cpex-encoded-exfil-detection 0.2.1

High-performance encoded exfiltration detection for MCP Gateway

Encoded Exfiltration Detection (Rust)

High-performance encoded exfiltration detection for ContextForge and MCP Gateway.

Features

• Detects suspicious encoded payloads in prompt args, tool outputs, and resource content
• Scans common exfil encodings:
  • base64
  • base64url
  • hex
  • percent-encoding
  • escaped hex
• Scores candidates using decoded length, entropy, printable ratio, sensitive keywords, and egress hints
• Optional redaction instead of hard blocking
• Recursive scanning of nested dicts, lists, and JSON-like string payloads
• Allowlist regex support for known-safe encoded strings
• Decode-depth and recursion-depth guardrails

Build

make install

Usage

The plugin scans these hooks:

• prompt_pre_fetch
• tool_post_invoke
• resource_post_fetch

Typical uses:

• block suspicious encoded payloads before they leave the gateway
• redact encoded secrets or staged exfil fragments from tool results
• surface findings metadata for review and tuning

Detection Model

Each candidate encoded segment is decoded and scored. The detector looks for combinations of:

• sufficient decoded length
• suspicious entropy
• printable decoded content
• sensitive markers such as password, secret, token, authorization, or private key
• egress hints such as curl, wget, webhook, upload, socket, or pastebin

The plugin can also inspect JSON strings recursively so encoded content nested inside serialized blobs is still visible to the detector.

Configuration

Important settings include:

• enabled: per-encoding enable flags
• min_encoded_length
• min_decoded_length
• min_entropy
• min_printable_ratio
• min_suspicion_score
• max_scan_string_length
• max_findings_per_value
• redact
• redaction_text
• block_on_detection
• min_findings_to_block
• allowlist_patterns
• extra_sensitive_keywords
• extra_egress_hints
• max_decode_depth
• max_recursion_depth
• parse_json_strings

Returned Metadata

When detections occur, the plugin can emit:

• encoded_exfil_count
• encoded_exfil_findings
• encoded_exfil_redacted
• implementation

Blocking responses use the ENCODED_EXFIL_DETECTED violation code.

Security Notes

• Guardrails reject Rust-incompatible allowlist regexes at engine initialization time (during plugin construction). Features such as lookaround and backreferences are not supported.
• Scan and recursion caps exist to keep detection bounded on large payloads.
• Detailed findings can be reduced or sanitized before metadata emission depending on configuration.

Testing

make ci