fix: escape script vars

[mattzcarey]

we should do this

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Latest commit: ff2973f

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[claude]

Claude Code Review

PR #788: fix: escape script vars

This PR fixes a potential XSS vulnerability in OAuth error handling where user-controlled error messages could be injected into inline script tags without proper escaping.

Issues Found:

1. Incomplete XSS prevention (examples/mcp-client/src/server.ts:15-17, site/ai-playground/src/server.ts:78-80):

   • Using JSON.stringify() and string concatenation (+) in inline scripts does not fully prevent XSS
   • If result.authError contains </script>, it will still break out of the script context
   • Example: authError = '</script><img src=x onerror=alert(1)>'

   Recommended fix: Use double JSON encoding:

   const safeError = JSON.stringify(JSON.stringify(result.authError || "Unknown error"));
   return new Response(
     `<script>alert("Authentication failed: " + JSON.parse(SAFEVAR)); window.close();</script>`.replace("SAFEVAR", safeError),
     { headers: { "content-type": "text/html" }, status: 200 }
   );

2. No test coverage: The OAuth error path lacks tests to verify XSS protection works correctly.

Severity: High - This is security-critical code handling authentication errors that could contain attacker-controlled input.

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commit: ff2973f