CVE-2026-77640: CWE-1284 Improper Validation of Specified Quantity in Input in torproject Tor
Description
A vulnerability in Tor before version 0.4.9.9 causes an infinite loop when decompressing a truncated zlib/gzip stream with done=1. The issue arises because the decompression never reaches the expected end state, leading to a retry loop. This flaw was fixed by returning an error to allow clean aborting of the operation.
CVSS v3.1
Score 3.7low
Affected software
torproject
Tor
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Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
Tor versions prior to 0.4.9.9 contain a flaw in the decompression logic where a truncated zlib/gzip stream with done=1 never reaches Z_STREAM_END. As a result, zlib returns Z_BUF_ERROR with no input remaining, which buf_add_compress() misinterprets as a full output buffer and retries indefinitely, causing an infinite loop. The fix involves returning TOR_COMPRESS_ERROR to enable the caller to abort cleanly.
Potential Impact
The vulnerability can cause a denial of service condition by triggering an infinite loop during decompression, leading to resource exhaustion. There is no impact on confidentiality or integrity. The CVSS score is 3.7 (medium severity), indicating low impact beyond availability disruption.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. The issue is fixed in Tor version 0.4.9.9 and later by returning an error to abort the decompression loop cleanly. Users should upgrade to version 0.4.9.9 or later once available.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- mitre
- Date Reserved
- 2026-08-20T21:02:36.115Z
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 6a876c9aacd9273b4920be04
Added to database: 08/20/2026, 21:07:38 UTC
Last enriched: 09/10/2026, 13:33:48 UTC
Last updated: 10/05/2026, 06:48:18 UTC
Views: 75
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