Add support for latest HTTP Signatures spec draft (#14556)

* Add support for latest HTTP Signatures spec draft

https://www.ietf.org/id/draft-ietf-httpbis-message-signatures-00.html

- add support for the “hs2019” signature algorithm (assumed to be equivalent
  to RSA-SHA256, since we do not have a mechanism to specify the algorithm
  within the key metadata yet)
- add support for (created) and (expires) pseudo-headers and related
  signature parameters, when using the hs2019 signature algorithm
- adjust default “headers” parameter while being backwards-compatible with
  previous implementation
- change the acceptable time window logic from 12 hours surrounding the “date”
  header to accepting signatures created up to 1 hour in the future and
  expiring up to 1 hour in the past (but only allowing expiration dates up to
  12 hours after the creation date)
  This doesn't conform with the current draft, as it doesn't permit accounting
  for clock skew.
  This, however, should be addressed in a next version of the draft:
  https://github.com/httpwg/http-extensions/pull/1235

* Add additional signature requirements

* Rewrite signature params parsing using Parslet

* Make apparent which signature algorithm Mastodon on verification failure

Mastodon uses RSASSA-PKCS1-v1_5, which is not recommended for new applications,
and new implementers may thus unknowingly use RSASSA-PSS.

* Add workaround for PeerTube's invalid signature header

The previous parser allowed incorrect Signature headers, such as
those produced by old versions of the `http-signature` node.js package,
and seemingly used by PeerTube.

This commit adds a workaround for that.

* Fix `signature_key_id` raising an exception

Previously, parsing failures would result in `signature_key_id` being nil,
but the parser changes made that result in an exception.

This commit changes the `signature_key_id` method to return `nil` in case
of parsing failures.

* Move extra HTTP signature helper methods to private methods

* Relax (request-target) requirement to (request-target) || digest

This lets requests from Plume work without lowering security significantly.

1 files changed, 108 insertions, 55 deletions

diff --git a/app/controllers/concerns/signature_verification.rb b/app/controllers/concerns/signature_verification.rb
index 10efbf2e0..18f549de9 100644
--- a/app/controllers/concerns/signature_verification.rb
+++ b/app/controllers/concerns/signature_verification.rb
@@ -7,6 +7,44 @@ module SignatureVerification
 
   include DomainControlHelper
 
+  EXPIRATION_WINDOW_LIMIT = 12.hours
+  CLOCK_SKEW_MARGIN       = 1.hour
+
+  class SignatureVerificationError < StandardError; end
+
+  class SignatureParamsParser < Parslet::Parser
+    rule(:token)         { match("[0-9a-zA-Z!#$%&'*+.^_`|~-]").repeat(1).as(:token) }
+    rule(:quoted_string) { str('"') >> (qdtext | quoted_pair).repeat.as(:quoted_string) >> str('"') }
+    # qdtext and quoted_pair are not exactly according to spec but meh
+    rule(:qdtext)        { match('[^\\\\"]') }
+    rule(:quoted_pair)   { str('\\') >> any }
+    rule(:bws)           { match('\s').repeat }
+    rule(:param)         { (token.as(:key) >> bws >> str('=') >> bws >> (token | quoted_string).as(:value)).as(:param) }
+    rule(:comma)         { bws >> str(',') >> bws }
+    # Old versions of node-http-signature add an incorrect "Signature " prefix to the header
+    rule(:buggy_prefix)  { str('Signature ') }
+    rule(:params)        { buggy_prefix.maybe >> (param >> (comma >> param).repeat).as(:params) }
+    root(:params)
+  end
+
+  class SignatureParamsTransformer < Parslet::Transform
+    rule(params: subtree(:p)) do
+      (p.is_a?(Array) ? p : [p]).each_with_object({}) { |(key, val), h| h[key] = val }
+    end
+
+    rule(param: { key: simple(:key), value: simple(:val) }) do
+      [key, val]
+    end
+
+    rule(quoted_string: simple(:string)) do
+      string.to_s
+    end
+
+    rule(token: simple(:string)) do
+      string.to_s
+    end
+  end
+
   def require_signature!
     render plain: signature_verification_failure_reason, status: signature_verification_failure_code unless signed_request_account
   end
@@ -24,72 +62,40 @@ module SignatureVerification
   end
 
