Executive brief
Tor is an anonymity network used by millions worldwide to protect their privacy. A vulnerability in Tor's circuit handshake allows a remote attacker to trigger a crash by sending a crafted message, even without any prior authentication. This could cause Tor clients to disconnect from the network, disrupting their ability to browse anonymously.
Technical details
A logic error in Tor's circuit handshake implementation allows the CC_RESPONSE extension to be processed even when the CC_REQUEST extension was not sent. This occurs in the congestion_control_parse_ext_response() function, which lacks a check for whether congestion control was actually requested before accepting the response. An unauthenticated remote attacker can send a malformed CC_RESPONSE extension to put the congestion control state machine into an invalid state, leading to a possible crash. The vulnerability was introduced in Tor 0.4.9.3-alpha and is fixed in 0.4.9.12 by validating the cc_requested flag before processing the CC_RESPONSE extension.
Affected products
- The Tor Project Tor 0.4.9.3-alpha to 0.4.9.11
Timeline
- 2026-09-09: disclosed: CVE-2026-87724 published
- 2026-09-08: patched: Fix committed to master branch
- 2026-08-24: other: Patch authored