VSV00007 Varnish HTTP/2 Request Smuggling Attack


Date: 2021-07-13

A request smuggling attack can be performed on Varnish Cache and Varnish Cache Plus servers that have the HTTP/2 protocol enabled. The smuggled requests do not go through normal VCL processing, and any authorization steps implemented in VCL would be bypassed.

The responses to the smuggled requests can under some circumstances also be obtained by the attacker. Also, it may be possible for an attacker to use this for cache poisoning, where the response to a smuggled request is inserted as the cached content.

Identifying smuggled requests

Smuggled requests will not show in any logs generated by Varnish, but would show in the backend logs. It may be possible to identify the smuggled requests in the backend logs by missing Varnish inserted artifacts, like the X-Varnish header. Though a determined attacker may spoof these artifacts in the smuggled requests.

Versions affected

  • Varnish Cache releases 6.0.0, 6.0.1, 6.0.2, 6.0.3, 6.0.4, 6.0.5, 6.1.0, 6.1.1, 6.2.0, 6.2.1, 6.2.2, 6.2.3, 6.3.0, 6.3.1, 6.3.2, 6.4.0, 6.5.0, 6.5.1, 6.6.0.

  • Varnish Cache releases 5.x.x. Notice that the experimental HTTP/2 support in these releases are known to have several issues, and enabling HTTP/2 is not recommended.

  • Varnish Cache 6.0 LTS by Varnish Software up to and including 6.0.7

Versions not affected

  • All versions of Varnish Cache prior to version 5.0.0

Fixed in

  • Varnish Cache 6.6.1

  • Varnish Cache 6.5.2

  • Varnish Cache 6.0 LTS version 6.0.8

  • GitHub Varnish Cache master branch at commit 450961a019d1c1955ca1851d51940ff2c87bdc98


Mitigation is possible by either disabling the HTTP/2 protocol, or preventing backend connection reuse.

Turning off support for HTTP/2:

The problem only affects servers that have HTTP/2 support enabled. This support can be turned off at runtime. To disable HTTP/2 on a server do:

sudo varnishadm param.set feature -http2

To verify that HTTP/2 is disabled on a server, execute this command and make sure the current value does not list http2:

sudo varnishadm param.show feature

When using Hitch (or any other TLS termination proxy) in front of Varnish to handle TLS termination, you should also unlist the h2 token as a possible protocol in the ALPN advertisement sent to connecting clients.

To unlist h2 as a supported protocol in Hitch, remove or comment out the line stating alpn-protos = “h2, http/1.1” in your Hitch configuration file. Then restart the Hitch service (reload is not sufficient).

Preventing connection reuse

On compliant backends it is possible to prevent the execution of smuggled requests by disabling connection reuse of backend requests. Note that for this workaround to be effective, it relies on the backend to refuse any additional requests after seeing a Connection: close header.

To disable backend connection reuse, add a Connection: close header on the outgoing backend requests:

sub vcl_backend_fetch {
      set bereq.http.Connection = "close";


Martin Blix Grydeland at Varnish Software for identifying and handling the issue.