VSV00004 Workspace information leak

Martin Blix Grydeland martin at varnish-software.com
Mon Oct 21 10:11:22 UTC 2019

VSV00004 Workspace information leak

Date: 2019-10-21

A bug has been discovered in Varnish Cache where we fail to clear a
pointer between the handling of one client requests and the next on the
same connection. This can under specific circumstances lead to information
being leaked from the connection workspace.

The impact is an information leak, limited to the data in the workspace
used for handling the connection. This will typically be data structures
and stale header data from previous requests on the same connection, but
could also be temporary headers set during processing of VCL.

For the leak to occur, Varnish needs to switch to synthetic handling due
to an internal error, in order to generate an error response to the client
request. Note that it will not be a problem when synthetic response
handling is reached explicitly through VCL statements.

The leaked data will be pre populated in the `resp.reason` variable on
entering `vcl_synth`, and will unless overridden show up in the response
field of the HTTP response sent.

The following error situations will cause Varnish to go directly to
synthetic handling for the current client request, and can cause data to
be leaked:

* Reaching max_restarts. There is a maximum limit on how many times the
  VCL is allowed to do a `return(restart)`, and when this limit is reached
  the request is forced to synthetic handling to send an error response.

* Requests initiating backend fetches, there is no grace candidate, and
  the backend responds, but fails to send valid HTTP headers in return. In
  this situation, specifically for the single request initiating the
  backend fetch, the request is forced to synthetic handling to send an
  error response.

* Attempting to switch to another VCL label after a restart. Switching to
  another VCL is only allowed on the first run through `vcl_recv`, and
  attempts to do it later will force the request to synthetic handling to
  send an error response.

To successfully attack a vulnerable Varnish server, the system must either
be vulnerable from its VCL configuration (max restarts or erroneous VCL
switching), or the attacker must be able to cause backend response issues.

Mitigation is possible from VCL or by updating to a fixed version
of Varnish Cache.

Versions affected

* 5.0 and forward
* 6.0 LTS by Varnish Software up to and including 6.0.4

Fixed in

* 6.3.1
* 6.2.2
* 6.0.5 LTS by Varnish Software
* GitHub Varnish Cache master branch at commit

Mitigation from VCL

It is possible to mitigate the problem through VCL, by making sure that
the `resp.reason` field is overridden in the `vcl_synth` function. That
will update the stale pointer value to a known value, eliminating the

The following does this by setting `resp.status` to itself, updating
`resp.reason` to the generic value for that status code in the process::

sub vcl_synth {
   # This line should be added last in vcl_synth,
   # or just before any return statements
   set resp.status = resp.status;

Thankyous and credits

Trygve Tønnesland of VG/Schibsted for discovering the vulnerability, and
responsible disclosure.

Varnish Software for handling this security incident.

Martin Blix Grydeland
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