Fix possibility of unrightful webfinger redirect (mastodon#2147)

* Fix possibility of unrightful webfinger redirect

* Add more tests for FollowRemoteAccountService

app/services/follow_remote_account_service.rb

@@ -10,7 +10,7 @@ class FollowRemoteAccountService < BaseService
   # important information from their feed
   # @param [String] uri User URI in the form of username@domain
   # @return [Account]
-  def call(uri)
+  def call(uri, redirected = nil)
     username, domain = uri.split('@')
 
     return Account.find_local(username) if TagManager.instance.local_domain?(domain)
@@ -24,8 +24,14 @@ def call(uri)
 
     raise Goldfinger::Error, 'Missing resource links' if data.link('http://schemas.google.com/g/2010#updates-from').nil? || data.link('salmon').nil? || data.link('http://webfinger.net/rel/profile-page').nil? || data.link('magic-public-key').nil?
 
+    # Disallow account hijacking
     confirmed_username, confirmed_domain = data.subject.gsub(/\Aacct:/, '').split('@')
 
+    unless confirmed_username.casecmp(username).zero? && confirmed_domain.casecmp(domain).zero?
+      return call("#{confirmed_username}@#{confirmed_domain}", true) if redirected.nil?
+      raise Goldfinger::Error, 'Requested and returned acct URI do not match'
+    end
+
     return Account.find_local(confirmed_username) if TagManager.instance.local_domain?(confirmed_domain)
 
     confirmed_account = Account.find_remote(confirmed_username, confirmed_domain)

spec/services/follow_remote_account_service_spec.rb

@@ -3,10 +3,35 @@
 RSpec.describe FollowRemoteAccountService do
   subject { FollowRemoteAccountService.new }
 
-  it 'returns nil if no such user can be resolved via webfinger'
-  it 'returns nil if the domain does not have webfinger'
-  it 'returns nil if remote user does not offer a hub URL'
-  it 'returns an already existing remote account'
-  it 'returns a new remote account'
-  it 'fills the remote account with profile information'
+  before do
+    stub_request(:get, "https://quitter.no/.well-known/host-meta").to_return(request_fixture('.host-meta.txt'))
+    stub_request(:get, "https://example.com/.well-known/host-meta").to_return(status: 404)
+    stub_request(:get, "https://quitter.no/.well-known/webfinger?resource=acct:gargron@quitter.no").to_return(request_fixture('webfinger.txt'))
+    stub_request(:get, "https://quitter.no/.well-known/webfinger?resource=acct:catsrgr8@quitter.no").to_return(status: 404)
+    stub_request(:get, "https://quitter.no/api/statuses/user_timeline/7477.atom").to_return(request_fixture('feed.txt'))
+    stub_request(:get, "https://quitter.no/avatar/7477-300-20160211190340.png").to_return(request_fixture('avatar.txt'))
+  end
+
+  it 'raises error if no such user can be resolved via webfinger' do
+    expect { subject.call('catsrgr8@quitter.no') }.to raise_error Goldfinger::Error
+  end
+
+  it 'raises error if the domain does not have webfinger' do
+    expect { subject.call('catsrgr8@example.com') }.to raise_error Goldfinger::Error
+  end
+
+  it 'returns an already existing remote account' do
+    old_account      = Fabricate(:account, username: 'gargron', domain: 'quitter.no')
+    returned_account = subject.call('gargron@quitter.no')
+
+    expect(old_account.id).to eq returned_account.id
+  end
+
+  it 'returns a new remote account' do
+    account = subject.call('gargron@quitter.no')
+
+    expect(account.username).to eq 'gargron'
+    expect(account.domain).to eq 'quitter.no'
+    expect(account.remote_url).to eq 'https://quitter.no/api/statuses/user_timeline/7477.atom'
+  end
 end