mixed-language-monorepos

name	mixed-language-monorepos
description	This skill should be used when the user is auditing a "polyglot monorepo", "multi-language codebase", "microservices with different languages", "Go + Python + TypeScript", or any codebase with services written in different programming languages. Provides strategies for cross-service security analysis and unified threat modeling. Use when this capability is needed.
metadata	{"author":"allsmog"}

Mixed-Language Monorepo Security Analysis

Purpose

Provide comprehensive guidance for security analysis of polyglot codebases where multiple services are written in different programming languages. These are increasingly common in modern architectures (Go APIs + Python ML + TypeScript frontends).

When to Use

Activate this skill when:

• Monorepo contains services in 2+ different languages
• Microservices architecture with polyglot stack
• User mentions "Go + Python", "TypeScript + Java", etc.
• Cross-service API security analysis needed
• gRPC/protobuf, OpenAPI, or GraphQL schemas present

Core Challenges

Why Mixed-Language is Different

Single-Language	Mixed-Language
One compression strategy	Per-service strategy needed
Unified sink database	Multiple sink databases
Same vulnerability patterns	Language-specific + cross-service patterns
Simple trust boundaries	Complex inter-service boundaries
One toolchain	Multiple toolchains (Semgrep rules vary)

Common Polyglot Patterns

Pattern	Example	Security Concern
API Gateway + Services	Go gateway → Python/Java services	Gateway bypass, auth propagation
BFF + Backend	TS frontend → Go API → Python ML	Input validation gaps
Event-Driven	Services communicate via Kafka/RabbitMQ	Message injection, schema drift
Sidecar/Service Mesh	Envoy/Istio + any language	mTLS config, RBAC policies

Detection

Step 1: Identify All Languages

# Count files by language
echo "=== Language Distribution ==="
echo "Go:         $(find . -name '*.go' ! -path '*/vendor/*' ! -name '*_test.go' 2>/dev/null | wc -l)"
echo "Python:     $(find . -name '*.py' ! -path '*/.venv/*' ! -path '*/venv/*' ! -name 'test_*.py' 2>/dev/null | wc -l)"
echo "TypeScript: $(find . \( -name '*.ts' -o -name '*.tsx' \) ! -path '*/node_modules/*' ! -name '*.test.*' ! -name '*.spec.*' 2>/dev/null | wc -l)"
echo "JavaScript: $(find . \( -name '*.js' -o -name '*.jsx' \) ! -path '*/node_modules/*' ! -name '*.test.*' ! -name '*.spec.*' 2>/dev/null | wc -l)"
echo "Java:       $(find . -name '*.java' ! -path '*/test/*' ! -name '*Test.java' 2>/dev/null | wc -l)"
echo "Rust:       $(find . -name '*.rs' ! -path '*/target/*' ! -name '*_test.rs' 2>/dev/null | wc -l)"
echo "PHP:        $(find . -name '*.php' ! -path '*/vendor/*' ! -name '*Test.php' 2>/dev/null | wc -l)"
echo "C#:         $(find . -name '*.cs' ! -path '*/bin/*' ! -path '*/obj/*' ! -name '*Test*.cs' 2>/dev/null | wc -l)"
echo "Ruby:       $(find . -name '*.rb' ! -path '*/vendor/*' ! -name '*_spec.rb' ! -name '*_test.rb' 2>/dev/null | wc -l)"
echo "Solidity:   $(find . -name '*.sol' ! -path '*/node_modules/*' ! -name '*Test.sol' 2>/dev/null | wc -l)"

Step 2: Map Services to Languages

# Find service boundaries (look for entrypoints)
echo "=== Service Discovery ==="

# Go services (main.go or cmd/)
find . -name 'main.go' -o -type d -name 'cmd' 2>/dev/null | head -20

# Python services (main.py, app.py, manage.py, __main__.py)
find . \( -name 'main.py' -o -name 'app.py' -o -name 'manage.py' -o -name '__main__.py' \) 2>/dev/null | head -20

# TypeScript/JS services (package.json with "start" script)
find . -name 'package.json' ! -path '*/node_modules/*' -exec grep -l '"start"' {} \; 2>/dev/null | head -20

# Java services (Application.java, pom.xml, build.gradle)
find . \( -name '*Application.java' -o -name 'pom.xml' -o -name 'build.gradle' \) ! -path '*/test/*' 2>/dev/null | head -20

# Rust services (Cargo.toml with [[bin]])
find . -name 'Cargo.toml' -exec grep -l '\[\[bin\]\]' {} \; 2>/dev/null | head -20

# Docker/container indicators
find . -name 'Dockerfile*' -o -name 'docker-compose*.yml' -o -name 'docker-compose*.yaml' 2>/dev/null | head -20

