pay-rails/pay: non-constant-time HMAC comparison in Paddle Billing webhook signature verifier

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.4 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary Pay::Webhooks::PaddleBillingController#validsignature? (app/controllers/pay/webhooks/paddlebillingcontroller.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.fixedlengthsecurecompare / ActiveSupport::SecurityUtils.securecompare). Impact - CWE-208 — Observable Timing Discrepancy on the webhook signature verifier. - An attacker who can deliver requests to the /pay/webhooks/paddlebilling 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. -…

Published by the source

1 Jul 2026, 18:57 UTC

Last modified by the source

1 Jul 2026, 18:57 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-mjgf-xj26-9qf9

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: GHSA-mjgf-xj26-9qf9

Advisory: pay-rails/pay: non-constant-time HMAC comparison in Paddle Billing webhook signature verifier
IDs: GHSA-mjgf-xj26-9qf9
Record: https://patchnext.com/advisories/ghsa-mjgf-xj26-9qf9 (revision 1, source checked 26 Oct 2026, 15:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems pay: affected <= 11.6.1

## Suggested inspection (read-only)
- RubyGems pay: check the installed version with your package manager.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on pay.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-mjgf-xj26-9qf9 (GitHub-reviewed advisory GHSA-mjgf-xj26-9qf9 lists affected ranges: RubyGems pay >=0 <=11.6.1)

Commands are suggestions: review each one before running it.