# Reachability Drift Detection - Architecture

**Module:** Scanner
**Version:** 1.0
**Status:** Implemented (Sprint 3600.2-3600.3)
**Last Updated:** 2025-12-22

---

## 1. Overview

Reachability Drift Detection tracks function-level reachability changes between scans to identify when code modifications create new paths to vulnerable sinks or mitigate existing risks. This enables security teams to:

- **Detect regressions** when previously unreachable vulnerabilities become exploitable
- **Validate fixes** by confirming vulnerable code paths are removed
- **Prioritize triage** based on actual exploitability rather than theoretical risk
- **Automate VEX** by generating evidence-backed justifications

---

## 2. Key Concepts

### 2.1 Call Graph

A directed graph representing function/method call relationships in source code:

- **Nodes**: Functions, methods, lambdas with metadata (file, line, visibility)
- **Edges**: Call relationships with call kind (direct, virtual, delegate, reflection, dynamic)
- **Entrypoints**: Public-facing functions (HTTP handlers, CLI commands, message consumers)
- **Sinks**: Security-sensitive APIs (command execution, SQL, file I/O, deserialization)

### 2.2 Reachability Analysis

Multi-source BFS traversal from entrypoints to determine which sinks are exploitable:

```
Entrypoints (HTTP handlers, CLI)
        │
        ▼ BFS traversal
    [Application Code]
        │
        ▼
    Sinks (exec, query, writeFile)
        │
        ▼
    Reachable = TRUE if path exists
```

### 2.3 Drift Detection

Compares reachability between two scans (base vs head):

| Transition | Direction | Risk Impact |
|------------|-----------|-------------|
| Unreachable → Reachable | `became_reachable` | **Increased** - New exploit path |
| Reachable → Unreachable | `became_unreachable` | **Decreased** - Mitigation applied |

### 2.4 Cause Attribution

Explains *why* drift occurred by correlating with code changes:

| Cause Kind | Description | Example |
|------------|-------------|---------|
| `guard_removed` | Conditional check removed | `if (!authorized)` deleted |
| `guard_added` | New conditional blocks path | Added null check |
| `new_public_route` | New entrypoint created | Added `/api/admin` endpoint |
| `visibility_escalated` | Internal → Public | Method made public |
| `dependency_upgraded` | Library update changed behavior | lodash 4.x → 5.x |
| `symbol_removed` | Function deleted | Removed vulnerable helper |
| `unknown` | Cannot determine | Multiple simultaneous changes |

---

## 3. Data Flow

```mermaid
flowchart TD
    subgraph Scan["Scan Execution"]
        A[Source Code] --> B[Call Graph Extractor]
        B --> C[CallGraphSnapshot]
    end

    subgraph Analysis["Drift Analysis"]
        C --> D[Reachability Analyzer]
        D --> E[ReachabilityResult]

        F[Base Scan Graph] --> G[Drift Detector]
        E --> G
        H[Code Changes] --> G
        G --> I[ReachabilityDriftResult]
    end

    subgraph Output["Output"]
        I --> J[Path Compressor]
        J --> K[Compressed Paths]
        I --> L[Cause Explainer]
        L --> M[Drift Causes]

        K --> N[Storage/API]
        M --> N
    end

    subgraph Integration["Integration"]
        N --> O[Policy Gates]
        N --> P[VEX Emission]
        N --> Q[Web UI]
    end
```

---

## 4. Component Architecture

### 4.1 Call Graph Extractors

Per-language AST analysis producing `CallGraphSnapshot`:

| Language | Extractor | Technology | Status |
|----------|-----------|------------|--------|
| .NET | `DotNetCallGraphExtractor` | Roslyn semantic model | **Done** |
| Java | `JavaCallGraphExtractor` | ASM bytecode analysis | **Done** |
| Go | `GoCallGraphExtractor` | golang.org/x/tools SSA | **Done** |
| Python | `PythonCallGraphExtractor` | Python AST | **Done** |
| Node.js | `NodeCallGraphExtractor` | Babel (planned) | Skeleton |
| PHP | `PhpCallGraphExtractor` | php-parser | **Done** |
| Ruby | `RubyCallGraphExtractor` | parser gem | **Done** |

