# CVE-2026-77640: Denial of service in Tor

> An attacker can cause a denial of service in Tor by sending a truncated compressed stream, triggering an infinite loop during decompression.

- **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 20 Aug 2026
- **Updated:** 21 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-77640, GHSA-5g6h-2vr8-m7f4, UBUNTU-CVE-2026-77640

## What is CVE-2026-77640?

CVE-2026-77640 is a low-severity denial of service vulnerability in torproject Tor, published on 20 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

tor before 0.4.9.9 was prone to an infinite loop when decompressing a truncated zlib/gzip  stream with done=1. A truncated stream never reaches Z_STREAM_END,  causing zlib to return Z_BUF_ERROR with no input remaining, which  buf_add_compress() mistook for a full output buffer and retried forever. Fixed by returning TOR_COMPRESS_ERROR in that case so the caller can abort cleanly. This is TROVE-2026-021.

Weakness: CWE-1284 (Improper Validation of Specified Quantity in Input)

## Which versions are affected by CVE-2026-77640?

It affects 1 product.


Products: Torproject Tor.

## How do I fix CVE-2026-77640?

Upgrade to a fixed version.

## Is CVE-2026-77640 being exploited?

There is no known exploitation of CVE-2026-77640 in the wild and no public exploit code.

## Other vulnerabilities in torproject Tor

- [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-2026-77640 and fix it if it is affected.

What it is: CVE-2026-77640 is a low-severity denial of service vulnerability in torproject Tor, published on 20 Aug 2026. 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-2026-77640 in the wild and no public exploit code.

Affected product: torproject Tor. 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-2026-77640/ (Markdown: https://insights.sbomify.com/CVE-2026-77640/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-77640
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77640
- OSV: https://osv.dev/vulnerability/CVE-2026-77640

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-77640/
Summaries are generated and may contain mistakes; the official description above is the record.