Hugo: security.http.urls deny rules bypassed by alternate IPv4 encodings (SSRF)

Impact

The default security.http.urls policy denies requests to loopback, internal, and cloud-metadata IPv4 literals (e.g. http://127.0.0.1/, http://169.254.169.254/). The deny rule only matched dotted-decimal notation, so alternate IPv4 encodings of the same addresses — integer, hex, or octal, which contain no dot — passed the policy:

• http://2130706433/ → 127.0.0.1
• http://2852039166/ → 169.254.169.254 (cloud metadata)
• http://0x7f000001/, http://017700000001/, http://0/

When a template passes an untrusted or data-derived URL to resources.GetRemote and the host platform uses the cgo system resolver, these encodings resolve to the blocked address — allowing build-time server-side requests to loopback and internal services, including the cloud-metadata endpoint in hosted/CI builds. The same check is reused on redirects, so the gap also applies to each redirect hop.

This affects sites that rely on security.http.urls as a security boundary while fetching attacker-influenced remote URLs; it does not affect sites that fully trust the URLs they fetch.

Patches

Fixed in v0.163.1. Integer/hex/octal IPv4 hosts are now canonicalized to dotted-decimal before the policy is applied, so every encoding of an address is treated alike. No configuration change is required.

Workarounds

Avoid passing untrusted URLs to resources.GetRemote, or tighten security.http.urls to an explicit allow-list of trusted hosts.

Affected versions

v0.162.0 – v0.163.0 (patched in v0.163.1).