## Slack Bot Security Gap: Unverified Requests Still Incur Lambda Costs, Prompting API Gateway or WAF Fix
A critical cost-control vulnerability has been identified in a Slack bot's authentication system. While recent updates correctly reject unauthorized requests with a 401 status, these invalid requests still trigger full AWS Lambda executions, generating unnecessary and potentially exploitable cloud costs. A malicious actor could bombard the `/slack/commands` endpoint with unsigned payloads, incurring charges for every blocked invocation without ever gaining access.

The issue stems from the placement of HMAC signature verification. PR #73 successfully implemented validation, but it occurs within the Lambda function itself. This means the compute resource is already provisioned and billed before the request is denied. The core problem is not a security breach—the authentication logic is sound—but a financial leak. The system is paying for the privilege of telling bad actors 'no.'

Three mitigation paths are under review to move rejection upstream. The primary options are implementing an API Gateway request validator to block requests missing the `X-Slack-Signature` and `X-Slack-Request-Timestamp` headers before they reach Lambda, or deploying a WAF rule for the same purpose, which would provide valuable CloudWatch metrics for alerting. A secondary option is a per-source-IP throttle via API Gateway usage plans. While classified as a low-priority cost-containment issue, it highlights a common architectural blind spot where security logic fails to align with financial controls, leaving systems open to budget-draining attacks even when functionally secure.
---
- **Source**: GitHub Issues
- **Sector**: The Lab
- **Tags**: AWS Lambda, API Gateway, Cloud Security, Cost Optimization, Web Application Firewall
- **Credibility**: unverified
- **Published**: 2026-04-20 06:22:36
- **ID**: 71887
- **URL**: https://whisperx.ai/en/intel/71887