"The wireless network appears to have been compromised..."

I, too, started getting this warning after upgrading to El Capitan. I spoke with AppleCare today who assured me that this message is not generated by Apple and is not in their library of warnings at all. After the tech (through screen sharing) examined all of my network settings, nothing was deemed out of line. Apple also ran a malware check and found nothing wrong. The suggestion was made to call my wireless provider. ATT switched me to a different wireless channel. Apple also told me to have ATT change my security setting to WPA2 which they did. So far so good ...

Will--ATT said they switched me to "channel 9"--not sure what that means but they kept saying it is the "good" channel and would not be affected by neighborhood use ... And they added the WPA2 security. Not really sure which is helping more, but I have not had the issue since.

AND ... my speed has remained consistently high. Before it would dive way down as verified by frequent speed tests. The only solution was a reboot. Now it stays up all the time. I might also add that before this (on Yosemite) I got consistent "resolving host" issues where the only solution was to either reboot or turn off/on the wireless. I believe the "compromised" message with El Capitan was the same thing as "resolving host" was with Yosemite. So happy to see neither of those anymore. My router is 6 years old. ATT said we would try the new settings and if that continued to be problematic, the next step would be to change the router.

The one thing I did make note of when speaking to Apple was that they had not heard of this specific message before, and it doesn't seem to be prevalent on their help forums. I also noticed that I never had this problem when traveling and being on various hotel or airport Wi-fi, which made me less suspicious of the OS and more of my home service. I appreciate that ATT was eager to resolve the problem and that they have. So far anyway. 🙂 (And it was all done remotely while I was on the phone with them.)

I am having this problem as now. Only started after upgrading to OS X El Capitan.

i have the same problem and have WPA2 and continue to have the problem. Its not all the time, so its hard to troubleshoot. I either turn off my Wifi or restart. But i believe its and El Capitan issue as well.

I think they may have been mistaken. That warning has been popping up from time to time since at least OS X Tiger.

It's possible that it's a symptom of your WiFi access point packing up and the signal getting corrupted. If you have a router with iffy capacitors you'll get the message more and more frequently over a few months as they fail completely.

I keep a spare router in the cupboard for such occasions. It's very difficult for an amateur to diagnose problems other than by substitution.

C.