CVE-2026-77584: CWE-821 Incorrect Synchronization in torproject Tor
Description
CVE-2026-77584 is a high-severity vulnerability in Tor before version 0.4.9.10 involving incorrect synchronization. The flaw allows a malicious client to send a RELAY_COMMAND_BEGIN before a CONFLUX_LINK on the same circuit, causing an exit stream to become orphaned and leading to a use-after-free condition when the circuit is freed. This can impact the integrity and availability of the Tor process.
CVSS v3.1
Score 7.0high
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 0.4.9.10 do not properly reject a CONFLUX_LINK cell arriving on a circuit that already has attached streams. A malicious client can exploit this by sending a RELAY_COMMAND_BEGIN before the CONFLUX_LINK on the same circuit, attaching an exit stream that later becomes orphaned. This results in a dangling circuit back-pointer and a use-after-free (UAF) vulnerability when the circuit is freed, classified as CWE-821 Incorrect Synchronization.
Potential Impact
The vulnerability can lead to a use-after-free condition, which may cause memory corruption, potentially impacting the integrity and availability of the Tor service. The CVSS v3.1 score is 7.0 (high), reflecting network attack vector, high impact on integrity, and low impact on confidentiality and availability. There are no known exploits in the wild at this time.
Mitigation Recommendations
A fix is available in Tor version 0.4.9.10 and later. Users should upgrade to version 0.4.9.10 or newer to remediate this vulnerability. No vendor advisory content was provided, so patch status is inferred from the version range indicating fixed in 0.4.9.10.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- mitre
- Date Reserved
- 2026-08-20T20:38:41.768Z
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 6a876922acd9273b491d0679
Added to database: 08/20/2026, 20:52:50 UTC
Last enriched: 09/10/2026, 23:02:03 UTC
Last updated: 10/05/2026, 06:48:18 UTC
Views: 70
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