The signature ET USER_AGENTS Node XMLHTTP User-Agent (Emerging Threats sid:2027388) matches on the User-Agent string “node-XMLHttpRequest” only. It does not inspect the destination, payload, or behaviour. Its classification is “unknown” with severity “Minor” and performance impact “Low”. It is a generic fingerprint for the Node.js xmlhttprequest library, not a malware indicator.
The endpoint can be safely whitelisted, or the signature suppressed at the firewall, depending on which approach your IDS prefers.
Worth noting that the same Emerging Threats ruleset includes several other minor User-Agent fingerprint rules from the same 2019 batch. If your firewall is flagging this one but not the others, that is a configuration outcome on your side, not a difference in the traffic itself. Why not flag MS Update?