Direct k-anonymity lookup: pass the first 5 hex chars of a SHA-1 password hash, get back all SHA-1 suffixes with their pwned counts. Use this if you're hashing client-side and only want to send the prefix.
Part of the Hibp server.
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AI agents call check_password_prefix to retrieve information from Hibp without modifying any data. This is common in research, monitoring, and reporting workflows where the agent needs context before taking action. Because read operations don't change state, they are generally safe to allow without restrictions -- but you may still want rate limits to control API costs.
Even though check_password_prefix only reads data, uncontrolled read access can leak sensitive information or rack up API costs. An agent caught in a retry loop could make thousands of calls per minute. A rate limit gives you a safety net without blocking legitimate use.
Read-only tools are safe to allow by default. No rate limit needed unless you want to control costs.
{
"version": "1",
"default": "deny",
"tools": {
"check_password_prefix": {}
}
}See the full Hibp policy for all 26 tools.
These attack patterns abuse exactly the kind of access check_password_prefix gives an agent. Each links to the full case and the policy that stops it:
Other read tools across the catalogue. The same approach applies to each: allow, with a rate cap to control cost.
Direct k-anonymity lookup: pass the first 5 hex chars of a SHA-1 password hash, get back all SHA-1 suffixes with their pwned counts. Use this if you're hashing client-side and only want to send the prefix.. It is categorised as a Read tool in the Hibp MCP Server, which means it retrieves data without modifying state.
Register the Hibp MCP server in PolicyLayer and add a rule for check_password_prefix: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Hibp. Nothing to install.
check_password_prefix is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the check_password_prefix rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for check_password_prefix. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
check_password_prefix is provided by the Hibp MCP server (https://gateway.pipeworx.io/hibp/mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 26 Hibp tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
4,600+ MCP servers and 31,000+ tools scanned and risk-classified.