EEF-CVE-2026-82757

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EEF-CVE-2026-82757

Summary: ash_authentication_oauth2_server CIMD fetcher classifies IPv4-in-IPv6 and site-local addresses as public, allowing SSRF

Details: ## Summary Server-Side Request Forgery (SSRF) vulnerability in ash-project ash\_authentication\_oauth2\_server allows an attacker who controls a client metadata URL and its DNS to make the server connect to internal or loopback addresses. public\_ip?/1 in AshAuthentication.Oauth2Server.CIMD.ReqFetcher enforces the outbound policy for CIMD metadata fetches. It classified several address forms as publicly routable that are not: IPv4-compatible ::/96 (for example ::127.0.0.1), SIIT IPv4-translated ::ffff:0:0:0/96, and deprecated site-local fec0::/10. A returned AAAA record in one of these ranges passed the policy, so a fetch pinned to that address reached space the policy was meant to block. This issue affects ash\_authentication\_oauth2\_server: from 0.3.0 before 0.3.1. ## Configuration Reachable only when Client ID Metadata Documents are enabled (cimd\_enabled?: true), so the authorize endpoint fetches attacker-suppliable metadata URLs, and an internal or loopback target resolves to one of the affected IPv6 address forms.

References: https://github.com/ash-project/ash_authentication_oauth2_server/security/advisories/GHSA-wprp-8gvj-p6cv, https://cna.erlef.org/cves/CVE-2026-82757.html, https://github.com/ash-project/ash_authentication_oauth2_server/commit/268b591261a3473ab9b87272963e4dd2fd99d972, https://hex.pm/packages/ash_authentication_oauth2_server