Denial Of Service in shared VCL deployments Security

Overview

The headerplus.write_req0() function from vmod_headerplus updates the underlying req0, which is normally the original read-only request from which req is derived (readable and writable from VCL).

This is useful in the active VCL, after amending req, to prepare a refined req0 before switching to a different VCL with the return (vcl(<label>)) action. This is for example how the Varnish Controller operates shared VCL deployments.

If the amended req contained too many header fields for req0, this would have resulted in a workspace overflow that would in turn trigger a panic and crash the Varnish Enterprise server.

Impact

This could be used as a Denial of Service attack vector by malicious clients.

Status

Affected software versions

• Varnish Enterprise 6.0 series from version 6.0.9r5 up to and including 6.0.16r12.

Resolved in

• Varnish Enterprise 6.0.16r13

Solution

The recommended solution is to upgrade Varnish to one of the versions where this issue has been resolved, and then ensure that Varnish is restarted.

References

• CVE: pending