   def signature_key_id
-    raw_signature    = request.headers['Signature']
-    signature_params = {}
-
-    raw_signature.split(',').each do |part|
-      parsed_parts = part.match(/([a-z]+)="([^"]+)"/i)
-      next if parsed_parts.nil? || parsed_parts.size != 3
-      signature_params[parsed_parts[1]] = parsed_parts[2]
-    end
-
     signature_params['keyId']
+  rescue SignatureVerificationError
+    nil
   end
 
   def signed_request_account
     return @signed_request_account if defined?(@signed_request_account)
 
-    unless signed_request?
-      @signature_verification_failure_reason = 'Request not signed'
-      @signed_request_account = nil
-      return
-    end
-
-    if request.headers['Date'].present? && !matches_time_window?
-      @signature_verification_failure_reason = 'Signed request date outside acceptable time window'
-      @signed_request_account = nil
-      return
-    end
+    raise SignatureVerificationError, 'Request not signed' unless signed_request?
+    raise SignatureVerificationError, 'Incompatible request signature. keyId and signature are required' if missing_required_signature_parameters?
+    raise SignatureVerificationError, 'Unsupported signature algorithm (only rsa-sha256 and hs2019 are supported)' unless %w(rsa-sha256 hs2019).include?(signature_algorithm)
+    raise SignatureVerificationError, 'Signed request date outside acceptable time window' unless matches_time_window?
 
-    raw_signature    = request.headers['Signature']
-    signature_params = {}
-
-    raw_signature.split(',').each do |part|
-      parsed_parts = part.match(/([a-z]+)="([^"]+)"/i)
-      next if parsed_parts.nil? || parsed_parts.size != 3
-      signature_params[parsed_parts[1]] = parsed_parts[2]
-    end
-
-    if incompatible_signature?(signature_params)
-      @signature_verification_failure_reason = 'Incompatible request signature'
-      @signed_request_account = nil
-      return
-    end
+    verify_signature_strength!
 
     account = account_from_key_id(signature_params['keyId'])
 
-    if account.nil?
-      @signature_verification_failure_reason = "Public key not found for key #{signature_params['keyId']}"
-      @signed_request_account = nil
-      return
-    end
+    raise SignatureVerificationError, "Public key not found for key #{signature_params['keyId']}" if account.nil?
 
     signature             = Base64.decode64(signature_params['signature'])
-    compare_signed_string = build_signed_string(signature_params['headers'])
+    compare_signed_string = build_signed_string
 
     return account unless verify_signature(account, signature, compare_signed_string).nil?
 
     account = stoplight_wrap_request { account.possibly_stale? ? account.refresh! : account_refresh_key(account) }
 
-    if account.nil?
-      @signature_verification_failure_reason = "Public key not found for key #{signature_params['keyId']}"
-      @signed_request_account = nil
-      return
-    end
+    raise SignatureVerificationError, "Public key not found for key #{signature_params['keyId']}" if account.nil?
 
     return account unless verify_signature(account, signature, compare_signed_string).nil?
 
-    @signature_verification_failure_reason = "Verification failed for #{account.username}@#{account.domain} #{account.uri}"
+    @signature_verification_failure_reason = "Verification failed for #{account.username}@#{account.domain} #{account.uri} using rsa-sha256 (RSASSA-PKCS1-v1_5 with SHA-256)"
+    @signed_request_account = nil
+  rescue SignatureVerificationError => e
+    @signature_verification_failure_reason = e.message
     @signed_request_account = nil
   end
 
