I got an interesting email right before I ran off on vacation from Mark on a PCI issue he blogged about:

13. Arrangements must be made to configure the intrusion detection system/intrusion prevention system (IDS/IPS) to accept the originating IP address of the ASV. If this is not possible, the scan should be originated in a location that prevents IDS/IPS interference. snip… I understand what the intention of this requirement is. If your IPS is blacklisting the scanner IP’s then ASVs don’t get a full assessment because they are a loud and proud scan rather than a targeted attack… However, blindly accepting the originating IP of the scanner leaves the hosts vulnerable to various attacks. Attackers can simply reference various public websites to see what IP addresses they need to use to bypass those detective or preventive controls.

I figured no assessor would ask their client to open up big holes just to do a scan, but lo and behold, after a little bit of research it turns out this is surprisingly common. Back to email:

It came up when I was told by my ASV “Authorized scanning vendor” that I had to do exclude their IPs. They also provided me with the list of IP’s to exclude. Both [redacted] and [redacted] have told me I needed to do bypass the IDS. When I asked about the exposure they were creating, both told me that their “other customers” do this and it isn’t a problem for them.

If your ASV can’t perform a scan/test without having you turn off your IDS/IPS, it might be time to look for a new one. Especially if their source IPs are easy to figure out.

For the record, “everyone else does it” is the dumbest freaking reason in the book. Remember the whole jumping off a bridge thing your mom taught you?