VSV00013 Varnish HTTP/2 Rapid Reset Attack


Date: 2023-11-13

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 volume 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 is a vulnerability of the protocol itself, HTTP/2 has no provisions for servers to pace the creation of new streams from clients.

The Rapid Reset Attack was disclosed on October 10th 2023.

Versions affected

  • Varnish Cache releases 5.x, 6.x, 7.0.x, 7.1.x, 7.2.x, 7.3.0, 7.4.0, 7.4.1.

  • Varnish Cache 6.0 LTS series up to and including 6.0.11.

  • Varnish Enterprise by Varnish Software 6.0.x up to and including 6.0.11r7.

Versions not affected

  • Varnish Cache 7.3.1 (released 2023-11-13)

  • Varnish Cache 7.4.2 (released 2023-11-13)

  • Varnish Cache 6.0 LTS version 6.0.12 (released 2023-11-13)

  • GitHub Varnish Cache master branch at commit 0c166fb5cc04cb379f7e1b6089beba7e0f75a9cc.

  • Varnish Enterprise by Varnish Software version 6.0.12r1.


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.

After upgrading, two mitigations are enabled by default.

The first one closes HTTP/2 connections when a rapid reset attack is detected. This can be tuned with the following parameters:

  • h2_rapid_reset (duration under which a reset is considered rapid)

  • h2_rapid_reset_limit (budget for rapid resets over a sliding period)

  • h2_rapid_reset_period (the sliding period enforcing the reset limit)

A new vmod_h2 module can tweak these parameters on a per-session basis, allowing for example to relax the limit or period of trusted clients.

The second mitigation adds a circuit breaker before the execution of VCL subroutines to fail the transaction if the client reset its stream. This will also avoid processing VCL legitimate resets or prematurely closed connections, and help free resources sooner.

This is guarded by a feature flag vcl_req_reset that can be disabled for VCL relying on code to always be reached, for example for accounting purposes with the help of a VMOD:

varnishadm param.set feature -vcl_req_reset

There is always the possibility of a VCL transaction failing for other reasons, like a workspace overflow, so there are no guarantees that VCL always completes, even when this feature is disabled.

It is also possible that the second mitigation triggers before any VCL is executed for the HTTP/2 stream.

Monitoring the mitigations

When a VCL transaction is interrupted because a stream was reset, a Timestamp record is emitted with the Reset prefix. The second field after the prefix tells how much time elapsed between the start of the VCL task and the moment when the circuit breaker bypassed VCL execution.

This may not reflect how fast the RST_FRAME was received, as it is subject to time spent executing VCL, which can be significant depending on what the code is doing or how far the execution went, for example triggering a backend fetch.

A high cache hit ratio can also reduce the effect of this attack.

The new time stamp makes it possible to collect access logs dedicated to interrupted request transactions:

varnishncsa -q 'Timestamp:Reset[2] < 1.0' -F '%{VSL:Begin[2]}x ...'

The command above collects all transactions reset in less than a second and output logs with a format starting with the session VXID. It becomes possible to observe resets across HTTP/2 connections and find a pattern of attack if applicable.

When a client VCL task is reset, it increments the MAIN.req_reset counter, which can be used to raise an alarm.

Likewise, when h2_rapid_reset_limit is exceeded, the connection is closed with the reason SC_RAPID_RESET visible in raw Varnish logs, and the MAIN.sc_rapid_reset counter is incremented and can also be used to raise an alarm.

Thankyous and credits

None of the members of the Varnish projects were part of the one month embargo of the CVE, so we learned about this vulnerability when it was disclosed.

Varnish Software initially assessed the vulnerability and started two efforts in parallel. Dag Haavi Finstad implemented the Rapid Reset Attack detection and Dridi Boukelmoune implemented the VCL circuit breaker.

Nils Goroll of UPLEX implemented vmod_h2. He reviewed the two mitigations along with Poul-Henning Kamp.

And Tomas Korbar tested both mitigations as soon as they became public and provided early positive feedback on Github.