CVE-2026-77640: Torproject Tor

Medium severity, CVSS 5.3. EPSS: 0.4% chance of exploitation in the next 30 days.

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.

Affected products

• Torproject Tor: before 0.4.9.9 (fixed in 0.4.9.9)

Published 2026-08-20. Last modified 2026-09-08.