SANS ISC: Attackers Can Use Hostnames to Bypass IP-String Filters for Cloud Metadata SSRF

The SANS Internet Storm Center reported that IP-address-based filtering — such as blocking or matching the literal string 169.254.169.254 — can be bypassed because software that accepts an IP address will typically also accept a hostname that resolves to it. The diary follows up on the ISC's earlier reporting of scans targeting the cloud metadata service at 169.254.169.254, which attempted to exploit Server-Side Request Forgery (SSRF) vulnerabilities.

Why It Matters

Many organizations mitigate SSRF risk against cloud metadata endpoints by blocklisting the literal IP string 169.254.169.254 in request filters, WAF rules, or application-layer validation. If an attacker can instead supply a hostname that resolves to that same address via DNS, string-based defenses that never resolve the hostname will not recognize the target and can be silently bypassed. This affects any environment relying on IP-metadata endpoints (AWS, Azure, GCP, and similar) where SSRF is a viable attack path.

What Defenders Should Watch For

• Audit SSRF mitigations to confirm whether they filter on literal IP strings only, or whether they resolve and validate hostnames before allowing outbound requests.
• Consider network-layer controls (e.g., blocking access to the metadata IP range at the host/network level, or requiring IMDSv2-style token-based metadata access) rather than relying solely on string matching in application code.
• Hunt for DNS resolutions to hostnames pointing at 169.254.169.254 or other link-local/metadata addresses in DNS logs.
• Review outbound request validation logic for any service that accepts user-supplied URLs or hostnames as parameters.

This is based on a single SANS ISC diary entry and represents an observation about a filtering weakness rather than a confirmed active exploitation campaign; details may evolve as the ISC or others follow up. Read the original diary at isc.sans.edu.