Cross-Origin Resource Abuse Detection

Detect and block MCP tool calls that access resources outside their declared origin scope — addressing OWASP MCP Top 10 #4. ShieldAgent evaluates the target of every tool call against per-tool origin allowlists and flags violations before they reach upstream servers.

The Threat

OWASP MCP Top 10 #4 (Cross-Origin Resource Abuse) describes scenarios where an AI agent is manipulated — through prompt injection, a compromised tool response, or a malicious task — into calling an MCP tool with parameters that target a resource it was never intended to reach.

SSRF via MCP tools

critical

An agent is tricked into calling a web-fetch or HTTP tool with an internal IP address (e.g. 192.168.x.x, 169.254.169.254) or cloud metadata endpoint, causing the MCP server to make requests on the attacker's behalf from inside the network perimeter.

Cross-tenant resource access

high

A multi-tenant MCP server is accessed with path traversal or parameter manipulation so one tenant's agent reads another tenant's data.

DNS rebinding

high

A domain the agent is permitted to access resolves to an internal IP after the allowlist check, bypassing origin controls at the DNS layer.

Unauthorized scheme usage

medium

A tool intended for HTTPS is called with a file://, ftp://, or data: URI to read local filesystem content or bypass transport-layer controls.

How ShieldAgent Detects It

For each MCP tool call request that passes through the proxy, ShieldAgent extracts all URL-shaped values from the tool arguments and evaluates them against the tool's configured origin scope. Evaluation runs in the policy pipeline before the request reaches the upstream MCP server.

Tool call arguments

→

Extract URLs / origins

→

Resolve IPs(DNS rebind check)

→

Match allowlist

→

Allowed → pass|Denied → block + event

Abuse patterns detected

ShieldAgent evaluates every URL-shaped argument against the tool's configured origin scope, blocking calls that target private networks, cloud infrastructure endpoints, disallowed URI schemes, or origins outside the configured allowlist. DNS lookups are performed to detect rebinding attacks where an allowlisted hostname resolves to a restricted address.

Configuration

Origin scope is configured per tool in your ShieldAgent policy YAML. The global settings below control detection defaults when no per-tool scope is defined.

Global settings

Setting	Default	Description
Cross-origin detection	true	Enable cross-origin resource abuse detection.
Block private IPs	true	Block tool calls targeting private/loopback IP ranges by default.
Block metadata endpoints	true	Block calls to known cloud metadata endpoints.
Resolve DNS	true	Resolve DNS to detect rebinding. Disable only if your proxy cannot perform DNS lookups.
Action on violation	block	Action when a violation is detected: block or alert.

Per-tool origin scope (policy YAML)

yaml

tools:
  - name: fetch_url
    origin_scope:
      allowed_origins:
        - "https://api.example.com"
        - "https://cdn.example.com"
      allowed_schemes:
        - "https"
      block_private_ips: true
      block_metadata_endpoints: true

Audit Events & API

Every violation is persisted as a cross_origin_abuse audit event including the offending argument, the detected pattern, and the resolved IP (when DNS rebinding is suspected).

json

{
  "id": "aev_...",
  "agentId": "agt_...",
  "tenantId": "ten_...",
  "eventType": "cross_origin_abuse",
  "toolName": "fetch_url",
  "action": "block",
  "riskScore": 88,
  "details": {
    "offendingArgument": "url",
    "offendingValue": "http://192.168.1.1/admin",
    "resolvedIp": "192.168.1.1",
    "allowedOrigins": ["https://api.example.com"]
  },
  "timestamp": "2026-04-25T10:00:00.000Z"
}

API endpoints

GET/tenants/:tenantId/audit-events?eventType=cross_origin_abuse—List cross-origin abuse events. Supports ?agentId=, ?toolName=, ?from=, ?to= filters.

GET/tenants/:tenantId/anomalies?anomalyType=cross_origin_burst—Anomaly events for repeated cross-origin attempts in a short window.

Policy Integration

Use security.crossOrigin.detected as a policy condition to block tool calls that violate origin controls:

json

{
  "name": "Block cross-origin resource abuse",
  "priority": 3,
  "conditions": [
    { "field": "security.crossOrigin.detected", "op": "eq", "value": true }
  ],
  "action": "block",
  "response": {
    "code": 403,
    "message": "Cross-origin resource access blocked by security policy."
  }
}