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

What is CVE-2026-113799?

CVE-2026-113799 is a medium-severity vulnerability in abdolence slack-morphism-rust, published on 21 Oct 2026.

Official description

Slack Morphism is a client library for Slack Web/Events API/Socket Mode and Block Kit. Versions 1.0.0 through 2.27.0 expose 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. Version 2.28.0 patches the issue.

Weakness: CWE-532

Which versions are affected by CVE-2026-113799?

2 open source packages ship an affected version, including slack-morphism.

How do I fix CVE-2026-113799?

Upgrade slack-morphism to 2.28.0 or later.

Is CVE-2026-113799 being exploited?

There is no known exploitation of CVE-2026-113799 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-26w4-8w32-4x3v.