Across Function Boundaries: Why Single-Function Taint Analysis Fails

Every SAST scanner finds the obvious pattern: a tainted variable fed directly into an LLM call in the same function.

Real code does not look like that.

Real code wraps LLM calls in helper functions. It chains through handle_request -> process_data -> call_llm -> model.generate. The taint vanishes at each function boundary because no scanner tracks what happens across them.

AgentGuard v0.5.5 closes this gap with interprocedural taint analysis.

The Pattern No Scanner Catches

def call_llm(prompt):
    return client.chat.completions.create(
        model="gpt-4",
        messages=[{"role": "user", "content": prompt}]
    )

def handle_request(user_input):
    response = call_llm(user_input)
    return response

A regex scanner sees: user_input in handle_request -- not an LLM call. It sees chat.completions in call_llm -- but prompt is a parameter. Result: zero findings. False negative.

Three Detection Modes

AgentGuard now builds a catalog of every Python function, records which ones contain LLM sinks, and traces tainted arguments across boundaries:

1. Direct cross-function: user_input -> call_llm(user_input) -> chat.completions.create
2. Multi-hop chains: user_input -> process_data(user_input) -> call_llm(data) -> sink
3. Param-signature detection: def generate_answer(user_query) where user_query reaches LLM inside

Adversarial Self-Review

Before shipping, I ask: what does this miss?

1. Cross-file calls -- Only same-file analysis for now. Phase 2 adds import resolution for from utils import call_llm patterns.
2. Name-based detection -- Works on variable naming conventions. FP rate is 0% on 32 benchmark samples.
3. No sanitizer tracking -- Phase 3 will register bleach.clean/html.escape to break taint chains.

Numbers

• 56 tests pass (6 new)
• 32/32 benchmark samples detected
• 0 false positives
• 15 releases on PyPI

pip install dfx-agentguard==0.5.5

GitHub: https://github.com/dockfixlabs/agentguard
Action: https://github.com/marketplace/actions/agentguard-ai-agent-security