AI agents use submit_flag to create or update resources in CTFd MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your CTFd MCP Server environment.
This tool creates or modifies competition state (submission records, user progress, scores) in a reversible manner. While submissions cannot be undone within a single attempt, they do not permanently destroy data. This is Write rather than Destructive because the effects are not irreversible—users can resubmit, and the underlying challenge data remains intact.
From the tool's definition submit_flag submits a flag for a challenge ID, which modifies the state of a CTFd competition by recording an attempt and potentially advancing a user's progress in the competition.
Documented attack patterns abuse exactly the kind of access submit_flag gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and CTFd MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for submit_flag:
{
"version": "1",
"default": "deny",
"tools": {
"submit_flag": {
"limits": [
{
"counter": "submit_flag_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}submit_flag stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.
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Submit a flag for a challenge ID. It is categorised as a Write tool in the CTFd MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the CTFd MCP Server MCP server in PolicyLayer and add a rule for submit_flag: 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 CTFd MCP Server. Nothing to install.
submit_flag is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the submit_flag 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 submit_flag. 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.
submit_flag is provided by the CTFd MCP Server MCP server (umbra2728/ctfd-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from CTFd MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
5 CTFd MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.