What is CVE-2026-77639?

CVE-2026-77639 is a medium-severity vulnerability in Torproject Tor, classified under Unprotected Alternate Channel. CVSS score: 5.3/10. Published 2026-08-20.

How severe is CVE-2026-77639?

Medium severity. CVSS v3 base score is 5.3 out of 10.

Vulnerability in Torproject Tor

CVE-2026-77639

Tor before 0.4.9.9 was prone to a compression bomb bypass where an attacker could concatenate many gzip or zlib sub-streams, each just under the per-stream detection threshold, to avoid the compression bomb check entirely. This is TROVE-2026-022.

CVSS v3 metric

CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.

Affected products

Torproject Tor

Weakness classification (CWE)

CWE-420 — Unprotected Alternate Channel

References

cve@mitre.org

Frequently asked questions

What is CVE-2026-77639?: CVE-2026-77639 is a medium-severity vulnerability in Torproject Tor, classified under Unprotected Alternate Channel. CVSS score: 5.3/10. Published 2026-08-20.
How severe is CVE-2026-77639?: Medium severity. CVSS v3 base score is 5.3 out of 10.