HTTP/2 Made You Reset Attack Security

Published August 13, 2025.

Overview

A denial of service attack can be performed on Varnish Cache and Varnish Enterprise 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 upstream 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.

This issue was found and reported by Gal Bar Nahum, Anat Bremler-Barr, and Yaniv Harel of Tel Aviv University.

Impact

The primary risk of this vulnerability is enabling Denial of Service attacks.

The vulnerability has been given a severity rating of low.

Versions affected

Varnish Cache releases up to and including 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 7.7.2 (released 2025-08-13)
Varnish Cache 7.6.4 (released 2025-08-13)
Varnish Cache 6.0 LTS version 6.0.15 (released 2025-08-13)
Varnish Enterprise 6.0.14r5 (released 2025-06-19)

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.

Timeline

2025-05-28

Varnish Software is invited to join a multi vendor coordinated effort to patch an HTTP/2 security vulnerability.

2025-06-19

A patched release of Varnish Enterprise closing the vulnerability is released.

2025-08-13

Coordinated public disclosure of the vulnerability.
Varnish Cache packages released to the official Varnish Cache package repositories.
Public announcement.