Quick answers
- Is CVE-2026-77639 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-77639 being exploited in the wild.
- How severe is CVE-2026-77639?
- Medium severity, with a CVSS score of 5.3, published 20 August 2026.
- Which products and versions are affected?
- Tor Project Tor before 0.4.9.9.
- Is there a fix for CVE-2026-77639?
- A patch was published on 2026-08-20: Fixed in Tor 0.4.9.9 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2026-77639 attacked?
- It is attacked over the network, in a hosted service.
- Am I affected by CVE-2026-77639?
- Check whether you run Torproject Tor. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
Technical details
The vulnerability is a compression bomb bypass in Tor's gzip/zlib stream decompression logic. An attacker can concatenate multiple gzip or zlib sub-streams, each individually below the per-stream compression detection threshold, allowing them to collectively exceed the intended limit and evade the compression bomb protection. The attack requires network access to a Tor relay or client that processes the malicious compressed data. A successful exploit can cause excessive CPU and memory consumption on the target Tor node, leading to denial of service. The issue was fixed in Tor version 0.4.9.9.
Affected products
- Tor Project Tor before 0.4.9.9
Timeline
- 2026-08-20: disclosed
- 2026-08-20: patched: Fixed in Tor 0.4.9.9