URL Reputation (Rust)

Static URL policy checks for ContextForge and MCP Gateway resource fetches.

Features

• Blocks resource fetches before execution with the resource_pre_fetch hook
• Allows trusted domains or URL regex patterns to bypass later checks
• Blocks configured domains, subdomains, or URL regex patterns
• Blocks non-HTTPS URLs by default
• Optional domain heuristics for high entropy, static IANA TLD validity, and Unicode security
• Case-insensitive domain normalization for allowlist and blocklist entries
• Pure static policy checks; no external reputation provider or threat-intel feed calls

Build

make install

Runtime Requirements

This plugin depends on cpex>=0.1.0,<0.2 and imports hook models from cpex.framework. The compiled Rust extension is mandatory; there is no Python fallback implementation.

Usage

The plugin runs on resource_pre_fetch before a resource URI is fetched.

Typical uses:

• block known bad domains and subdomains
• allow trusted internal URL patterns before enforcing HTTPS
• reject insecure http:// resource fetches
• enable lightweight domain heuristics for suspicious generated or Unicode domains

Configuration

config:
  whitelist_domains:
    - "example.com"
  allowed_patterns:
    - "^https://trusted\\.internal/.*"
  blocked_domains:
    - "malicious.example.com"
  blocked_patterns:
    - "casino"
    - "crypto"
  use_heuristic_check: false
  entropy_threshold: 3.65
  block_non_secure_http: true

Logic Workflow

1. Trim and parse the URL.
2. Extract the host/domain.
3. Detect IPv4 or IPv6 hosts so domain heuristics can be skipped.
4. Allow exact or parent-domain matches in whitelist_domains.
5. Allow matches in allowed_patterns; this also bypasses HTTPS enforcement.
6. Block non-HTTPS schemes when block_non_secure_http=true.
7. Block exact or parent-domain matches in blocked_domains.
8. Block matches in blocked_patterns.
9. If heuristics are enabled for a non-IP host, block high-entropy domains, illegal static TLDs, or unsafe Unicode domains.

Returned Metadata

Allowed URLs return continue_processing=true.

Blocked URLs return continue_processing=false with a PluginViolation using code URL_REPUTATION_BLOCK. Violation details include the URL or domain involved in the decision.

resource_pre_fetch accepts an optional extensions parameter carrying OpenTelemetry trace context. When a trace context is present (via extensions.request.trace_id), the plugin emits operational metrics on result.metadata["url_reputation"] with the following schema:

result.metadata["url_reputation"] = {
    "total_checked": 1,                    # int — always 1; one URL is checked per call
    "total_blocked": 0,                     # int — 0 or 1; this call's outcome, not a running total
    "reputation_categories": [],            # list[str] — category slugs; empty when allowed
}

resource_pre_fetch evaluates exactly one URL per call with no running counter, so — mirroring rate_limiter's per-call allowed/throttled semantics — total_checked and total_blocked describe only the current call's outcome; the gateway aggregates counts across spans/time. When the URL is blocked, reputation_categories contains exactly one slug describing why (e.g. blocked_domain, insecure_scheme, high_entropy_domain, illegal_tld, unicode_spoofing, blocked_pattern, malformed_url, malformed_domain, internal_error).

Gating: Metrics are only emitted when a valid trace_id is present in the trace context (extensions.request.trace_id). No trace context means no result.metadata write at all.

Security Note (S1): The plugin never includes the raw URL or domain in result.metadata. Only counts and category slugs (derived from the plugin's static, hardcoded violation reasons) are reported; the actual URL/domain stays confined to PluginViolation.details on the blocking path, a separate channel unaffected by this metrics addition.

Limitations

• Reputation data is static configuration only; there are no external provider lookups.
• The IANA TLD list is compiled into the plugin and can lag newly delegated TLDs.
• allowed_patterns intentionally runs before HTTPS enforcement, so trusted patterns can allow http:// URLs.
• IP addresses skip domain heuristics.

Testing

make ci