CVE-2026-113799: Slack Morphism verifier errors disclose valid HMACs in the logs

Slack Morphism exposes the correct Slack request HMAC when signature verification fails.

For an invalid request, slack-morphism computes the correct signature for the attacker-controlled timestamp and request body and stores it in SlackEventWrongSignatureError.generated_hash. The error’s Debug and Display representations expose that value, and the default Axum listener error handler logs it at error level.

An attacker who can both reach the application’s Slack callback endpoint and read its application/service logs can therefore use the verifier as a chosen-message HMAC oracle. The attacker can send an arbitrary request with an invalid signature, recover the correct signature from the resulting log entry, and resend the same request within the five-minute freshness window. The second request passes Slack Morphism’s signature verification and reaches the protected application callback without the attacker knowing the Slack signing secret.

I reproduced this end-to-end against both the Axum and Hyper integrations in slack-morphism 2.26.0 and 2.27.0.