# CVE-2019-8955: Denial of service in n/a

> Remote attackers can cause denial of service against Tor clients and relays by exhausting memory in the KIST cell scheduler.

- **Severity:** High 7.5 (`CVSS:3.0/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:** 21 Feb 2019
- **Updated:** 4 Aug 2024
- **Also known as:** DEBIAN-CVE-2019-8955, GHSA-h3x5-p32h-w7q2, UBUNTU-CVE-2019-8955, openSUSE-SU-2019:1107-1

## What is CVE-2019-8955?

CVE-2019-8955 is a high-severity denial of service vulnerability in n/a, published on 21 Feb 2019. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In Tor before 0.3.3.12, 0.3.4.x before 0.3.4.11, 0.3.5.x before 0.3.5.8, and 0.4.x before 0.4.0.2-alpha, remote denial of service against Tor clients and relays can occur via memory exhaustion in the KIST cell scheduler.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2019-8955?

It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `tor` | SUSE:Package Hub 12 | All versions | 0.3.4.11-bp150.3.6.1 |
| `tor` | SUSE:Package Hub 15 | All versions | 0.3.4.11-bp150.3.6.1 |
| `tor` | openSUSE:Leap 15.0 | All versions | 0.3.4.11-bp150.3.6.1 |

Products: Torproject Tor.

## How do I fix CVE-2019-8955?

Upgrade to a fixed version.

## Is CVE-2019-8955 being exploited?

There is no known exploitation of CVE-2019-8955 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-2019-8955 and fix it if it is affected.

What it is: CVE-2019-8955 is a high-severity denial of service vulnerability in n/a, published on 21 Feb 2019. 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-2019-8955 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-2019-8955/ (Markdown: https://insights.sbomify.com/CVE-2019-8955/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2019-8955
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2019-8955
- OSV: https://osv.dev/vulnerability/CVE-2019-8955

---
From sbomify Insights: https://insights.sbomify.com/CVE-2019-8955/
Summaries are generated and may contain mistakes; the official description above is the record.