Vulnerability in Torproject Tor
CVE-2026-77640
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 3.7 (Low). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L.
EPSS exploit prediction
EPSS: 0.004 (29.4th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 29th percentile — 29.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.004 · change: +0.002
Affected products
Weakness classification (CWE)
References
- gitlab.torproject.org/tpo/core/tor/-/raw/tor-0.4.9.9/ChangeLog (Permissions Required, Release Notes, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-77640?
- CVE-2026-77640 is a low-severity vulnerability in Torproject Tor, classified under Improper Validation of Specified Quantity in Input. CVSS score: 3.7/10. Published 2026-08-20.
- How severe is CVE-2026-77640?
- Low severity. CVSS v3 base score is 3.7 out of 10.