fn v4_is_invalid(ip: &Ipv4Addr) -> bool {
    ip.is_loopback()
        || ip.is_private()
        || ip.is_link_local()
        || ip.is_broadcast()
        // MISSING: || ip.is_unspecified()  <- 0.0.0.0 not blocked
}

fn v4_is_invalid(ip: &Ipv4Addr) -> bool {
    ip.is_loopback()
        || ip.is_private()
        || ip.is_link_local()
        || ip.is_broadcast()
        || ip.is_unspecified()  // <- Now blocks 0.0.0.0
}

+---------------------------------------------------+
|           SSRF via 0.0.0.0 Bypass                 |
+---------------------------------------------------+
|                                                   |
|  1. Attacker hosts ActivityPub object with        |
|     URL pointing to http://0.0.0.0:<port>/path    |
|                                                   |
|  2. Lemmy instance fetches the object via         |
|     activitypub-federation-rust                   |
|                                                   |
|  3. v4_is_invalid() checks IP address:            |
|     x is_loopback()     -> false (not 127.x)     |
|     x is_private()      -> false (not RFC1918)    |
|     x is_link_local()   -> false (not 169.254.x) |
|     x is_broadcast()    -> false (not 255.x)     |
|     x is_unspecified()  -> NOT CHECKED            |
|     Result: 0.0.0.0 PASSES validation            |
|                                                   |
|  4. Request sent to 0.0.0.0 -> resolves to       |
|     localhost -> accesses internal services       |
|                                                   |
|  5. Internal service data returned to attacker    |
|     via ActivityPub federation response           |
|                                                   |
+---------------------------------------------------+