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.

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

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

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.