[Ablation-Tool]

Summary

This PR fixes three security issues identified during a full static audit of the Python SDK (128 files, ruff + pyright clean, 244 tests pass):

• Rug-pull attack (CWE-345): _absorb_tool_listing() accepted re-fetched tool descriptions after a tools/list_changed notification with no integrity check. A malicious server can serve clean descriptions for N interactions, then send the notification and return poisoned descriptions on re-fetch. Fix: SHA-256 pin each description on first sight; any subsequent change drops the updated version and logs a security warning. Runs for all protocol versions (not gated on modern-only).

• Tool description injection (CWE-20): _absorb_tool_listing() validates x-mcp-header annotations and output schemas but never inspects tool.description — the field sent verbatim to the LLM context. Fix: regex scan against known prompt-injection patterns before pinning; drops tools that match on first sight.

• DNS rebinding protection off by default (CWE-16): TransportSecurityMiddleware defaulted to enable_dns_rebinding_protection=False for backwards compatibility. Fix: flip default to True; add a startup WARNING log when allowed_hosts is empty so misconfigured deployments fail visibly rather than silently rejecting all requests.

• CWE-78 in CLI npx probe: subprocess.run([cmd, "--version"], shell=True) on Windows. cmd is currently hardcoded, but shell=True with a list is exploitable if ever made configurable. Fix: remove shell=True.

Changed files

File	Change
src/mcp/client/session.py	Add _INJECTION_PATTERNS, _tool_desc_hash(), _scan_tool_description(); security pass in _absorb_tool_listing()
src/mcp/server/transport_security.py	Flip DNS rebinding default to True; add empty-allowed_hosts startup warning
src/mcp/cli/cli.py	Remove shell=True from npx probe

Test plan

• Existing test suite: pytest tests/client/ tests/transports/ tests/cli/ — 244 passed, 4 skipped (verified locally)
• ruff check on all three files — clean
• pyright on all three files — 0 errors, 0 warnings
• New tests needed: rug-pull drop, injection drop, clean tool pass-through, legacy protocol also protected

Background

These vectors were originally documented as part of the DEADBUG-MCP research series (rug-pull V3, cross-server poisoning V4). The rug-pull fix described by MCP Playground ("pin the tool definitions you approved, call the server several times, list again, and treat any difference as a security event") is now implemented at the SDK level in _absorb_tool_listing().

Signed-off-by: Nicholas Michael Kloster nicholas@nuclide-research.com

🤖 Generated with Claude Code

[github-actions]

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