VSV00017 Varnish HTTP/2 Made You Reset Attack

CVE-2025-8671 VU#767506

Date: 2025-08-13

A denial of service attack can be performed on Varnish Cache servers that have the HTTP/2 protocol turned on. An attacker can create a large number of streams and immediately reset them without ever reaching the maximum number of concurrent streams allowed for the session, causing the Varnish server to consume unnecessary resources processing requests for which the response will not be delivered.

This attack is a variant of the HTTP/2 Rapid Reset Attack, which was handled as VSV00013. The countermeasure put in place for VSV00013 was to implement a rate limit on the number of stream resets allowed before the session would be closed by the Varnish server. The new variant of the attack bypasses this rate limit by executing a benign protocol violation, causing the stream to be reset by the server rather than by the client. This would effectively bypass the rate limit, reopening the attack vector.

The Made You Reset Attack was disclosed on August 13 2025.

Versions affected

  • Varnish Cache releases 5.x, 6.x, 7.0.x, 7.1.x, 7.2.x, 7.3.x, 7.4.x, 7.5.x, 7.6.0, 7.6.1, 7.6.2, 7.6.3, 7.7.0, 7.7.1

  • Varnish Cache 6.0 LTS series up to and including 6.0.14.

  • Varnish Enterprise by Varnish Software 6.0.x up to and including 6.0.14r4.

Versions not affected

  • Varnish Cache 7.6.4 (released 2025-08-13)

  • Varnish Cache 7.7.2 (released 2025-08-13)

  • Varnish Cache 6.0 LTS version 6.0.15 (released 2025-08-13)

  • GitHub Varnish Cache master branch at commit 5202a6e329651cd0121e9eac78e60b66351a50bf.

  • Varnish Enterprise by Varnish Software version 6.0.14r5.

Mitigation

If upgrading Varnish is not possible, it is possible to mitigate the problem by simply disabling HTTP/2 support:

varnishadm param.set feature -http2

You must also remove h2 from the list of protocols if your TLS terminator is advertising it with ALPN.

The attack is also partly mitigated by the second line of defense that was put in place when the similar VSV00013 Rapid Reset Attack was patched. A circuit breaker before the execution of VCL subroutines was put in place to fail the transaction if the client reset its stream. This mechanism is controlled by a feature flag vcl_req_reset that defaults to on. With this mechanism in place, Varnish servers are much more resilient to this type of attack.

Monitoring the mitigations

The monitoring strategies described in VSV00013 are also applicable to this issue.

Thankyous and credits

Thanks to the reporters, Gal Bar Nahum, Anat Bremler-Barr, and Yaniv Harel of Tel Aviv University.