Step 3: Detect Inter-Service Communication

# Protocol definitions
echo "=== Communication Protocols ==="

# gRPC/Protobuf
find . -name '*.proto' 2>/dev/null | head -10
echo "Proto files: $(find . -name '*.proto' 2>/dev/null | wc -l)"

# OpenAPI/Swagger
find . \( -name 'openapi*.yaml' -o -name 'openapi*.yml' -o -name 'openapi*.json' -o -name 'swagger*.yaml' -o -name 'swagger*.yml' -o -name 'swagger*.json' \) 2>/dev/null | head -10

# GraphQL
find . \( -name '*.graphql' -o -name '*.gql' -o -name 'schema.graphql' \) 2>/dev/null | head -10

# Thrift
find . -name '*.thrift' 2>/dev/null | head -5

# Message queues (Kafka, RabbitMQ, etc.)
grep -rniE "(kafka|rabbitmq|amqp|pulsar|nats)" --include="*.yaml" --include="*.yml" --include="*.json" --include="*.toml" . 2>/dev/null | head -10

Service Mapping Output

After detection, produce a service map:

## Service Architecture Map

| Service | Path | Language | Entry Point | Communication |
|---------|------|----------|-------------|---------------|
| api-gateway | services/gateway/ | Go | cmd/gateway/main.go | gRPC (internal), REST (external) |
| auth-service | services/auth/ | Go | cmd/auth/main.go | gRPC |
| ml-pipeline | services/ml/ | Python | src/main.py | REST, Kafka consumer |
| web-frontend | apps/web/ | TypeScript | src/index.tsx | REST client |
| data-processor | services/processor/ | Java | src/.../Application.java | Kafka producer/consumer |

### Inter-Service Flows

```mermaid
graph LR
    A[web-frontend<br/>TypeScript] -->|REST| B[api-gateway<br/>Go]
    B -->|gRPC| C[auth-service<br/>Go]
    B -->|gRPC| D[ml-pipeline<br/>Python]
    D -->|Kafka| E[data-processor<br/>Java]
    E -->|Kafka| D

## Per-Service Scoping Strategy

### Multi-Service Scope Command

For polyglot monorepos, scope each service with its language-appropriate strategy:

```bash
# Step 1: Scope each service with language-appropriate compression
# Go service (97% compression)
npx repomix services/gateway --compress --style markdown \
  --include "**/interfaces/**/*.go,**/handler/**/*.go,**/svc/**/*.go,**/api/**/*.go" \
  --ignore "*_test.go,**/testing/**,**/*.pb.go" \
  --output .claude/scope-gateway.md

# Python service (85-90% compression)
npx repomix services/ml --compress --style markdown \
  --include "**/api/**/*.py,**/routes/**/*.py,**/models/**/*.py,**/schemas/**/*.py,**/services/**/*.py" \
  --ignore "**/*_test.py,**/tests/**,**/__pycache__/**" \
  --output .claude/scope-ml.md

# TypeScript frontend (80% compression)
npx repomix apps/web --compress --style markdown \
  --ignore "**/node_modules/**,**/*.test.*,**/*.spec.*,**/dist/**" \
  --output .claude/scope-web.md

# Java service (80-85% compression)
npx repomix services/processor --compress --style markdown \
  --include "**/*Controller*.java,**/*Service*.java,**/*Repository*.java,**/model/**/*.java" \
  --ignore "**/test/**,**/*Test.java,**/target/**" \
  --output .claude/scope-processor.md

# Step 2: Scope shared protocol definitions (always include)
npx repomix proto/ --style markdown --output .claude/scope-proto.md

Unified Architecture Scope

Create a lightweight cross-service architecture view:

# Architecture-only scope (all languages, minimal detail)
npx repomix . --compress --style markdown \
  --include "**/proto/**/*.proto,**/openapi*.yaml,**/swagger*.yaml,**/*.graphql,**/docker-compose*.yml,**/Dockerfile*,**/**/main.go,**/**/main.py,**/index.ts,**/*Application.java" \
  --ignore "**/node_modules/**,**/vendor/**,**/target/**,**/.venv/**,**/dist/**,**/build/**" \
  --output .claude/scope-architecture-overview.md

Cross-Service Security Analysis

Trust Boundaries

In polyglot monorepos, trust boundaries exist at:

Boundary	Location	Security Concern
External → Gateway	API Gateway ingress	Input validation, rate limiting, auth
Gateway → Services	gRPC/REST internal calls	Auth token propagation, authz checks
Service → Service	Inter-service communication	Mutual TLS, service identity
Service → Data Store	Database connections	Connection string secrets, query safety
Service → Queue	Message broker	Message validation, poison message handling
Service → External API	Third-party integrations	SSRF, credential exposure

