Why Fiddler Classic Changes Windows’ System Proxy

Fiddler Classic registers itself as Windows’ WinINET system proxy when it starts capturing. A localhost endpoint appearing while the debugger runs can therefore be an intentional part of capture.

Fiddler normally unregisters itself before closing. If the process terminates abnormally, inspect the effective proxy setting and whether the referenced local listener still exists. A browser pointing at a stopped debugger cannot use that route.

Stop capture through Fiddler’s normal controls when ending a session. If a stale setting remains, restore the known intended configuration for that user. A managed machine may need its corporate proxy or automatic configuration restored rather than having every proxy option disabled.

Identify the process associated with a change before attributing every rewritten proxy value to malware.