# Pentesting gRPC-Web

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## **Manipulating gRPC-Web Payloads**
gRPC-Web uses Content-Type: `application/grpc-web-text` in requests which is kind of protobuf in base64 encoded form, you can use [gprc-coder](https://github.com/nxenon/grpc-pentest-suite) tool ,and you can also install its [Burp Suite Extension](https://github.com/nxenon/grpc-pentest-suite).  

### **Manual with gGRPC Coder Tool**
1. First decode the payload:
```bash
echo "AAAAABYSC0FtaW4gTmFzaXJpGDY6BVhlbm9u" | python3 grpc-coder.py --decode --type grpc-web-text | protoscope > out.txt
```
2. Edit the content of decoded payload
```
nano out.txt
2: {"Amin Nasiri Xenon GRPC"}
3: 54
7: {"<script>alert(origin)</script>"}
```
3. Encode the new payload
```bash
protoscope -s out.txt | python3 grpc-coder.py --encode --type grpc-web-text
```
4. Use output in Burp interceptor:
```
AAAAADoSFkFtaW4gTmFzaXJpIFhlbm9uIEdSUEMYNjoePHNjcmlwdD5hbGVydChvcmlnaW4pPC9zY3JpcHQ+
```

### **Manual with gRPC-Web Coder Burp Suite Extension**
You can use gRPC-Web Coder Burp Suite Extension in [gRPC-Web Pentest Suite](https://github.com/nxenon/grpc-pentest-suite) which is easier. You can read the installation and usage instruction in its repo.

## **Analysing gRPC-Web Javascript Files**
There is at least one Javascript file in every gRPC-Web application. You can analyse the file to find new messages, endpoints and services. Try using [gRPC-Scan](https://github.com/nxenon/grpc-pentest-suite) tool.
1. Download the Javascript gRPC-Web File
2. Scan it with grpc-scan.py:
```bash
python3 grpc-scan.py --file main.js
```
3. Analyse output and test the new endpoints and new services:
```
Output:
Found Endpoints:
  /grpc.gateway.testing.EchoService/Echo
  /grpc.gateway.testing.EchoService/EchoAbort
  /grpc.gateway.testing.EchoService/NoOp
  /grpc.gateway.testing.EchoService/ServerStreamingEcho
  /grpc.gateway.testing.EchoService/ServerStreamingEchoAbort

Found Messages:

grpc.gateway.testing.EchoRequest:
+------------+--------------------+--------------+
| Field Name |     Field Type     | Field Number |
+============+====================+==============+
| Message    | Proto3StringField  | 1            |
+------------+--------------------+--------------+
| Name       | Proto3StringField  | 2            |
+------------+--------------------+--------------+
| Age        | Proto3IntField     | 3            |
+------------+--------------------+--------------+
| IsAdmin    | Proto3BooleanField | 4            |
+------------+--------------------+--------------+
| Weight     | Proto3FloatField   | 5            |
+------------+--------------------+--------------+
| Test       | Proto3StringField  | 6            |
+------------+--------------------+--------------+
| Test2      | Proto3StringField  | 7            |
+------------+--------------------+--------------+
| Test3      | Proto3StringField  | 16           |
+------------+--------------------+--------------+
| Test4      | Proto3StringField  | 20           |
+------------+--------------------+--------------+

grpc.gateway.testing.EchoResponse:
+--------------+--------------------+--------------+
|  Field Name  |     Field Type     | Field Number |
+==============+====================+==============+
| Message      | Proto3StringField  | 1            |
+--------------+--------------------+--------------+
| Name         | Proto3StringField  | 2            |
+--------------+--------------------+--------------+
| Age          | Proto3IntField     | 3            |
+--------------+--------------------+--------------+
| IsAdmin      | Proto3BooleanField | 4            |
+--------------+--------------------+--------------+
| Weight       | Proto3FloatField   | 5            |
+--------------+--------------------+--------------+
| Test         | Proto3StringField  | 6            |
+--------------+--------------------+--------------+
| Test2        | Proto3StringField  | 7            |
+--------------+--------------------+--------------+
| Test3        | Proto3StringField  | 16           |
+--------------+--------------------+--------------+
| Test4        | Proto3StringField  | 20           |
+--------------+--------------------+--------------+
| MessageCount | Proto3IntField     | 8            |
+--------------+--------------------+--------------+

grpc.gateway.testing.ServerStreamingEchoRequest:
+-----------------+-------------------+--------------+
|   Field Name    |    Field Type     | Field Number |
+=================+===================+==============+
| Message         | Proto3StringField | 1            |
+-----------------+-------------------+--------------+
| MessageCount    | Proto3IntField    | 2            |
+-----------------+-------------------+--------------+
| MessageInterval | Proto3IntField    | 3            |
+-----------------+-------------------+--------------+

grpc.gateway.testing.ServerStreamingEchoResponse:
+------------+-------------------+--------------+
| Field Name |    Field Type     | Field Number |
+============+===================+==============+
| Message    | Proto3StringField | 1            |
+------------+-------------------+--------------+

grpc.gateway.testing.ClientStreamingEchoRequest:
+------------+-------------------+--------------+
| Field Name |    Field Type     | Field Number |
+============+===================+==============+
| Message    | Proto3StringField | 1            |
+------------+-------------------+--------------+

grpc.gateway.testing.ClientStreamingEchoResponse:
+--------------+----------------+--------------+
|  Field Name  |   Field Type   | Field Number |
+==============+================+==============+
| MessageCount | Proto3IntField | 1            |
+--------------+----------------+--------------+
```

## References

* [Hacking into gRPC-Web Article by Amin Nasiri](https://infosecwriteups.com/hacking-into-grpc-web-a54053757a45)
* [gRPC-Web Pentest Suite](https://github.com/nxenon/grpc-pentest-suite)

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<details>

<summary>Support HackTricks</summary>

* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
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* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.

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