Attack Chain

1. Initial Access / Injection: An attacker injects a specially crafted URL containing an alternate IPv4 encoding (e.g., http://2130706433/ for 127.0.0.1 or http://2852039166/ for cloud metadata) into a Hugo template or data source.
2. Vulnerable Processing: During a Hugo site build, a template attempts to fetch content from this untrusted URL using the resources.GetRemote function.
3. Policy Bypass Attempt: Hugo's security.http.urls policy is consulted to determine if the URL should be denied, but it only checks for dotted-decimal IPv4 formats.
4. Encoding Misinterpretation: Due to the vulnerability, the policy fails to recognize the integer, hexadecimal, or octal IPv4 encoding as a disallowed internal, loopback, or cloud-metadata address.
5. DNS/Resolver Resolution: The host platform's cgo system resolver resolves the alternate IPv4 encoding (e.g., 2130706433) to its standard dotted-decimal equivalent (127.0.0.1).
6. Internal Request Execution: Hugo proceeds to make an outbound HTTP GET request to the now-resolved internal IP address (e.g., 127.0.0.1, 169.254.169.254, or another internal service).
7. Information Disclosure/Internal Access: The build environment's internal services or cloud metadata endpoint respond to Hugo's request, potentially disclosing sensitive configuration data, credentials, or allowing access to internal resources that should have been protected.

Impact

The primary impact of this vulnerability is the potential for Server-Side Request Forgery (SSRF) during the Hugo site build process. If exploited, an attacker can coerce the build server to make outbound HTTP requests to arbitrary internal network resources, including loopback addresses, internal hosts, or cloud metadata endpoints (e.g., 169.254.169.254). This can lead to the exposure of sensitive information such as cloud instance credentials, internal network topology, or other confidential data accessible from the build environment. While no specific victim counts or sectors were noted, organizations using Hugo in CI/CD pipelines or environments where untrusted content influences builds are at risk of unauthorized data access and potential lateral movement within their internal infrastructure.

Recommendation

• Upgrade Hugo to version v0.163.1 or newer immediately to apply the patch that correctly canonicalizes IPv4 addresses.
• Review CI/CD pipeline configurations and Hugo site templates to avoid passing untrusted or data-derived URLs directly to resources.GetRemote.
• Harden security.http.urls in Hugo configurations to implement an explicit allow-list of trusted hosts for resources.GetRemote calls.
• Deploy the provided Sigma rules to detect unexpected outbound network connections from build servers and similar environments.
• Ensure network connection logging is enabled on build servers and developer workstations to capture attempts to access internal or cloud metadata IPs.