Update: As it turns out, it was nothing nefarious. An engineer simply uploaded an incorrect ephemeris and as soon as it was noticed, they started rectifying the situation by uploading the correct one. The system had a problem for approximately 11 hours.
Update: In looking at the chart on the IAC website (link below), all satellites are now green, so it appears that the crisis has been averted.
Update: Unanswered is why? Was it a hack? Or was it a massive failure in command and control in the uploading of the ephemerides? Why does the IAC’s status page even have a category for Illegal Ephemeris? One problem GLONASS has had is a lack of ground stations for uploading ephemerides and detecting problems. This is being rectified, most recently with an agreement with Nicaragua to place a ground station in that country, but absent widely, globally-dispersed ground stations such as GPS has, the GLONASS satts have to wait as much as 12 hours to orbit back over the existing ground stations in Russia.
Original post: We have had a report from JAVAD GNSS that the Russian GLONASS constellation is not working. This will have a major impact on the ease with which GNSS receivers can be used, and will cause RTK initialization times to increase. The good news is that the GPS constellation continues to function just as it has for decades.
Neil Vancans of Altus Positioning confirmed the report and said, "Our experience at the moment indicates GLONASS is not functioning correctly and we are advising users to use GPS only." He added, "This issue affects all receiver types."
From the Russian Federal Space Agency 24 hour GLONASS status monitoring:
http://glonass-iac.ru/en/GLONASS/dayMonitoringNew.html it can be seen that most of the satellites have an "illegal ephemeris," indicating a severe problem.