CVE-2026-33693: Activitypub federation Activitypub-Federation Has SSRF Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-q537-8fr5-cw35",
  "modified" : "2026-03-27T21:36:54Z",
  "published" : "2026-03-25T20:23:38Z",
  "aliases" : [ "CVE-2026-33693" ],
  "summary" : "Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid()",
  "details" : "### Summary\n\nThe `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.\n\n### Details\n\n**File:** `src/utils.rs` in `activitypub-federation-rust`\n**Function:** `v4_is_invalid(v4: Ipv4Addr) -> bool`\n\nThe 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).\n\nAdditionally, `::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.\n\n**Independent secondary finding — DNS Rebinding TOCTOU:**\n`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).\n\n### PoC\n\n**1. Logic Proof (reproduced from source):**\n\n```rust\nfn v4_is_invalid(v4: Ipv4Addr) -> bool {\n    v4.is_private()\n        || v4.is_loopback()\n        || v4.is_link_local()\n        || v4.is_multicast()\n        || v4.is_documentation()\n    // BUG: Missing || v4.is_unspecified()\n}\n\nassert_eq!(v4_is_invalid(Ipv4Addr::UNSPECIFIED), false);  // 0.0.0.0 PASSES validation\nassert_eq!(v4_is_invalid(Ipv4Addr::LOCALHOST), true);     // 127.0.0.1 correctly blocked\n```\n\n**2. OS Routing Verification:**\n\n```\n$ connect(0.0.0.0:80) → ConnectionRefused\n```\n\nConnectionRefused proves the OS routed to localhost (port 80 not listening). Any service on 0.0.0.0:PORT is reachable.\n\n**3. Attack Chain:**\n\n1. Attacker configures DNS: `evil.com A → 0.0.0.0`\n2. 2. Attacker sends ActivityPub activity referencing `https://evil.com/actor`\n3. 3. Library calls `verify_url_valid()` → `is_invalid_ip()` → resolves to 0.0.0.0\n4. 4. `v4_is_invalid(0.0.0.0)` returns `false` (BYPASS)\n5. 5. `reqwest` connects to 0.0.0.0 → reaches localhost services\n### Impact\n\n- **Direct:** Bypasses the SSRF protection layer for all ActivityPub federation traffic\n- - **Downstream:** 6+ dependent projects affected including Lemmy (13.7k stars), hatsu, gill, ties, fediscus, fediverse-axum\n- - **Attacker can:** Access cloud instance metadata (169.254.169.254 via rebinding), reach internal services on localhost, port scan internal infrastructure\n### Suggested Fix\n\n```rust\nfn v4_is_invalid(v4: Ipv4Addr) -> bool {\n    v4.is_private()\n        || v4.is_loopback()\n        || v4.is_link_local()\n        || v4.is_multicast()\n        || v4.is_documentation()\n        || v4.is_unspecified()    // ADD: blocks 0.0.0.0\n        || v4.is_broadcast()      // ADD: blocks 255.255.255.255\n}\n```\n\nFor DNS rebinding TOCTOU, pin the resolved IP:\n\n```rust\nlet resolved_ip = lookup_host((domain, 80)).await?;\n// validate resolved_ip...\nlet client = reqwest::Client::builder()\n    .resolve(domain, resolved_ip)  // pin resolution\n    .build()?;\n```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "activitypub_federation"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.7.0-beta.9"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/LemmyNet/lemmy/security/advisories/GHSA-q537-8fr5-cw35"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-33693"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/LemmyNet/activitypub-federation-rust/commit/4ae8532b17bc35755240b7f55d4a5b7665351599"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/LemmyNet/activitypub-federation-rust"
  }, {
    "type" : "ADVISORY",
    "url" : "https://github.com/advisories/GHSA-7723-35v7-qcxw"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-918" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-25T20:23:38Z",
    "nvd_published_at" : "2026-03-27T01:16:18Z"
  }
}