Lemmy is a link aggregator and forum for the fediverse. Prior to version 0.7.0-beta.9, the `v4_is_invalid()` function in `activitypub-federation-rust` (`src/utils.rs`) does not check for `Ipv4Addr::UNSPECIFIED` (0.0.0.0). An unauthenticated attacker controlling a remote domain can point it to 0.0.0.0, bypass the SSRF protection introduced by the fix for CVE-2025-25194 (GHSA-7723-35v7-qcxw), and reach localhost services on the target server. Version 0.7.0-beta.9 patches the issue.

Affected: crates.io: activitypub_federation

### Summary

The `v4_is_invalid()` function in `activitypub-federation-rust` (`src/utils.rs`) does not check for `Ipv4Addr::UNSPECIFIED` (0.0.0.0). An unauthenticated attacker controlling a remote domain can point it to 0.0.0.0, bypass the SSRF protection introduced by the fix for CVE-2025-25194 (GHSA-7723-35v7-qcxw), and reach localhost services on the target server.

### Details

**File:** `src/utils.rs` in `activitypub-federation-rust` **Function:** `v4_is_invalid(v4: Ipv4Addr) -> bool`

The function checks `is_private()`, `is_loopback()`, `is_link_local()`, `is_multicast()`, and `is_documentation()` — but omits `is_unspecified()`. On Linux, macOS, and Windows, TCP connections to 0.0.0.0 are routed to localhost (127.0.0.1).

Additionally, `::ffff:0.0.0.0` (IPv4-mapped IPv6) also bypasses because `v6_is_invalid()` calls `to_ipv4_mapped().is_some_and(v4_is_invalid)`, inheriting the same gap. Notably, `v6_is_invalid()` already includes `is_unspecified()` for native IPv6, making this an asymmetric oversight.

**Independent secondary finding — DNS Rebinding TOCTOU:** `is_invalid_ip()` resolves DNS via `lookup_host()` for validation, but `reqwest` resolves DNS again for the actual connection. With TTL=0 DNS responses, an attacker can return a legitimate IP for the first resolution (passes check) and 127.0.0.1 for the second (reqwest connects to localhost). CVSS for rebinding alone: 4.8 (AC:H).

### PoC

**1. Logic Proof (reproduced from source):**

```rust fn v4_is_invalid(v4:…