CVE-2026-77640: Low severity Tor Project Tor vulnerability
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 ZSTREAMEND, causing zlib to return ZBUFERROR with no input remaining, which bufaddcompress() mistook for a full output buffer and retried forever. Fixed by returning TORCOMPRESSERROR in that case so the caller can abort cleanly. This is TROVE-2026-021.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
torto a version that resolves this vulnerability.Fixed in 0.4.9.9Patch TROVE-2026-021
Event History
Frequently Asked Questions
What does an attacker need to do to trigger the issue?
They need to cause Tor to decompress a truncated zlib or gzip stream in the affected code path, with done=1. Exploitation requires high attack complexity according to the supplied vector.
What is the impact if the issue is triggered?
Tor can enter an infinite retry loop while decompressing the malformed stream, resulting in an availability impact. The supplied vector indicates no confidentiality or integrity impact.
Which versions are affected and what fixes it?
Tor versions before 0.4.9.9 are affected. Version 0.4.9.9 changes the handling of an input-exhausted Z_BUF_ERROR so decompression returns an error instead of retrying indefinitely.