Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add UnboundCompute/security-agent-skills --skill finding-fail-open-flawsgit clone --depth 1 https://github.com/UnboundCompute/security-agent-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/unboundcompute/security-agent-skills/finding-fail-open-flaws)<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/finding-fail-open-flaws"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/finding-fail-open-flaws.svg" alt="Measured on agentmods" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00107 | $0.01405 |
| Opus 5 | $0.00053 | $0.00702 |
| Sonnet 5 | $0.00021 | $0.00281 |
| Haiku 4.5 | $0.00011 | $0.00140 |
Grade A, and why
finding-fail-open-flaws scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 3d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 121 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Finding fail-open flaws: prove the gate denies by default
A control fails open when its error, empty, or default path grants access instead of refusing it. The check looks present and even passes its happy-path tests, but on the branch that runs when something goes wrong, when a lookup errors, a list is empty, a value is missing, an exception is caught, it lets the request through. These flaws hide in the paths tests rarely cover, and they turn any upstream failure into an authorization bypass.
When to use
- You are reviewing authentication, authorization, or any access gate.
- A check "passes" and you have not confirmed why, or what it does on failure.
- A control depends on an external service, a list, or an input that could be absent.
Scope check
Test gates in code you own or are authorized to test, inducing failures against your own environment. If you can't name the authorization, stop.
The loop
-
Enumerate the gates and their failure branches. For each authentication or authorization check in scope, find not just the allow/deny decision but what happens when the check cannot be completed: the lookup throws, the policy service is unreachable, the input is null or unrecognized, the list is empty. Every gate has a failure branch; find it.
-
Determine the default. For each gate, is the default deny (access requires an explicit, successful allow) or allow (access proceeds unless something explicitly denies)? Default-allow is the root fail-open shape: anything that prevents the deny from firing grants access. State the default for every gate.
-
Test the error path. Force the check to fail (an unreachable dependency, a malformed input, an exception) and observe the outcome. If a caught exception, a timeout, or an error returns or falls through to allow, the gate fails open. A try/catch around a permission check that continues in the catch is the classic instance.
-
Test the empty and wildcard cases. Does an empty allowlist match nothing (safe) or everything (fail-open)? Does a wildcard, a missing filter, or an unset scope collapse to allow-all? Check what the control does with no rules and with a catch-all rule; both are common allow-everything defaults.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 3d ago First seen · 121 lines · 107 tokens per session scan A 1585d68ba73d
finding-fail-open-flaws is a skill published in the GitHub repository UnboundCompute/security-agent-skills (5 stars, last pushed 5d ago), licensed MIT. It adds 107 tokens to every session and 1,405 once invoked, about $0.0005 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-05.
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