Fiddler as a Man-in-the-Middle Proxy

Why not to use fiddler classic to capture https?

Fiddler Classic uses HTTP/1.1 and TLS 1.2 by default. However, the current HTTPS requests use HTTP/2 and TLS 1.3, which is why Fiddler Classic is unable to decrypt the traffic.

To fix this issue, you can try using Fiddler Everywhere, which supports HTTP/2 and TLS 1.3. Fiddler Classic, on the other hand, has limitations with newer protocols.

Fiddler as a Man-in-the-Middle Proxy

How Fiddler Captures Traffic

Fiddler works by intercepting network traffic and forwarding it to the intended server. It modifies system proxy settings to route traffic through itself.

1. Capturing Local Traffic

• Fiddler sets a proxy on localhost:8866.
• Modifies system settings to route traffic through this proxy.
• Logs HTTP and HTTPS traffic (with SSL decryption enabled).

2. Capturing Remote Machine Traffic

For remote traffic interception:

1. Manually configure the client’s proxy to point to Fiddler’s machine.
2. For HTTPS traffic, install Fiddler's root certificate on the client machine.
3. The remote machine’s requests now pass through Fiddler before reaching the server.

Setting Proxy for Windows Services

Why Use This Method?

By default, Windows services (especially those running as LocalSystem or NetworkService) do not use user-level proxy settings. Instead, they rely on system-wide proxy settings stored in the registry under HKEY_USERS\S-1-5-18.

To force a specific proxy for Windows services, we manually edit the registry.

Step-by-Step Guide to Set Proxy in Registry for Windows Services

Step 1: Open Registry Editor

1. Press Win + R, type regedit, and hit Enter.
2. Navigate to the following registry path:
   1. HKEY_USERS\S-1-5-18\Software\Microsoft\Windows\CurrentVersion\Internet Settings

• S-1-5-18 refers to the SYSTEM account, under which many Windows services run.
• If your service runs under a different account, find its corresponding SID under HKEY_USERS.

Step 2: Modify Proxy Settings in the Registry

Inside Internet Settings, locate or create the following values:

| Value Name | Type | Value | |---------------|--------|-------| | ProxyEnable | DWORD | 1 (Enables proxy) | | ProxyServer | String | http=proxyserver:8866;https=proxyserver:8866 |

Step 3: Restart Your Service

After making changes:

1. Restart your service using:

net stop "YourServiceName" && net start "YourServiceName"

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