Sniffing an Android app to find API URL

Sniffing Android App Traffic to Discover API Endpoints

Learn how to capture and analyze network traffic from Android applications to identify hidden API URLs, understand data exchange, and troubleshoot connectivity issues.

Understanding how an Android application communicates with its backend servers is crucial for various reasons, including security analysis, reverse engineering, debugging, and integration testing. This article will guide you through the process of sniffing an Android app's network traffic to uncover the API URLs it uses, the data it sends, and the responses it receives. We'll cover common tools and techniques, focusing on methods that work for both rooted and non-rooted devices.

The Basics of Network Sniffing

Network sniffing, also known as packet capturing or protocol analysis, involves intercepting and logging data packets that pass over a computer network. For Android apps, this typically means capturing traffic between your device and the internet. The goal is to observe the HTTP/HTTPS requests and responses to identify API endpoints, request parameters, and data formats. While HTTP traffic is relatively easy to intercept, HTTPS traffic, due to its encryption, requires additional steps like setting up a Man-in-the-Middle (MitM) proxy to decrypt the communication.

sequenceDiagram
    participant AndroidApp as Android App
    participant Proxy as Proxy Server (e.g., Burp Suite)
    participant BackendAPI as Backend API

    AndroidApp->>Proxy: Encrypted HTTPS Request
    activate Proxy
    Proxy->>AndroidApp: Proxy's CA Certificate
    AndroidApp->>Proxy: Trust Proxy's CA
    Proxy->>BackendAPI: Decrypted HTTPS Request
    activate BackendAPI
    BackendAPI-->>Proxy: Encrypted HTTPS Response
    deactivate BackendAPI
    Proxy-->>AndroidApp: Re-encrypted HTTPS Response
    deactivate Proxy

Sequence diagram of HTTPS traffic interception using a proxy server.

Setting Up Your Environment for Sniffing

To effectively sniff Android app traffic, you'll need a few key components. A proxy server is essential for intercepting and decrypting HTTPS traffic. Popular choices include Burp Suite, OWASP ZAP, and Fiddler. You'll also need to configure your Android device to route its traffic through this proxy. For HTTPS decryption, installing the proxy's root certificate on your Android device is mandatory. This allows your device to trust the proxy as an intermediary, enabling it to decrypt and re-encrypt traffic.

1. Step 1: Install and Configure a Proxy Server

Download and install a proxy tool like Burp Suite Community Edition on your computer. Configure its listener to accept connections on a specific port (e.g., 8080) and bind it to an IP address accessible from your Android device (e.g., your computer's local IP address on the same network).

2. Step 2: Configure Android Device Proxy Settings

On your Android device, go to Wi-Fi settings, long-press your connected network, select 'Modify network', and then 'Advanced options'. Set 'Proxy' to 'Manual' and enter your computer's IP address and the proxy port (e.g., 8080). Save the settings.

3. Step 3: Install Proxy's CA Certificate on Android

Open a browser on your Android device and navigate to http://burp/cert (for Burp Suite) or the equivalent URL for your proxy. Download the CA certificate. For Android 7.0+, you'll typically need to install it as a user-installed CA certificate. For older Android versions, it might be installed directly as a trusted credential. Note that user-installed CAs are not trusted by apps targeting API level 24 (Android 7.0) and higher by default, requiring additional steps for those apps.

4. Step 4: Start Sniffing Traffic

With the proxy configured and certificate installed, open your proxy tool (e.g., Burp Suite's 'Proxy' tab, 'HTTP history'). Launch the Android app you want to analyze and interact with it. You should see HTTP/HTTPS requests and responses appearing in your proxy's history. Look for requests to external domains, especially those containing 'api' or similar keywords, to identify API endpoints.

Analyzing Captured Traffic with Wireshark

While proxy tools are excellent for HTTP/HTTPS analysis, Wireshark offers a deeper dive into network protocols at various layers. You can capture traffic directly on a rooted Android device or, more commonly, capture traffic from your computer's network interface while the Android device routes through it. Wireshark can help identify non-HTTP/HTTPS traffic, DNS queries, and other low-level network interactions that a proxy might miss. However, for encrypted HTTPS traffic, Wireshark alone won't decrypt the payload without pre-shared keys or other advanced methods, making a proxy the preferred tool for API endpoint discovery.

# Example of capturing traffic on a rooted Android device using tcpdump
# Requires tcpdump binary on the device and root access

# Push tcpdump to device (if not already present)
adb push tcpdump /data/local/tmp/

# Grant execute permissions
adb shell "chmod 777 /data/local/tmp/tcpdump"

# Start capturing traffic to a .pcap file
adb shell "/data/local/tmp/tcpdump -s 0 -w /sdcard/capture.pcap"

# Stop capture (Ctrl+C in adb shell, then pull file)
adb pull /sdcard/capture.pcap ./

# Open capture.pcap with Wireshark on your computer

Capturing network traffic on a rooted Android device using tcpdump.