CVE-2026-79913

**Summary** Cloudreve's server-side request forgery guard `ValidateExternalURL` (`pkg/request/ssrf.go`) resolves a user-supplied URL host and rejects it when any resolved IP is a loopback, private, link-local, multicast, unspecified, CGNAT, or the cloud-metadata address. The classification is performed by `checkIP`, which uses Go's `net.IP` builtins (`IsLoopback`, `IsPrivate`, `IsLinkLocalUnicast`, ...) directly on the resolved address. These builtins inspect only the outer IPv6 address and do not decode IPv4-in-IPv6 transition wrappers. An attacker who controls a hostname's AAAA record (or, on a DNS64/NAT64 network, any hostname) can point the remote-download URL at a NAT64 well-known-prefix address (`64:ff9b::a.b.c.d`, RFC 6052), an IPv4-compatible address (`::a.b.c.d`, RFC 4291), or a 6to4 address (`2002:AABB:CCDD::`, RFC 3056) that wraps an internal IPv4. Go classifies these wrappers as ordinary global IPv6 addresses, so `checkIP` accepts them; the network then delivers the request to the embedded internal IPv4 (loopback, RFC 1918, or the cloud instance metadata service `169.254.169.254`). This bypasses the SSRF guard that was added to block direct access to internal services. **Affected component and versions** - Component: `pkg/request/ssrf.go` (`ValidateExternalURL` / `checkIP`), reached from the remote-download workflow `pkg/filemanager/workflows/remote_download.go` (`RemoteDownloadTask.createDownloadTask`, which passes the user-supplied `SrcUri` to `ValidateExternalURL`). - Affected: Cloudreve `<= 4.17.0` (latest release at time of report) and current `main`. - Reachable by an authenticated remote-download user; administrative privileges are not required. **Vulnerable form vs correctly-guarded sibling** `checkIP` DOES block IPv4-mapped IPv6 (`::ffff:a.b.c.d`), because Go's `net.IP.To4()` returns the embedded IPv4 for that form and the standard checks then fire. It does NOT block the other IPv4-in-IPv6 transition forms, because for those `To4()` returns