How to Secure Webhooks: HMAC Signing, Replay Attacks, and SSRF Risks

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Webhooks power most modern software integrations but are vulnerable to several serious security threats if not properly hardened. Replay attacks can cause duplicate actions like double-crediting wallets, while unencrypted payloads risk exposing sensitive data in transit. Timing attacks on naive signature comparisons can allow attackers to reconstruct valid signatures byte-by-byte over repeated attempts. Outbound webhooks also pose a server-side request forgery risk, where malicious customers can redirect delivery workers to internal network endpoints, a threat OWASP warns cannot be mitigated by one-time URL validation alone. Industry best practices recommend HMAC-SHA256 signature verification on raw request payloads, mutual TLS in regulated environments, IP allowlisting as a supplementary filter, and isolated network segments for outbound webhook delivery workers.

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