CVE-2026-33693: 
Rust vulnerability analysis and mitigation

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):

fn v4_is_invalid(v4: Ipv4Addr) -> bool {
    v4.is_private()
        || v4.is_loopback()
        || v4.is_link_local()
        || v4.is_multicast()
        || v4.is_documentation()
    // BUG: Missing || v4.is_unspecified()
}
assert_eq!(v4_is_invalid(Ipv4Addr::UNSPECIFIED), false);  // 0.0.0.0 PASSES validation
assert_eq!(v4_is_invalid(Ipv4Addr::LOCALHOST), true);     // 127.0.0.1 correctly blocked

2. OS Routing Verification:

$ connect(0.0.0.0:80) → ConnectionRefused

ConnectionRefused proves the OS routed to localhost (port 80 not listening). Any service on 0.0.0.0:PORT is reachable. 3. Attack Chain:

1. Attacker configures DNS: evil.com A → 0.0.0.0
2. 2. Attacker sends ActivityPub activity referencing https://evil.com/actor
3. 3. Library calls verify_url_valid() → is_invalid_ip() → resolves to 0.0.0.0
4. 4. v4_is_invalid(0.0.0.0) returns false (BYPASS)
5. 5. reqwest connects to 0.0.0.0 → reaches localhost services

Impact

• Direct: Bypasses the SSRF protection layer for all ActivityPub federation traffic
• • Downstream: 6+ dependent projects affected including Lemmy (13.7k stars), hatsu, gill, ties, fediscus, fediverse-axum
• • Attacker can: Access cloud instance metadata (169.254.169.254 via rebinding), reach internal services on localhost, port scan internal infrastructure

Suggested Fix

fn v4_is_invalid(v4: Ipv4Addr) -> bool {
    v4.is_private()
        || v4.is_loopback()
        || v4.is_link_local()
        || v4.is_multicast()
        || v4.is_documentation()
        || v4.is_unspecified()    // ADD: blocks 0.0.0.0
        || v4.is_broadcast()      // ADD: blocks 255.255.255.255
}

For DNS rebinding TOCTOU, pin the resolved IP:

let resolved_ip = lookup_host((domain, 80)).await?;
// validate resolved_ip...
let client = reqwest::Client::builder()
    .resolve(domain, resolved_ip)  // pin resolution
    .build()?;