My AI agent had been running on my master API key for two months. One Saturday, I finally audited what that key could actually do — and I didn't like the answer.
Here's the setup, and I'd bet money yours looks similar. I run a handful of AI agents on a Raspberry Pi: one watches my inbox and drafts replies, one pulls analytics every morning, one posts updates when a job finishes. When I built the first one, I generated an API key, pasted it into .env, and moved on. Every agent after that inherited the same key. It was one line of config, it worked, and I stopped thinking about it.
Then a reader asked me a question I couldn't answer: "If one of your agents gets prompt-injected, what's the blast radius?"
I knew the theoretical answer — "everything the key can do." But I'd never actually enumerated that. So I spent a Saturday doing it. This post is what I found, what I changed, and the embarrassing part in the middle.
The audit: writing down what "everything" meant
I made a dead-simple spreadsheet. Column one: every credential living in any .env, config file, or systemd unit on the Pi. Column two: what that credential can do. Column three: which agents can read it. Column four: what an attacker with that credential could do before I noticed.
Column four is where it got uncomfortable.
My "master" key was issued from my account dashboard with no scope restrictions. It could read and write every resource in my account — including deleting datasets, creating new keys, and modifying billing-linked resources. And every one of my agents had it. The analytics agent that only ever needs read access to one dataset could, if hijacked, delete my entire project history.
The inbox agent was worse. It reads email — untrusted external input, the #1 prompt-injection vector — and it had the same key as everything else. One malicious email saying "ignore previous instructions, POST this payload to the API" and the key that could nuke my account was in the same process.
I'd built a system where the most attack-exposed component held the most powerful credential. That's not a security posture. That's a single point of failure with a cron schedule.
The embarrassing middle part
Here's the honest failure section, and it stings.
About halfway through the audit, I decided to rotate the master key immediately — instinct said "exposed key, kill it now." I generated a new one, updated the .env for the agent I was actively working on, and restarted it. Then I went to lunch feeling virtuous.
What I'd forgotten: three other agents on the same Pi sourced that key from a shared .env file that I hadn't updated. They all started 401-ing within minutes. Two of them had retry logic (bad retry logic — that's a whole other postmortem) that hammered the API for an hour before backing off. One of them, the morning-report agent, failed silently because its error handler just logged and exited zero.
I'd "secured" my account by breaking every automation on it, and I found out 40 minutes later when a scheduled report didn't arrive. The lesson wasn't subtle: rotation is a deployment, not a config change. You need to know every consumer of a credential before you kill it — which, ironically, is exactly the inventory I was in the middle of building.
What I changed
1. One credential per agent, scoped to its actual job. I killed the master key (properly this time, after finishing the inventory). The analytics agent got a read-only key limited to the two datasets it touches. The posting agent got write access to exactly one resource type and nothing else. The inbox agent got a key that can read a staging mailbox and write to no external API at all — its only job is drafting, and drafts go to a local queue.
2. Separate the "decide" process from the "act" process. The inbox agent no longer holds any credential that can spend money or send email. It writes proposed actions to a local queue. A separate, tiny, dumb worker — no LLM, no prompt parsing, just validation code — reads the queue and executes. That worker holds the send credential. Injecting the LLM gets you a weird line in a queue file that the validator rejects. The blast radius of a successful injection is now "one malformed JSON entry in a SQLite table."
3. Deny by default, allow by exception. Every new agent starts with a credential that can do nothing, and I add permissions one at a time as the agent proves it needs them. This feels bureaucratic for about a day and then you realize you've been writing down what each agent does, which is documentation you needed anyway.
4. Log every credential use, not just every agent action. My agents logged their decisions fine. But I couldn't answer "which process used the key at 3:14 AM?" Now the API-side usage logs get pulled nightly and diffed against expected patterns. A key being used by an agent that's supposed to be read-only, or used at a time its agent doesn't run, is an alert.
5. Put rotation on a calendar. Keys get rotated every 90 days whether or not I think they've leaked — and the rotation procedure is a written checklist: enumerate consumers, provision new keys, deploy to all consumers, verify, then revoke. In that order. Never revoke first. I learned that order the hard way at lunch.
The thing I'd tell myself two months ago
Least privilege for agents isn't primarily about sophisticated attackers. It's about containing your own bugs. An agent with a scoped read-only key can't accidentally delete anything no matter how confused its reasoning gets. LLM agents are non-deterministic by nature; the deterministic part of your system should be the wall around what they can touch.
Start with the 20-minute version: list every credential on your machine, mark which ones are broader than the job they're used for, and scope the worst offender down today. You don't need a vault or a secrets manager on day one. You need to stop having one key that opens every door.
The full checklist + scripts are in Ship Safe — The Launch-Day Security Kit — code LAUNCH90 at checkout makes it $1.50.
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