CVE-2026-77640 Denial of service in Tor

Low 3.7 Published 20 Aug 2026 Updated 21 Aug 2026

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

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

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

Which versions are affected by CVE-2026-77640?

It affects 1 product.

Products include Torproject Tor.

How do I fix CVE-2026-77640?

Upgrade to a fixed version.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-77640 being exploited?

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

Sources

Also known as DEBIAN-CVE-2026-77640, GHSA-5g6h-2vr8-m7f4, UBUNTU-CVE-2026-77640.