{"id":"CVE-2026-33693","aliases":["EUVD-2026-16494","GHSA-q537-8fr5-cw35"],"status":"published","title":"Lemmy's Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid()","description":"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.","severity":{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-09","score":0.00373,"percentile":0.31524,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":true,"fix_available":true},"cwes":["CWE-918"],"published":"2026-03-25T20:23:38Z","modified":"2026-10-28T02:31:01Z","exploits":[{"source":"pocgithub","id":"SnailSploit/CVE-2026-33693","title":"SnailSploit/CVE-2026-33693","date":"2026-03-23","url":"https://github.com/SnailSploit/CVE-2026-33693"}],"fixes":[{"ecosystem":"crates.io","package":"activitypub_federation","fixed_in":["0.7.0-beta.9"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"crates.io","package":"activitypub_federation","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.7.0-beta.9"}]}]},{"source":"osv","ecosystem":"crates.io","package":"activitypub_federation","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"0.7.0-beta.9"}]}]}],"products":[{"vendor":"lemmynet","product":"lemmy"}],"references":[{"url":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-q537-8fr5-cw35","tags":["x_refsource_CONFIRM","advisory","web"]},{"url":"https://github.com/LemmyNet/activitypub-federation-rust/commit/4ae8532b17bc35755240b7f55d4a5b7665351599","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/advisories/GHSA-7723-35v7-qcxw","tags":["x_refsource_MISC","advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-33693","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-q537-8fr5-cw35"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33693.json","tags":["advisory"]},{"url":"https://github.com/LemmyNet/activitypub-federation-rust","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-33693","title":"Lemmy's Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid()","description":"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.","status":"published","published":"2026-03-27T00:03:35.946Z","modified":"2026-03-30T11:51:10.425Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium","assessor":"cna"}],"cwes":["CWE-918"],"updated_at":"2026-11-02T14:20:07.609948Z"}],"euvd":[{"id":"EUVD-2026-16494","description":"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.","status":"published","published":"2026-03-27T00:03:35Z","modified":"2026-03-30T11:51:10Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium"}],"updated_at":"2026-11-02T19:25:26.376466Z"}],"ghsa":[{"id":"GHSA-q537-8fr5-cw35","title":"Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid()","description":"### 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) -\u003e 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) -\u003e 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) -\u003e 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```","status":"published","published":"2026-03-25T20:23:38Z","modified":"2026-03-27T21:36:54Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium"}],"cwes":["CWE-918"],"updated_at":"2026-11-02T14:39:43.747579Z"}],"nvd":[{"id":"CVE-2026-33693","description":"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.","status":"published","published":"2026-03-27T01:16:18.983Z","modified":"2026-06-17T10:37:56.163Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium","assessor":"secondary"}],"cwes":["CWE-918"],"updated_at":"2026-11-02T14:12:11.678711Z"}],"osv":[{"id":"CVE-2026-33693","title":"Lemmy's Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid()","description":"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.","status":"published","published":"2026-03-27T00:03:35.946Z","modified":"2026-10-28T02:31:01.957619807Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium"}],"cwes":["CWE-918"],"updated_at":"2026-11-02T15:45:01.896044Z"},{"id":"GHSA-q537-8fr5-cw35","title":"Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid()","description":"### 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) -\u003e 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) -\u003e 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) -\u003e 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```","status":"published","published":"2026-03-25T20:23:38Z","modified":"2026-04-02T13:44:32.200396077Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N","level":"medium"}],"cwes":["CWE-918"],"updated_at":"2026-11-02T15:46:54.055966Z"}]}}