CVE-2026-77640: CWE-1284 Improper Validation of Specified Quantity in Input in torproject Tor
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 decompression never completes because the truncated stream does not reach the expected end state, leading to repeated retries. This issue results in a denial of service due to resource exhaustion during decompression.
AI Analysis
Technical Summary
Tor versions prior to 0.4.9.9 contain a flaw in the decompression logic where a truncated zlib/gzip stream with done=1 causes zlib to return Z_BUF_ERROR without input remaining. The function buf_add_compress() incorrectly interprets this as a full output buffer and retries indefinitely, causing an infinite loop. The issue is fixed by returning TOR_COMPRESS_ERROR to allow the caller to abort cleanly.
Potential Impact
This vulnerability can cause a denial of service by triggering an infinite loop during decompression, leading to resource exhaustion. There is no impact on confidentiality or integrity, and no known exploits in the wild.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. The issue is fixed by returning an error to abort decompression cleanly, so upgrading to version 0.4.9.9 or later is expected to resolve the problem once an official fix is released.
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 decompression never completes because the truncated stream does not reach the expected end state, leading to repeated retries. This issue results in a denial of service due to resource exhaustion during decompression.
CVSS v3.1
Score 3.7low
Affected software
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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 causes zlib to return Z_BUF_ERROR without input remaining. The function buf_add_compress() incorrectly interprets this as a full output buffer and retries indefinitely, causing an infinite loop. The issue is fixed by returning TOR_COMPRESS_ERROR to allow the caller to abort cleanly.
Potential Impact
This vulnerability can cause a denial of service by triggering an infinite loop during decompression, leading to resource exhaustion. There is no impact on confidentiality or integrity, and no known exploits in the wild.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. The issue is fixed by returning an error to abort decompression cleanly, so upgrading to version 0.4.9.9 or later is expected to resolve the problem once an official fix is released.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- mitre
- Date Reserved
- 2026-08-20T21:02:36.115Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a876c9aacd9273b4920be04
Added to database: 08/20/2026, 21:07:38 UTC
Last enriched: 08/20/2026, 21:22:18 UTC
Last updated: 08/20/2026, 22:10:08 UTC
Views: 5
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