If you are helping a small team with no SOC, this is the first thing I suggest.
A canary is a credential, file, URL, or DNS name that has no legitimate use. Anyone touching it is by definition not supposed to be there. Zero false positives, near-zero cost.
Where I plant them:
- A fake AWS key pair in ~/.aws/credentials on jump hosts and in a "backup" S3 bucket. CloudTrail fires the moment it is used.
- A document named "passwords.xlsx" or "board-deck-final.docx" in shared drives, with a beacon.
- A DNS name in /etc/hosts and in internal docs that resolves nowhere useful, with query logging.
- A user account that nobody should ever log in as, alerting on any auth attempt.
- For agent deployments: a fake tool credential in the environment. If an agent gets prompt-injected into exfiltrating env vars, this is what tells you.
Tooling: canarytokens.org for hosted, or roll your own with a Worker and a KV namespace (a URL that logs and alerts on hit is 30 lines).
What they do not do: tell you how the attacker got in. They tell you that someone is in, early, which is the part most teams are missing.
Canary tokens are the highest signal per dollar detection I know
One caution from experience: document the canaries somewhere the responders can find. I watched a team spend two hours on an "incident" that was their own backup job reading the canary file. Label them in the alert text.
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