CVE-2026-42261: PromptHub SSRF via IPv6 private address bypass in skills fetch

Summary

PromptHub, from version 0.4.9 before 0.5.4, has an SSRF flaw in the authenticated POST /api/skills/fetch-remote endpoint in apps/web/src/routes/skills.ts. The private-address check in isPrivateIPv6 in apps/web/src/utils/remote-http.ts can be bypassed with alternate IPv6 representations, letting any authenticated user reach IPv4 loopback, RFC1918 and link-local addresses and read up to 5 MB of the response. On deployments with ALLOW_REGISTRATION=true, any internet user who can register can exploit it.

Mitigation

Fixed in 0.5.4.

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