CVE-2026-77639: CWE-420 Unprotected Alternate Channel in torproject Tor
Description
A vulnerability in Tor versions before 0.4.9.9 allows an attacker to bypass the compression bomb detection by concatenating multiple gzip or zlib sub-streams, each below the detection threshold. This flaw can lead to denial of service by causing resource exhaustion. The issue is tracked as CVE-2026-77639 and is classified under CWE-420 (Unprotected Alternate Channel).
CVSS v3.1
Score 5.3medium
Affected software
torproject
Tor
pkg:github/torproject/torRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
Tor versions prior to 0.4.9.9 are vulnerable to a compression bomb bypass where an attacker can concatenate many gzip or zlib sub-streams, each just under the per-stream detection threshold, thereby avoiding the compression bomb check entirely. This vulnerability is identified as CVE-2026-77639 and corresponds to CWE-420. The flaw can cause denial of service by exhausting system resources due to the decompression of crafted data streams.
Potential Impact
The vulnerability impacts availability by enabling denial of service attacks through resource exhaustion when processing specially crafted compressed data streams. There is no impact on confidentiality or integrity according to the CVSS vector. No known exploits are reported in the wild.
Mitigation Recommendations
A fix is available in Tor version 0.4.9.9 and later. Users should upgrade to version 0.4.9.9 or newer to mitigate this vulnerability. Patch status is confirmed by the affectedVersions data indicating versions before 0.4.9.9 are vulnerable.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- mitre
- Date Reserved
- 2026-08-20T20:57:49.855Z
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 6a876c9aacd9273b4920be02
Added to database: 08/20/2026, 21:07:38 UTC
Last enriched: 09/11/2026, 04:17:59 UTC
Last updated: 10/05/2026, 06:48:18 UTC
Views: 67
Community Reviews
0 reviewsCrowdsource mitigation strategies, share intel context, and vote on the most helpful responses. Sign in to add your voice and help keep defenders ahead.
Want to contribute mitigation steps or threat intel context? Sign in or create an account to join the community discussion.
Actions
Updates to AI analysis require Pro Console access. Upgrade inside Console → Billing.
External Links
Need more coverage?
Upgrade to Pro Console for AI refresh and higher limits.
For incident response and remediation, OffSeq services can help resolve threats faster.
Latest Threats
Check if your credentials are on the dark web
Instant breach scanning across billions of leaked records. Free tier available.