VSV00005 Varnish HTTP Proxy Protocol V2 Denial of Service


Date: 2020-02-04

An assert can be triggered in Varnish Cache when using Varnish with a TLS termination proxy, and the proxy and Varnish use the PROXY version 2 protocol to communicate connection details. Depending on the type of proxy used and the details it include in the proxy payload, it may be possible for remote clients to cause Varnish to assert and restart, making it a denial of service attack.

The assert will cause Varnish to restart, and the cache will be empty after the restart. This will reduce overall performance due to an increased number of cache misses, and may cause higher load on the backend servers.

There is no potential for remote code execution or data leaks related to this vulnerability.

Mitigation is possible by configuration tuning, or by updating to a fixed version of Varnish Cache.

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, 6.3.0, 6.3.1.

  • 6.0 LTS by Varnish Software up to and including 6.0.5

Fixed in

  • Varnish Cache 6.2.3

  • Varnish Cache 6.3.2

  • 6.0.6 LTS by Varnish Software

  • GitHub Varnish Cache master branch at commit 2d8fc1a784a1e26d78c30174923a2b14ee2ebf62


It is possible to mitigate the problem in several ways.

Switch to proxy protocol version 1

Setups using proxy protocol version 1 are not affected, so if the TLS termination proxy used supports both versions, switching to version 1 will mitigate the problem. This is the recommended mitigation method.

Note that version 1 of the proxy protocol will communicate less details about the connection than what is possible when using version 2. For example the SNI server name used by the connecting client will not be transferred, and can then not be queried using the Proxy VMOD in VCL.

When using the Hitch TLS proxy, version 1 of the proxy protocol can be selected by replacing any write-proxy or write-proxy-v2 options with write-proxy-v1, and restarting Hitch.

Disallow non-matching SNI names in Hitch

When using Hitch as the TLS termination proxy, one can work around the problem by disallowing client connects connecting using a SNI servername that does not match any of the configured certificates. This can be done by adding the sni-nomatch-abort option to the Hitch configuration.

Please note that this mitigation strategy is only effective if none of the configured certificates allow wildcard domain names.

Increase the session workspace

By increasing the session workspace, one can make sure that an attacker can not successfully exhaust the space available, thus not triggering the assert. The size it needs to be set at depends on the TLS proxy in use, and what fields it include in the proxy payload and how a remote client can influence its contents.

By default the session workspace is set to 512 bytes. The session workspace can be changed by setting the workspace_session Varnish parameter, and restarting the Varnish daemon.

When using Hitch as the TLS proxy, setting the session workspace to 34k will mitigate the problem completely. Add “-p workspace_session=34k” to the varnishd command line to set this value.

Note that increasing the session workspace will increase the amount of memory Varnish holds per connection, and you may want to decrease the memory cache size to compensate.

Thankyous and credits

Varnish Software for handling this security incident.