Cross-Service Vulnerability Patterns

1. Authentication Token Propagation

Risk: Gateway validates JWT but downstream services trust blindly

// Gateway (Go) - validates JWT
func authMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        token := r.Header.Get("Authorization")
        claims, err := validateJWT(token)  // Validated here
        // But how is identity passed to downstream services?
        ctx := context.WithValue(r.Context(), "user", claims.Subject)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

# ML Service (Python) - trusts X-User-ID header
@app.route('/predict')
def predict():
    user_id = request.headers.get('X-User-ID')  # VULNERABLE: Trusts header blindly
    # What if attacker bypasses gateway and calls directly?

Detection: Search for header-based identity passing:

# Go: Look for context value passing
grep -rniE "context\.WithValue.*user|X-User-ID|X-Authenticated" --include="*.go"

# Python: Look for header trust
grep -rniE "request\.headers\.get.*user|X-User-ID|X-Authenticated" --include="*.py"

# Java: Look for header trust
grep -rniE "getHeader.*[Uu]ser|X-User-ID|X-Authenticated" --include="*.java"

2. Input Validation Gaps at Boundaries

Risk: Each service assumes the previous one validated

// Frontend (TypeScript) - sends user input
const response = await fetch('/api/search', {
    method: 'POST',
    body: JSON.stringify({ query: userInput })  // Raw user input
});

// Gateway (Go) - passes through to ML service
func searchHandler(w http.ResponseWriter, r *http.Request) {
    var req SearchRequest
    json.NewDecoder(r.Body).Decode(&req)
    // No validation - assumes ML service will handle it
    mlResponse, _ := mlClient.Search(ctx, req.Query)
}

# ML Service (Python) - uses in SQL
def search(query: str):
    # VULNERABLE: No validation, trusts upstream
    results = db.execute(f"SELECT * FROM items WHERE name LIKE '%{query}%'")

Detection: Trace data flow across service boundaries:

# Find gRPC/REST client calls
grep -rniE "\.Call\(|\.Invoke\(|fetch\(|axios\.|requests\." --include="*.go" --include="*.py" --include="*.ts" --include="*.java"

# Find SQL execution points
grep -rniE "\.execute\(|\.query\(|\.Exec\(|\.Query\(" --include="*.go" --include="*.py" --include="*.java"

3. Schema Mismatch Attacks

Risk: Proto/OpenAPI schema drift between services

// Proto v1 (auth-service expects)
message UserRequest {
    string user_id = 1;
    bool is_admin = 2;  // Added in v2, not validated
}

# ML Service (Python) - older proto version
# Doesn't know about is_admin field, passes through unvalidated

Detection: Compare schema versions:

# Find all proto files and check for version comments
find . -name "*.proto" -exec grep -l "is_admin\|role\|permission" {} \;

# Check for schema version mismatches
diff <(grep "message\|field" services/auth/proto/*.proto) <(grep "message\|field" services/ml/proto/*.proto)

4. Message Queue Injection

Risk: Malicious messages from compromised service

// Data Processor (Java) - consumes Kafka messages
@KafkaListener(topics = "user-events")
public void processEvent(String message) {
    UserEvent event = objectMapper.readValue(message, UserEvent.class);
    // VULNERABLE: Deserialization without validation
    processUser(event.getUserId(), event.getAction());
}

Detection: Find message consumers without validation:

# Kafka consumers
grep -rniE "@KafkaListener|consume|subscribe" --include="*.java" --include="*.py" --include="*.go"

# Deserialization points
grep -rniE "objectMapper\.read|json\.loads|json\.Unmarshal|JSON\.parse" --include="*.java" --include="*.py" --include="*.go" --include="*.ts"

5. Service-to-Service SSRF

Risk: Internal service URLs constructed from user input

// Gateway (Go) - constructs internal URL
func proxyToService(w http.ResponseWriter, r *http.Request) {
    service := r.URL.Query().Get("service")  // User-controlled
    // VULNERABLE: Attacker can specify internal service URLs
    resp, _ := http.Get(fmt.Sprintf("http://%s.internal:8080/data", service))
}

Detection:

# Find URL construction with variables
grep -rniE "http\.Get\(.*\+|requests\.get\(.*\+|fetch\(.*\+|Sprintf.*http" --include="*.go" --include="*.py" --include="*.ts" --include="*.java"

Unified Threat Model

For polyglot monorepos, the threat model must include:

1. Per-Language Threats

Apply language-specific dangerous functions to each service:

