The National Cyber Security Centre (NCSC) has released an advisory relating to compromise of SolarWinds and FireEye. It can be accessed here.
SolarWinds, specifically their Orion IT system management platform, has been compromised and may be used for onward attacks against systems connected to the product. An attacker has been able to add a malicious, unauthorised modification to SolarWinds Orion products which allows them to send administrator-level commands to any affected installation. Please take time to review the advice provided above and see if you or key elements of your organisation (such as any managed service providers you are connected to) are affected. The NCSC guidance has detailed information that will help you find out if you have an affected system and the immediate steps you should take.