Stop opening the OAuth authorization URL through cmd.exe on Windows - #382
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The authorization URL is built from the authorization server's metadata, i.e. it is controlled by whatever server the user typed into `mcpc login`. On Windows it was handed to `cmd.exe /c start "" <url>`. libuv only quotes arguments containing whitespace or double quotes, so `&`, `|`, `^` and `%VAR%` inside the URL reached cmd.exe unescaped and were interpreted as commands: `https://evil.example/authorize?x=1&calc` ran `calc` once the user pressed Enter. Even a benign URL was truncated at its first `&`. The browser is now launched via `rundll32 url.dll,FileProtocolHandler` on Windows (no command interpreter in between, same approach as the GitHub CLI), and authorization URLs with a scheme other than http:/https: are refused on every platform before they are shown or opened. The launcher command is built by a pure, exported function with unit tests that assert no shell binary is ever used and that hostile URLs pass through as a single verbatim argument. Fixes H1 in docs/security-review-2026-09.md. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01HbTS6Mvbo9t5115trQQm6j
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Refs #382. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01HbTS6Mvbo9t5115trQQm6j
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mcpc loginhanded the server-supplied authorization URL tocmd.exe /c starton Windows. libuv only quotes arguments with whitespace or double quotes, so&,|,^and%VAR%in the URL ran as commands once the user pressed Enter, and even a benign URL was cut off at its first&. The browser is now launched without any shell, and non-http(s) authorization URLs are refused.rundll32 url.dll,FileProtocolHandler <url>(same approach as the GitHub CLI); macOS and Linux keepopen/xdg-open.http:/https:are rejected in both login flows before they are printed or opened.Securityentry and CLAUDE.md security note updated.Not exercised on Windows itself; the reasoning follows libuv's quoting rules and the rundll32 precedent.
Fixes H1 in docs/security-review-2026-09.md.
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https://claude.ai/code/session_01HbTS6Mvbo9t5115trQQm6j