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CVE-2026-50131 is a server-side request forgery protection bypass in Fedify, a TypeScript library for building ActivityPub-based federated server applications. The flaw is in the public-address validation logic used before runtime document and media fetching, specifically in validatePublicUrl() and its IPv4 classification helper isValidPublicIPv4Address(). Starting in version 0.11.2 and before fixed releases 1.9.12, 1.10.11, 2.0.19, 2.1.15, and 2.2.4, the implementation attempted to determine whether a destination was public by excluding only a limited set of private and local IPv4 ranges. While it blocked common ranges such as loopback, link-local, RFC1918 private space, and some local-use addresses, it failed to reject multiple other non-public or special-use ranges, including carrier-grade NAT, benchmarking, documentation, multicast, and future-reserved space. The IPv6 handling was also described as incomplete for transition and tunneling cases, including NAT64-wrapped private destinations. Because this validation served as an SSRF defense before outbound fetches, an attacker could supply crafted remote resource URLs that resolve to addresses incorrectly treated as public, thereby bypassing the intended restriction and reaching internal or local network resources.
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