@@ -99,6 +105,31 @@ module SignatureVerification
 
   private
 
+  def signature_params
+    @signature_params ||= begin
+      raw_signature = request.headers['Signature']
+      tree          = SignatureParamsParser.new.parse(raw_signature)
+      SignatureParamsTransformer.new.apply(tree)
+    end
+  rescue Parslet::ParseFailed
+    raise SignatureVerificationError, 'Error parsing signature parameters'
+  end
+
+  def signature_algorithm
+    signature_params.fetch('algorithm', 'hs2019')
+  end
+
+  def signed_headers
+    signature_params.fetch('headers', signature_algorithm == 'hs2019' ? '(created)' : 'date').downcase.split(' ')
+  end
+
+  def verify_signature_strength!
+    raise SignatureVerificationError, 'Mastodon requires the Date header or (created) pseudo-header to be signed' unless signed_headers.include?('date') || signed_headers.include?('(created)')
+    raise SignatureVerificationError, 'Mastodon requires the Digest header or (request-target) pseudo-header to be signed' unless signed_headers.include?(Request::REQUEST_TARGET) || signed_headers.include?('digest')
+    raise SignatureVerificationError, 'Mastodon requires the Host header to be signed' unless signed_headers.include?('host')
+    raise SignatureVerificationError, 'Mastodon requires the Digest header to be signed when doing a POST request' if request.post? && !signed_headers.include?('digest')
+  end
+
   def verify_signature(account, signature, compare_signed_string)
     if account.keypair.public_key.verify(OpenSSL::Digest::SHA256.new, signature, compare_signed_string)
       @signed_request_account = account
@@ -108,12 +139,20 @@ module SignatureVerification
     nil
   end
 
-  def build_signed_string(signed_headers)
-    signed_headers = 'date' if signed_headers.blank?
-
-    signed_headers.downcase.split(' ').map do |signed_header|
+  def build_signed_string
+    signed_headers.map do |signed_header|
       if signed_header == Request::REQUEST_TARGET
         "#{Request::REQUEST_TARGET}: #{request.method.downcase} #{request.path}"
+      elsif signed_header == '(created)'
+        raise SignatureVerificationError, 'Invalid pseudo-header (created) for rsa-sha256' unless signature_algorithm == 'hs2019'
+        raise SignatureVerificationError, 'Pseudo-header (created) used but corresponding argument missing' if signature_params['created'].blank?
+
+        "(created): #{signature_params['created']}"
+      elsif signed_header == '(expires)'
+        raise SignatureVerificationError, 'Invalid pseudo-header (expires) for rsa-sha256' unless signature_algorithm == 'hs2019'
+        raise SignatureVerificationError, 'Pseudo-header (expires) used but corresponding argument missing' if signature_params['expires'].blank?
+
+        "(expires): #{signature_params['expires']}"
       elsif signed_header == 'digest'
         "digest: #{body_digest}"
       else
@@ -123,13 +162,28 @@ module SignatureVerification
   end
 
   def matches_time_window?
+    created_time = nil
+    expires_time = nil
+
     begin
-      time_sent = Time.httpdate(request.headers['Date'])
+      if signature_algorithm == 'hs2019' && signature_params['created'].present?
+        created_time = Time.at(signature_params['created'].to_i).utc
+      elsif request.headers['Date'].present?
+        created_time = Time.httpdate(request.headers['Date']).utc
+      end
+
+      expires_time = Time.at(signature_params['expires'].to_i).utc if signature_params['expires'].present?
     rescue ArgumentError
       return false
     end
 
-    (Time.now.utc - time_sent).abs <= 12.hours
+    expires_time ||= created_time + 5.minutes unless created_time.nil?
+    expires_time = [expires_time, created_time + EXPIRATION_WINDOW_LIMIT].min unless created_time.nil?
+
+    return false if created_time.present? && created_time > Time.now.utc + CLOCK_SKEW_MARGIN
+    return false if expires_time.present? && Time.now.utc > expires_time + CLOCK_SKEW_MARGIN
+
+    true
   end
 
   def body_digest
@@ -140,9 +194,8 @@ module SignatureVerification
     name.split(/-/).map(&:capitalize).join('-')
   end
 
-  def incompatible_signature?(signature_params)
-    signature_params['keyId'].blank? ||
-      signature_params['signature'].blank?
+  def missing_required_signature_parameters?
+    signature_params['keyId'].blank? || signature_params['signature'].blank?
   end
 
   def account_from_key_id(key_id)