SearchA malicious client could send an HTTP/1 request, wait long enough until
the session releases its worker thread (timeout_linger) and resume traffic
before the session is closed (timeout_idle) sending more than one request
at once to trigger a pipelining operation between requests.
This vulnerability affecting Varnish Cache 9.0.0 emerged from a port of the Varnish Enterprise non-blocking architecture for HTTP/2. New code needed to adapt to a more recent workspace API that formalizes the pipelining operation. In addition to the workspace change on the Varnish Cache side, more differences created merge conflicts, like partial support for trailers in Varnish Enterprise.
The conflict resolution missed one code path configuring pipelining to perform
a complete workspace rollback, losing the guarantee that prefetched data would
fit inside workspace_client during the transition from one request to the
next.
This can result in a workspace overflow, triggering a panic and crashing the Varnish server.
The recommended solution is to upgrade Varnish Cache, and then ensure that Varnish is restarted.