ip-address: Address4 decodes leading-zero octets as decimal while resolvers decode them as octal, allowing SSRF and trust-boundary bypass

ip-address is a library for parsing and manipulating IPv4 and IPv6 addresses in JavaScript. Prior to 10.3.1, Address4 accepts an octet written with a leading zero and decodes it as decimal, while the WHATWG URL host parser, inet_aton, and getaddrinfo all decode a leading zero as octal. The library and the network stack therefore disagree about which host a string names. new Address4('012.0.0.1') reports correctForm() of 12.0.0.1 and isPrivate() of false, but fetch('http://012.0.0.1/') connects to 10.0.0.1. An application that builds a network trust-boundary decision on these checks, for example a filter intended to block Server-Side Request Forgery, or SSRF, will classify an internal target as external and allow the request. The defect is in the parse gate rather than in any one classifier, so every consumer of Address4 inherits it: isPrivate(), isLoopback(), isLinkLocal(), isCGNAT(), isInSubnet(), isHostInSubnet(), and correctForm() are all computed from the mis-decoded octets. This issue is fixed in version 10.3.1.

Source published

3 Aug 2026 19:56:13 UTC

Source modified

4 Aug 2026 13:59:43 UTC

Observed here

23 Oct 2026 15:48:27 UTC

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CVSS 4.0 · 7.7 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:H/SI:N/SA:N