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 hunting-mass-assignment-and-property-authzgit 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/hunting-mass-assignment-and-property-authz)<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/hunting-mass-assignment-and-property-authz"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/hunting-mass-assignment-and-property-authz/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/hunting-mass-assignment-and-property-authz"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/hunting-mass-assignment-and-property-authz.svg" alt="Reviewed on agentmods" width="80" 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 | $0.00156 | $0.01998 |
| Opus 5.5 | $0.00062 | $0.00799 |
| Sonnet 5.5 | $0.00031 | $0.00400 |
| Haiku 4.5 | $0.00016 | $0.00200 |
Grade A, and why
hunting-mass-assignment-and-property-authz 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 1mo 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 — 146 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Hunting mass assignment: which fields, not which object
Broken object-level authorization asks whether the caller may reach this object. Mass
assignment asks the next question: given an object the caller may write, which of its
fields may they set? The bug is a handler that binds a request payload onto a record
and trusts the client to send only the fields it should, so an attacker adds
"role":"admin", "owner_id": <someone else>, "price": 0, or "verified": true to a
payload the endpoint was never meant to accept, and the framework writes it. You find it
by listing what the client can put into the payload, following it to the record write,
and asking which of those fields the server, not the client, is supposed to own.
When to use
- You are reviewing a create or update handler that maps request fields onto a persisted object.
- The framework auto-binds or hydrates a payload onto a model, struct, or record.
- Some properties of the object are meant to be server-controlled: privilege, ownership, tenancy, money, state, or verification flags.
Scope check
Test property writes only in systems you own or are authorized to test, with accounts that let you observe a privilege, ownership, or state change take effect. If you can't name the authorization, stop.
The loop
-
List the writable fields the client can reach. For each create and update handler, determine how the payload becomes a record: an explicit field-by-field assignment, an allowlisted binder, or a whole-payload hydration. Whole-payload binding means every column is a candidate; a binder means whatever the allowlist admits. Enumerate the fields the client can actually set, not the ones the form shows.
-
Follow the payload to the record write. Trace the bound fields from the request to the persistence call, the object mutation and save. That write is the sink. Note whether the binding is an allowlist (only named fields) or a blocklist (everything except named fields); a blocklist is a standing bet that no sensitive field was ever added later.
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.
- 1mo ago First seen · 146 lines · 156 tokens per session scan A 475f5a95f8e7
hunting-mass-assignment-and-property-authz is a skill published in the GitHub repository UnboundCompute/security-agent-skills (5 stars, last pushed 1mo ago), licensed MIT. It adds 156 tokens to every session and 1,998 once invoked, about $0.0006 per session on Opus 5.5. 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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