internal/util: match "localhost" case-insensitively in IsLoopback - #1316
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Host names are case-insensitive (RFC 4343), but IsLoopback compared the host to "localhost" exactly. net/http keeps the Host header as the client sent it, so a client configured with http://LocalHost:8080/mcp got 403 "invalid Host header" from a streamable or SSE server listening on 127.0.0.1: the DNS rebinding check took the capitalized name for a foreign host. The same comparison made oauthex reject an HTTP discovery URL on LOCALHOST as non-loopback, and made the authorization code handler classify such a redirect URI as a web one. Accepting any capitalization does not weaken the rebinding protection. It rejects requests whose Host names an attacker's domain, and every spelling of localhost names the local machine. The new TestIsLoopback cases and the TestStreamableLocalhostProtection case fail without the change. Fixes modelcontextprotocol#1319 Signed-off-by: Akshita <110122283+akshita317@users.noreply.github.com>
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Host names are case-insensitive (RFC 4343), but IsLoopback compared the
host to "localhost" exactly. net/http keeps the Host header as the client
sent it, so a client configured with http://LocalHost:8080/mcp got 403
"invalid Host header" from a streamable or SSE server listening on
127.0.0.1: the DNS rebinding check took the capitalized name for a
foreign host. The same comparison made oauthex reject an HTTP discovery
URL on LOCALHOST as non-loopback, and made the authorization code
handler classify such a redirect URI as a web one.
Accepting any capitalization does not weaken the rebinding protection.
It rejects requests whose Host names an attacker's domain, and every
spelling of localhost names the local machine.
The new TestIsLoopback cases and the TestStreamableLocalhostProtection
case fail without the change.
Fixes #1319