Five Fail-Open Security Bugs Found in AI Agent That Defaulted to Allow on Missing Data

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Security researchers identified five related vulnerabilities in an MCP-mediated Slack AI agent called claude-code-slack-channel, each sharing the same root cause: when a security check lacked the data it needed, it defaulted to allowing the action instead of blocking it. In one case, a deny rule blocking access to /etc was silently bypassed because the policy engine received an empty input object with no structured arguments, causing scoped rules to never match. Another bug allowed an empty audit log to pass verification as clean, since zero events produced zero broken links. The core principle highlighted across all five bugs is that absence of data should trigger a fail-closed response, not a permissive one. Fixes included escalating to human approval when scoped rule inputs are unavailable, and enforcing a minimum event count threshold during audit log verification.

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