Fix webfinger redirect handling in ResolveAccountService (#15187)

* Fix webfinger redirect handling in ResolveAccountService

ResolveAccountService#process_webfinger! handled a one-step webfinger
redirection, but only accepting the result if it matched the exact URI passed
as input, defeating the point of a redirection check.

Instead, use the same logic as in `ActivityPub::FetchRemoteAccountService`,
updating the resulting `acct:` URI with the result of the first webfinger
query.

* Add tests

app/services/resolve_account_service.rb

@@ -29,6 +29,7 @@ class ResolveAccountService < BaseService
     # At this point we are in need of a Webfinger query, which may
     # yield us a different username/domain through a redirect
     process_webfinger!(@uri)
+    @domain = nil if TagManager.instance.local_domain?(@domain)
 
     # Because the username/domain pair may be different than what
     # we already checked, we need to check if we've already got
@@ -78,25 +79,31 @@ class ResolveAccountService < BaseService
     @uri = [@username, @domain].compact.join('@')
   end
 
-  def process_webfinger!(uri, redirected = false)
+  def process_webfinger!(uri)
     @webfinger                           = webfinger!("acct:#{uri}")
-    confirmed_username, confirmed_domain = @webfinger.subject.gsub(/\Aacct:/, '').split('@')
+    confirmed_username, confirmed_domain = split_acct(@webfinger.subject)
 
     if confirmed_username.casecmp(@username).zero? && confirmed_domain.casecmp(@domain).zero?
       @username = confirmed_username
       @domain   = confirmed_domain
-      @uri      = uri
-    elsif !redirected
-      return process_webfinger!("#{confirmed_username}@#{confirmed_domain}", true)
-    else
-      raise Webfinger::RedirectError, "The URI #{uri} tries to hijack #{@username}@#{@domain}"
+      return
     end
 
-    @domain = nil if TagManager.instance.local_domain?(@domain)
+    # Account doesn't match, so it may have been redirected
+    @webfinger         = webfinger!("acct:#{confirmed_username}@#{confirmed_domain}")
+    @username, @domain = split_acct(@webfinger.subject)
+
+    unless confirmed_username.casecmp(@username).zero? && confirmed_domain.casecmp(@domain).zero?
+      raise Webfinger::RedirectError, "The URI #{uri} tries to hijack #{@username}@#{@domain}"
+    end
   rescue Webfinger::GoneError
     @gone = true
   end
 
+  def split_acct(acct)
+    acct.gsub(/\Aacct:/, '').split('@')
+  end
+
   def process_account!
     return unless activitypub_ready?
 

spec/services/resolve_account_service_spec.rb

@@ -4,11 +4,8 @@ RSpec.describe ResolveAccountService, type: :service do
   subject { described_class.new }
 
   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/webfinger?resource=acct:catsrgr8@example.com").to_return(status: 404)
     stub_request(:get, "https://example.com/.well-known/host-meta").to_return(status: 404)
     stub_request(:get, "https://quitter.no/avatar/7477-300-20160211190340.png").to_return(request_fixture('avatar.txt'))
-    stub_request(:get, "https://quitter.no/.well-known/webfinger?resource=acct:catsrgr8@quitter.no").to_return(status: 404)
     stub_request(:get, "https://ap.example.com/.well-known/webfinger?resource=acct:foo@ap.example.com").to_return(request_fixture('activitypub-webfinger.txt'))
     stub_request(:get, "https://ap.example.com/users/foo").to_return(request_fixture('activitypub-actor.txt'))
     stub_request(:get, "https://ap.example.com/users/foo.atom").to_return(request_fixture('activitypub-feed.txt'))
@@ -16,13 +13,26 @@ RSpec.describe ResolveAccountService, type: :service do
     stub_request(:get, 'https://example.com/.well-known/webfinger?resource=acct:hoge@example.com').to_return(status: 410)
   end
 
-  it 'returns nil if no such user can be resolved via webfinger' do
-    expect(subject.call('catsrgr8@quitter.no')).to be_nil
+  context 'when there is an LRDD endpoint but no resolvable account' do
+    before do
+      stub_request(:get, "https://quitter.no/.well-known/host-meta").to_return(request_fixture('.host-meta.txt'))
+      stub_request(:get, "https://quitter.no/.well-known/webfinger?resource=acct:catsrgr8@quitter.no").to_return(status: 404)
     end
 
-  it 'returns nil if the domain does not have webfinger' do
+    it 'returns nil' do
+      expect(subject.call('catsrgr8@quitter.no')).to be_nil
+    end
+  end
+
+  context 'when there is no LRDD endpoint nor resolvable account' do
+    before do
+      stub_request(:get, "https://example.com/.well-known/webfinger?resource=acct:catsrgr8@example.com").to_return(status: 404)
+    end
+
+    it 'returns nil' do
       expect(subject.call('catsrgr8@example.com')).to be_nil
     end
+  end
 
   context 'when webfinger returns http gone' do
     context 'for a previously known account' do
@@ -48,6 +58,34 @@ RSpec.describe ResolveAccountService, type: :service do
     end
   end
 
+  context 'with a legitimate webfinger redirection' do
+    before do
+      webfinger = { subject: 'acct:foo@ap.example.com', links: [{ rel: 'self', href: 'https://ap.example.com/users/foo' }] }
+      stub_request(:get, 'https://redirected.example.com/.well-known/webfinger?resource=acct:Foo@redirected.example.com').to_return(body: Oj.dump(webfinger), headers: { 'Content-Type': 'application/jrd+json' })
+    end
+
+    it 'returns new remote account' do
+      account = subject.call('Foo@redirected.example.com')
+
+      expect(account.activitypub?).to eq true
+      expect(account.acct).to eq 'foo@ap.example.com'
+      expect(account.inbox_url).to eq 'https://ap.example.com/users/foo/inbox'
+    end
+  end
+
+  context 'with too many webfinger redirections' do
+    before do
+      webfinger = { subject: 'acct:foo@evil.example.com', links: [{ rel: 'self', href: 'https://ap.example.com/users/foo' }] }
+      stub_request(:get, 'https://redirected.example.com/.well-known/webfinger?resource=acct:Foo@redirected.example.com').to_return(body: Oj.dump(webfinger), headers: { 'Content-Type': 'application/jrd+json' })
+      webfinger2 = { subject: 'acct:foo@ap.example.com', links: [{ rel: 'self', href: 'https://ap.example.com/users/foo' }] }
+      stub_request(:get, 'https://evil.example.com/.well-known/webfinger?resource=acct:foo@evil.example.com').to_return(body: Oj.dump(webfinger2), headers: { 'Content-Type': 'application/jrd+json' })
+    end
+
+    it 'returns new remote account' do
+      expect { subject.call('Foo@redirected.example.com') }.to raise_error Webfinger::RedirectError
+    end
+  end
+
   context 'with an ActivityPub account' do
     it 'returns new remote account' do
       account = subject.call('foo@ap.example.com')