• Go services: Use Go sinks (exec.Command, sql.Query, etc.)
• Python services: Use Python sinks (eval, subprocess, SQLAlchemy raw)
• Java services: Use Java sinks (Runtime.exec, PreparedStatement issues)
• TypeScript: Use JS/TS sinks (eval, child_process, DOM manipulation)

2. Cross-Service Threats

Threat	Description	STRIDE
Auth Bypass via Direct Service Call	Attacker bypasses gateway, calls internal service directly	Spoofing, Elevation
Schema Exploitation	Exploit proto/OpenAPI schema mismatches	Tampering
Message Injection	Inject malicious messages into queue	Tampering, Spoofing
Internal SSRF	Pivot through services to reach internal endpoints	Information Disclosure
Credential Leakage	Secrets exposed in logs across services	Information Disclosure
Service Identity Spoofing	Impersonate internal service without mTLS	Spoofing

3. Data Flow Diagram (Cross-Service)

graph TB
    subgraph External
        User[User Browser]
        Attacker[Attacker]
    end

    subgraph DMZ
        Gateway[API Gateway<br/>Go]
    end

    subgraph Internal
        Auth[Auth Service<br/>Go]
        ML[ML Service<br/>Python]
        Processor[Data Processor<br/>Java]
        DB[(PostgreSQL)]
        Queue[Kafka]
    end

    User -->|HTTPS| Gateway
    Attacker -.->|Bypass?| Auth
    Gateway -->|gRPC+JWT| Auth
    Gateway -->|REST+Header| ML
    ML -->|Kafka| Queue
    Queue -->|Consume| Processor
    Processor -->|SQL| DB
    ML -->|SQL| DB

    classDef external fill:#f96,stroke:#333
    classDef dmz fill:#ff9,stroke:#333
    classDef internal fill:#9f9,stroke:#333
    class User,Attacker external
    class Gateway dmz
    class Auth,ML,Processor,DB,Queue internal

Audit Workflow for Polyglot Repos

Phase 0: Service Discovery

/vuln-scout:scope . --list --polyglot

Output:

## Polyglot Monorepo Analysis

| Service | Path | Language | Tokens | Risk | Communication |
|---------|------|----------|--------|------|---------------|
| api-gateway | services/gateway/ | Go | 85k | HIGH | gRPC, REST |
| auth-service | services/auth/ | Go | 45k | CRITICAL | gRPC |
| ml-pipeline | services/ml/ | Python | 120k | HIGH | REST, Kafka |
| web-frontend | apps/web/ | TypeScript | 200k | MEDIUM | REST client |

### Protocol Definitions
- Proto files: 15 (services/proto/)
- OpenAPI specs: 2 (docs/)

### Recommended Audit Order
1. auth-service (CRITICAL - handles authentication)
2. api-gateway (HIGH - external entry point)
3. ml-pipeline (HIGH - database access, Kafka producer)
4. web-frontend (MEDIUM - client-side only)

Phase 1: Per-Service Scoping

# Scope each service with language-appropriate strategy
/vuln-scout:scope services/auth --language go --name auth
/vuln-scout:scope services/gateway --language go --name gateway
/vuln-scout:scope services/ml --language python --name ml
/vuln-scout:scope apps/web --language typescript --name web

# Scope protocol definitions
/vuln-scout:scope services/proto --name protocols

Phase 2: Cross-Service Threat Model

/vuln-scout:threats --polyglot --save .claude/threat-model-polyglot.md

Phase 3: Per-Service Audit

# Audit each service with its language-specific sinks
/vuln-scout:full-audit --scope auth --language go
/vuln-scout:full-audit --scope ml --language python

Phase 4: Cross-Service Verification

# Verify cross-service data flows
/vuln-scout:trace gateway:handleRequest → ml:predict
/vuln-scout:trace ml:publishEvent → processor:consumeEvent

Integration Points

With Existing Commands

Command	Polyglot Enhancement
/scope --list	Add --polyglot to detect multi-language services
/scope [path]	Add --language to force language strategy
/full-audit	Auto-detect polyglot, audit services in priority order
/threats	Add --polyglot for cross-service threat model
/sinks	Use correct language sinks per service
/trace	Support cross-service traces (service:function notation)

With Dangerous Functions

For each service, use the appropriate language sink database:

• dangerous-functions/references/go-sinks.md
• dangerous-functions/references/python-sinks.md
• dangerous-functions/references/javascript-sinks.md
• dangerous-functions/references/java-sinks.md
• etc.

Notes

• Service boundaries are the highest-risk areas in polyglot systems
• Always audit protocol definitions (proto, OpenAPI) as contracts
• mTLS between services is critical - verify it's enforced
• Message queues need schema validation on both producer and consumer
• Consider using service mesh (Istio/Linkerd) findings if available

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Source: allsmog/vuln-scout — distributed by TomeVault.