**Location:** `src/Scanner/__Libraries/StellaOps.Scanner.CallGraph/Extraction/`

### 4.2 Reachability Analyzer

Multi-source BFS from entrypoints to sinks:

```csharp
public sealed class ReachabilityAnalyzer
{
    public ReachabilityResult Analyze(CallGraphSnapshot graph);
}

public record ReachabilityResult
{
    ImmutableHashSet<string> ReachableNodes { get; }
    ImmutableArray<string> ReachableSinks { get; }
    ImmutableDictionary<string, ImmutableArray<string>> ShortestPaths { get; }
}
```

**Location:** `src/Scanner/__Libraries/StellaOps.Scanner.CallGraph/Analysis/`

### 4.3 Drift Detector

Compares base and head graphs:

```csharp
public sealed class ReachabilityDriftDetector
{
    public ReachabilityDriftResult Detect(
        CallGraphSnapshot baseGraph,
        CallGraphSnapshot headGraph,
        IReadOnlyList<CodeChangeFact> codeChanges);
}
```

**Location:** `src/Scanner/__Libraries/StellaOps.Scanner.ReachabilityDrift/Services/`

### 4.4 Path Compressor

Reduces full paths to key nodes for storage/display:

```
Full Path (20 nodes):
  entrypoint → A → B → C → ... → X → Y → sink

Compressed Path:
  entrypoint → [changed: B] → [changed: X] → sink
  (intermediateCount: 17)
```

**Location:** `src/Scanner/__Libraries/StellaOps.Scanner.ReachabilityDrift/Services/PathCompressor.cs`

### 4.5 Cause Explainer

Correlates drift with code changes:

```csharp
public sealed class DriftCauseExplainer
{
    public DriftCause Explain(...);
    public DriftCause ExplainUnreachable(...);
}
```

**Location:** `src/Scanner/__Libraries/StellaOps.Scanner.ReachabilityDrift/Services/DriftCauseExplainer.cs`

---

## 5. Language Support Matrix

| Feature | .NET | Java | Go | Python | Node.js | PHP | Ruby |
|---------|------|------|-------|--------|---------|-----|------|
| Function extraction | Yes | Yes | Yes | Yes | Partial | Yes | Yes |
| Call edge extraction | Yes | Yes | Yes | Yes | Partial | Yes | Yes |
| HTTP entrypoints | ASP.NET | Spring | net/http | Flask/Django | Express* | Laravel | Rails |
| gRPC entrypoints | Yes | Yes | Yes | Yes | No | No | No |
| CLI entrypoints | Yes | Yes | Yes | Yes | Partial | Yes | Yes |
| Sink detection | Yes | Yes | Yes | Yes | Partial | Yes | Yes |

*Requires Sprint 3600.4 completion

---

## 6. Storage Schema

### 6.1 PostgreSQL Tables

**call_graph_snapshots:**
```sql
CREATE TABLE call_graph_snapshots (
    id UUID PRIMARY KEY,
    tenant_id UUID NOT NULL,
    scan_id TEXT NOT NULL,
    language TEXT NOT NULL,
    graph_digest TEXT NOT NULL,
    node_count INT NOT NULL,
    edge_count INT NOT NULL,
    entrypoint_count INT NOT NULL,
    sink_count INT NOT NULL,
    extracted_at TIMESTAMPTZ NOT NULL,
    snapshot_json JSONB NOT NULL
);
```

**reachability_drift_results:**
```sql
CREATE TABLE reachability_drift_results (
    id UUID PRIMARY KEY,
    tenant_id UUID NOT NULL,
    base_scan_id TEXT NOT NULL,
    head_scan_id TEXT NOT NULL,
    language TEXT NOT NULL,
    newly_reachable_count INT NOT NULL,
    newly_unreachable_count INT NOT NULL,
    detected_at TIMESTAMPTZ NOT NULL,
    result_digest TEXT NOT NULL
);
```

