Hugo Static Site Generator IPv4 Bypass in Security Policy

Executive Summary

This vulnerability in Hugo, a static site generator, involves a bypass of its security policy that blocks requests to certain internal IP addresses. The policy was designed to deny requests to loopback, internal, and cloud-metadata IPv4 addresses when expressed in standard dotted-decimal notation (e.g., 127.0.0.1). However, the vulnerability allowed alternate IPv4 address encodings—such as integer, hexadecimal, or octal forms—to bypass these restrictions.

When a template passes an untrusted or data-derived URL to the resources.GetRemote function, and the host platform uses the cgo system resolver, these alternate encodings resolve to the blocked addresses, enabling unauthorized server-side requests to internal services. This includes access to sensitive endpoints like cloud-metadata in hosted or continuous integration builds. The vulnerability also applies to redirect hops because the same check is reused on redirects.

The issue was fixed in Hugo version 0.163.1 by normalizing all IPv4 address encodings to the standard dotted-decimal form before applying the security policy, ensuring consistent blocking of disallowed addresses.

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Impact Analysis

This vulnerability can allow an attacker to perform server-side request forgery (SSRF) attacks by bypassing security restrictions on internal IP addresses. Specifically, it enables unauthorized build-time server-side requests to loopback, internal, and cloud-metadata endpoints.

Such unauthorized access could expose sensitive internal services or metadata endpoints, potentially leaking confidential information or enabling further attacks within the internal network or cloud environment.

The vulnerability affects hosted or continuous integration (CI) build environments where untrusted URLs might be processed, increasing the risk of exploitation in automated build pipelines.

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Detection Guidance

This vulnerability involves bypassing security.http.urls deny rules through alternate IPv4 encodings such as integer, hexadecimal, or octal representations of loopback, internal, and cloud-metadata addresses.

Detection can focus on identifying requests or logs where URLs use non-standard IPv4 encodings that resolve to restricted addresses.

• Inspect logs or network traffic for URLs containing integer (e.g., 2130706433), hex (e.g., 0x7f000001), or octal (e.g., 017700000001) IPv4 address formats.
• Use network monitoring tools or packet capture utilities (e.g., tcpdump, Wireshark) to filter HTTP requests and analyze the host portion of URLs for alternate IPv4 encodings.
• Example command to search logs for hex or integer IP encodings: grep -E '0x[0-9a-fA-F]+|[0-9]{7,10}' /path/to/logfile
• Use scripting or parsing tools to normalize IP addresses in URLs to dotted-decimal form and check if they resolve to loopback or internal addresses.

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Mitigation Strategies

The vulnerability is fixed in Hugo version 0.163.1 by canonicalizing IPv4 hosts to dotted-decimal notation before applying security policies.

Immediate mitigation steps include:

• Upgrade Hugo to version 0.163.1 or later where the fix is implemented.
• Avoid passing untrusted or data-derived URLs to resources.GetRemote to prevent SSRF risks.
• Tighten the security.http.urls configuration by using an explicit allow-list rather than relying solely on deny rules.

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