Vulnerability-Lookup

CVE-2026-33693 (GCVE-0-2026-33693)

Vulnerability from cvelistv5 – Published: 2026-03-27 00:03 – Updated: 2026-03-30 11:51

Title

Lemmy's Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid()

Summary

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 ?

6.5 (Medium)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

CWE

CWE-918 - Server-Side Request Forgery (SSRF)

Assigner

GitHub_M

References

URL

Tags

	https://github.com/LemmyNet/lemmy/security/adviso…	x_refsource_CONFIRM
	https://github.com/LemmyNet/activitypub-federatio…	x_refsource_MISC
	https://github.com/advisories/GHSA-7723-35v7-qcxw	x_refsource_MISC

Impacted products

	Vendor	Product	Version
	LemmyNet	lemmy	Affected: < 0.7.0-beta.9

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-33693

Vulnerability from fkie_nvd - Published: 2026-03-27 01:16 - Updated: 2026-03-30 13:26

Severity ?

6.5 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/LemmyNet/activitypub-federation-rust/commit/4ae8532b17bc35755240b7f55d4a5b7665351599
	security-advisories@github.com	https://github.com/LemmyNet/lemmy/security/advisories/GHSA-q537-8fr5-cw35
	security-advisories@github.com	https://github.com/advisories/GHSA-7723-35v7-qcxw

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "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."
    }
  ],
  "id": "CVE-2026-33693",
  "lastModified": "2026-03-30T13:26:29.793",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 6.5,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "LOW",
          "integrityImpact": "LOW",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 2.5,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-03-27T01:16:18.983",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/LemmyNet/activitypub-federation-rust/commit/4ae8532b17bc35755240b7f55d4a5b7665351599"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/LemmyNet/lemmy/security/advisories/GHSA-q537-8fr5-cw35"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/advisories/GHSA-7723-35v7-qcxw"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Awaiting Analysis",
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    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-918"
        }
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      "source": "security-advisories@github.com",
      "type": "Primary"
    }
  ]
}

GHSA-Q537-8FR5-CW35

Vulnerability from github – Published: 2026-03-25 20:23 – Updated: 2026-03-27 21:36

Summary

Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid()

Details

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:

Attacker configures DNS: evil.com A → 0.0.0.0
1. Attacker sends ActivityPub activity referencing https://evil.com/actor
1. Library calls verify_url_valid() → is_invalid_ip() → resolves to 0.0.0.0
1. v4_is_invalid(0.0.0.0) returns false (BYPASS)
1. 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()?;

Severity ?

6.5 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "crates.io",
        "name": "activitypub_federation"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.7.0-beta.9"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33693"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-918"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-25T20:23:38Z",
    "nvd_published_at": "2026-03-27T01:16:18Z",
    "severity": "MODERATE"
  },
  "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) -\u003e bool`\n\nThe function checks `is_private()`, `is_loopback()`, `is_link_local()`, `is_multicast()`, and `is_documentation()` \u2014 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 \u2014 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) \u2192 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 \u2192 0.0.0.0`\n2. 2. Attacker sends ActivityPub activity referencing `https://evil.com/actor`\n3. 3. Library calls `verify_url_valid()` \u2192 `is_invalid_ip()` \u2192 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 \u2192 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```",
  "id": "GHSA-q537-8fr5-cw35",
  "modified": "2026-03-27T21:36:54Z",
  "published": "2026-03-25T20:23:38Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Activitypub-Federation has SSRF via 0.0.0.0 bypass in activitypub-federation-rust v4_is_invalid()"
}

Sightings

Author	Source	Type	Date

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2026-33693 (GCVE-0-2026-33693)

FKIE_CVE-2026-33693

GHSA-Q537-8FR5-CW35

Summary

Details

PoC

Impact

Suggested Fix

Tags

Sightings

Nomenclature