue was discovered in Tor before 0.4.6.5, aka TROVE-2021-005. Hashing is mishandled for certain retrieval of circuit data. Consequently. an attacker can trigger the use of an attacker-chosen circuit ID to cause algorithm inefficiency.

Weakness: CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2021-34549?

It affects 1 product.


Products: Torproject Tor.

## How do I fix CVE-2021-34549?

Upgrade to a fixed version.

## Is CVE-2021-34549 being exploited?

There is no known exploitation of CVE-2021-34549 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2026-77638](https://insights.sbomify.com/CVE-2026-77638/): Race condition in Tor (high 8.9)
- [CVE-2026-77642](https://insights.sbomify.com/CVE-2026-77642/): Memory corruption in Tor (high 7.5)
- [CVE-2022-33903](https://insights.sbomify.com/CVE-2022-33903/): Denial of service in n/a (high 7.5)
- [CVE-2021-38385](https://insights.sbomify.com/CVE-2021-38385/): Denial of service in n/a (high 7.5)
- [CVE-2021-34548](https://insights.sbomify.com/CVE-2021-34548/): Security feature bypass in n/a (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-34549 and fix it if it is affected.

What it is: CVE-2021-34549 is a high-severity denial of service vulnerability in n/a, published on 29 Jun 2021. 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-2021-34549 in the wild and no public exploit code.

Affected product: n/a. 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-2021-34549/ (Markdown: https://insights.sbomify.com/CVE-2021-34549/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-34549
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-34549
- OSV: https://osv.dev/vulnerability/CVE-2021-34549

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-34549/
Summaries are generated and may contain mistakes; the official description above is the record.