AI agents invoke xss_check to trigger actions in CyberMCP. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.
Given the server's explicit purpose of security vulnerability testing and sibling tools that actively probe for vulnerabilities (path_traversal_check, auth_bypass_check, jwt_vulnerability_check), xss_check almost certainly executes Cross-Site Scripting payloads against target endpoints. This constitutes active execution of potentially malicious scripts against external systems.
From the tool's definition Tool name 'xss_check' on a server described as 'designed for testing backend APIs for security vulnerabilities like authentication bypass, injection attacks, and data leakage'; sibling tools include injection and traversal checks.
Documented attack patterns abuse exactly the kind of access xss_check gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and CyberMCP, and nothing reaches the server without passing your rules. This is the rule we recommend for xss_check:
{
"version": "1",
"default": "deny",
"tools": {
"xss_check": {
"limits": [
{
"counter": "xss_check_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}xss_check stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
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xss_check. It is categorised as a Execute tool in the CyberMCP MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Cyber MCP server in PolicyLayer and add a rule for xss_check: 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 CyberMCP. Nothing to install.
xss_check is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the xss_check 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 xss_check. 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.
xss_check is provided by the Cyber MCP server (ricauts/cybermcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from CyberMCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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14 CyberMCP tools catalogued and risk-classified — across an index of 43,000+ MCP servers.