How can security teams detect RainyDay?

What security teams should look for first

Start with behaviour that does not fit the host or the user context: short bursts of scripting, spawned interpreters, odd child processes, and execution paths that normally do not appear on that endpoint. For malware detection, a practical first pass is to combine endpoint telemetry with command-line logging and memory inspection, then compare that activity with the baseline for the same user, host role, and time of day. NHIMG’s Malware Guide is useful here because it helps map those behaviours to common malware tradecraft and evasion patterns.

Outbound traffic deserves the same baseline-driven treatment. Rare destinations, unusual DNS lookups, and proxy sessions that do not match the user’s normal workflow often provide the earliest confirmation that a suspicious process is active, even before a full payload is visible.

Which telemetry is most useful for RainyDay hunts

Focus on three layers together rather than in isolation: endpoint process creation, script execution, and network egress. A single strange command line can be ambiguous, but repeated evidence across those layers makes detection much stronger. Correlating process telemetry with DNS and proxy data helps separate benign admin activity from staged payload delivery, decoded content, or persistence activity.

Memory artefacts matter because some malware leaves little on disk after the initial run. If the process tree is messy, the command line is encoded, or the artefact disappears quickly, review live memory, parent-child relationships, and any signs of injected or unpacked code. Teams that only depend on file scanning tend to miss the execution stage that explains the rest of the chain.

Hunt logic should also look for the mechanics around stealth, not just the final payload. Suspicious decoding, repeated staging steps, and processes that create persistence-like behaviour with minimal user interaction are all useful signals when the campaign is trying to stay quiet. This is where command-line logs, script block telemetry, and network timing patterns usually add more value than signature-only detection.

How to turn findings into a usable detection rule

The best rules are anchored to clusters of weak signals rather than a single indicator. A good candidate pattern is suspicious interpreter activity plus outbound contact to a rare destination plus evidence of staged execution or persistence. That combination is far more actionable than any one field on its own, especially in environments where benign scripting is common.

When you build the rule, tune it to the environment’s normal automation and maintenance tools. The practical challenge is not finding one suspicious command, it is avoiding alerts on legitimate admin scripts while still catching encoded launchers, strange process ancestry, and host behaviour that looks externally directed. MITRE ATT&CK Enterprise Matrix is a useful reference for mapping those observations to adversary techniques and deciding which telemetry gaps still need coverage.

Risk and Threat Considerations

RainyDay-style activity is hard to see when defenders look at each event separately. The main exposure is that a short-lived process, an encoded command, or a rare network destination can look harmless until it is correlated with persistence, decoding, or staged payload behaviour across multiple logs.

Failure mechanism: Attackers exploit the gap between process execution, script activity, and outbound network visibility by spreading the campaign across telemetry sources that are often reviewed by different teams or tools.

Impact: That separation can delay detection long enough for persistence to settle in, payloads to stage, or follow-on activity to blend into normal operational traffic.

Practitioner Guidance

What to prioritise: Put correlation rules in place before you try to tune away noise. A single suspicious process is usually less useful than a linked chain of process, script, and egress events that all point to the same host and timeframe.

What to verify: Confirm that you are actually collecting command-line data, DNS visibility, proxy logs, and enough endpoint telemetry to reconstruct parent-child process trees. If one of those sources is missing, your detection will likely skew toward later-stage or noisier activity.

Decision rule: If you see encoding, decoding, or staged execution plus unusual outbound contact, treat the case as a likely active compromise and move to containment and timeline reconstruction rather than waiting for a single definitive signature.

Practitioner takeaway: RainyDay is best detected as a pattern across telemetry, not as a single IOC, so the highest-value defence is tight correlation between endpoint execution evidence and network behaviour.

Related resources from NHI Mgmt Group

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• Why is the abuse of NHIs a priority for security teams?
• What is the difference between SAST and DAST for security teams?