**drifted_sinks:**
```sql
CREATE TABLE drifted_sinks (
    id UUID PRIMARY KEY,
    tenant_id UUID NOT NULL,
    drift_result_id UUID NOT NULL REFERENCES reachability_drift_results(id),
    sink_node_id TEXT NOT NULL,
    symbol TEXT NOT NULL,
    sink_category TEXT NOT NULL,
    direction TEXT NOT NULL,
    cause_kind TEXT NOT NULL,
    cause_description TEXT NOT NULL,
    compressed_path JSONB NOT NULL,
    associated_vulns JSONB
);
```

**code_changes:**
```sql
CREATE TABLE code_changes (
    id UUID PRIMARY KEY,
    tenant_id UUID NOT NULL,
    scan_id TEXT NOT NULL,
    base_scan_id TEXT NOT NULL,
    language TEXT NOT NULL,
    file TEXT NOT NULL,
    symbol TEXT NOT NULL,
    change_kind TEXT NOT NULL,
    details JSONB,
    detected_at TIMESTAMPTZ NOT NULL
);
```

### 6.2 Valkey Caching

```
stella:callgraph:{scan_id}:{lang}:{digest}     → Compressed CallGraphSnapshot
stella:callgraph:{scan_id}:{lang}:reachable    → Set of reachable sink IDs
stella:callgraph:{scan_id}:{lang}:paths:{sink} → Shortest path to sink
```

TTL: Configurable (default 24h)
Circuit breaker: 5 failures → 30s timeout

---

## 7. API Endpoints

| Method | Path | Description |
|--------|------|-------------|
| GET | `/scans/{scanId}/drift` | Get drift results for a scan |
| GET | `/drift/{driftId}/sinks` | List drifted sinks (paginated) |
| POST | `/scans/{scanId}/compute-reachability` | Trigger reachability computation |
| GET | `/scans/{scanId}/reachability/components` | List components with reachability |
| GET | `/scans/{scanId}/reachability/findings` | Get reachable vulnerable sinks |
| GET | `/scans/{scanId}/reachability/explain` | Explain why a sink is reachable |

See: `docs/api/scanner-drift-api.md`

---

## 8. Integration Points

### 8.1 Policy Module

Drift results feed into policy gates for CI/CD blocking:

```yaml
smart_diff:
  gates:
    - condition: "delta_reachable > 0 AND is_kev = true"
      action: block
```

### 8.2 VEX Emission

Automatic VEX candidate generation on drift:

| Drift Direction | VEX Status | Justification |
|-----------------|------------|---------------|
| became_unreachable | `not_affected` | `vulnerable_code_not_in_execute_path` |
| became_reachable | — | Requires manual review |

### 8.3 Attestation

DSSE-signed drift attestations:

```json
{
  "_type": "https://in-toto.io/Statement/v1",
  "predicateType": "stellaops.dev/predicates/reachability-drift@v1",
  "predicate": {
    "baseScanId": "abc123",
    "headScanId": "def456",
    "newlyReachable": [...],
    "newlyUnreachable": [...],
    "resultDigest": "sha256:..."
  }
}
```

---

## 9. Performance Characteristics

| Metric | Target | Notes |
|--------|--------|-------|
| Graph extraction (100K LOC) | < 60s | Per language |
| Reachability analysis | < 5s | BFS traversal |
| Drift detection | < 10s | Graph comparison |
| Memory usage | < 2GB | Large projects |
| Cache hit improvement | 10x | Valkey lookup vs recompute |

---

## 10. References

- **Implementation Sprints:**
  - `docs/implplan/SPRINT_3600_0002_0001_call_graph_infrastructure.md`
  - `docs/implplan/SPRINT_3600_0003_0001_drift_detection_engine.md`
- **API Reference:** `docs/api/scanner-drift-api.md`
- **Operations Guide:** `docs/operations/reachability-drift-guide.md`
- **Original Advisory:** `docs/product-advisories/archived/17-Dec-2025 - Reachability Drift Detection.md`
- **Source Code:** `src/Scanner/__Libraries/StellaOps.Scanner.ReachabilityDrift/`