# CVE-2026-87724: Tor CC_RESPONSE extension denial of service in circuit handshake

Source: https://junglewise.ai/threats/cve-2026-87724-tor-cc-response-extension-denial-of-service-in-circuit-handshake  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-09. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-87724 · Severity: medium · CVSS 6.5 · Published 2026-09-09

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.

## Quick answers

**Is CVE-2026-87724 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-87724 being exploited in the wild.

**How severe is CVE-2026-87724?** Medium severity, with a CVSS score of 6.5, published 9 September 2026.

**Which products and versions are affected?** The Tor Project Tor 0.4.9.3-alpha to 0.4.9.11.

**Is there a fix for CVE-2026-87724?** A patch was published on 2026-09-08: Fix committed to master branch Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-87724 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-87724?** Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## 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

## References

- [https://gitlab.com/torproject/tor/-/commit/10d4b8ffefa7c00aab2b631ed7e7f15e42cd012d](https://gitlab.com/torproject/tor/-/commit/10d4b8ffefa7c00aab2b631ed7e7f15e42cd012d)
- [https://gitlab.torproject.org/tpo/core/tor/-/raw/tor-0.4.9.12/ChangeLog](https://gitlab.torproject.org/tpo/core/tor/-/raw/tor-0.4.9.12/ChangeLog)