Verifying Descope webhook HMAC means rebuilding the signature template

TL;DR

Capture the raw body in your framework (express.raw or the json verify callback) because JSON.stringify of a parsed body is not byte-identical to what was signed.

Steps

1. Read the x-descope-signature-template header on every request and rebuild the signed string in that order: substitute METHOD with the request method, URI with the request URL, each HEADER_name with the matching header value, and BODY with the raw request body bytes, not a re-stringified parse. Then HMAC-SHA256 the result with your HMAC secret, base64 it, and compare against x-descope-webhook-s256 with a timing-safe compare. Capture the raw body in your framework (express.raw or the json verify callback) because JSON.stringify of a parsed body is not byte-identical to what was signed.

When to use

You are seeing this: Read the x-descope-signature-template header on every request and rebuild the signed string in that order: substitute METHOD with the request method, URI with the request URL, each HEADER_name with the matching header value, and BODY with the raw request... Use this skill when you run into "Verifying Descope webhook HMAC means rebuilding the signature template".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Source: Official docs (Generic HTTP Connector): documents a gotcha that trips agents verifying Descope signatures. By default the HMAC secret signs the entire JSON request body and the result goes in the x-descope-webhook-s256 header, but the signed content can be customized with an x-descope-signature-template header, e.g. METHOD,URI,HEADER_x-descope-timestamp,BODY. An agent that hardcodes body-only verification will reject valid requests the moment a template is set, and re-stringifying a parsed JSON body may not reproduce the exact bytes that were signed.

Verified resolution: Read the x-descope-signature-template header on every request and rebuild the signed string in that order: substitute METHOD with the request method, URI with the request URL, each HEADER_name with the matching header value, and BODY with the raw request body bytes, not a re-stringified parse. Then HMAC-SHA256 the result with your HMAC secret, base64 it, and compare against x-descope-webhook-s256 with a timing-safe compare. Capture the raw body in your framework (express.raw or the json verify callback) because JSON.stringify of a parsed body is not byte-identical to what was signed.

Source URL: https://docs.descope.com/connectors/connector-configuration-guides/network/generic-http