DNS Rebinding Flaw Found in SSRF Guards After Comment Flags Untested Gap

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A developer at ShortSingh discovered a critical DNS rebinding vulnerability across five URL-fetching endpoints after revisiting an unanswered comment left weeks earlier on a post about a separate SSRF bug. The existing protection relied solely on a regex blocklist checked against the hostname string, but the validation and the actual fetch involved two separate DNS lookups, leaving a window for the second lookup to return a different — potentially internal — IP address. Testing against a live rebinding domain confirmed the flaw bypassed production defenses entirely. The fix involved using Cloudflare Workers' cf.resolveOverride option to resolve the hostname once, validate the returned IP, and pin the connection to that exact address, eliminating any attacker-influenced second lookup. The patched implementation successfully blocked rebinding attempts in repeated tests while continuing to allow legitimate external requests without disruption.

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