SearXNG Basic Authentication Credentials Exposed Through MCP Logs and JSON-RPC Error Responses

MCP logging notification disclosure

After the MCP client connects, src/index.ts:388-393 sends the complete URL through the MCP logging interface:

const searxngInstances = getSearxngInstances();

logMessage(
  mcpServer,
  "info",
  `SearXNG URLs: ${
    searxngInstances.length > 0
      ? searxngInstances.join("; ")
      : "not configured"
  }`
);

logMessage() passes this value to sendLoggingMessage() in src/logging.ts:15-25:

mcpServer.sendLoggingMessage({
  level,
  data: notificationData
});

As a result, the connected MCP client receives a message containing the username and password:

{
  "method": "notifications/message",
  "params": {
    "level": "info",
    "data": {
      "message": "SearXNG URLs: http://username:[email protected]"
    }
  },
  "jsonrpc": "2.0"
}

Configuration error disclosure

The URL validation function includes the complete unredacted value in error messages.

Relevant code in src/searxng-instances.ts:44-52:

export function validateSearxngInstanceUrl(
  value: string
): string | null {
  try {
    const url = new URL(value);

    if (!["http:", "https:"].includes(url.protocol)) {
      return `SEARXNG_URL invalid protocol for "${value}": ${url.protocol}`;
    }
  } catch {
    return `SEARXNG_URL invalid format: ${value}`;
  }

  return null;
}

The validation error is aggregated by validateEnvironment() in src/error-handler.ts:175-203:

const validationError =
  validateSearxngInstanceUrl(searxngUrl);

if (validationError) {
  issues.push(validationError);
}

The complete error is then thrown from src/search.ts:689-693:

const validationError = validateEnvironment();

if (validationError) {
  logMessage(mcpServer, "error", "Configuration invalid");
  throw new MCPSearXNGError(validationError);
}

The tool handler in src/index.ts:254-260 sends the error message and stack trace through MCP logging, then rethrows it:

logMessage(
  mcpServer,
  "error",
  `Tool execution error: ${
    error instanceof Error
      ? error.message
      : String(error)
  }`,
  {
    tool: name,
    args: args,
    error:
      error instanceof Error
        ? error.stack
        : String(error)
  }
);

throw error;

Rethrowing the error causes the same unredacted credential-bearing URL to be returned in the JSON-RPC error response.

Existing redaction function is not used

The project already contains a suitable redaction function in src/searxng-instances.ts:57-69:

export function redactSearxngInstanceUrl(
  raw: string
): string {
  try {
    const url = new URL(raw);

    if (!url.username && !url.password) {
      return raw;
    }

    url.username = "";
    url.password = "";
    return url.toString();
  } catch {
    return raw.replace(
      /^([a-zA-Z][a-zA-Z0-9+.-]*:\/\/)[^/]*@/,
      "$1"
    );
  }
}

However, this function is not applied before startup logging, MCP logging, or configuration error construction.

The MCP manifest also marks SEARXNG_URL as non-secret in .mcp/server.json:20-25:

{
  "name": "SEARXNG_URL",
  "description": "URL of your SearXNG instance",
  "isRequired": true,
  "isSecret": false,
  "format": "string"
}

Because credentials may be embedded in this variable, it should be classified as a secret.

PoC

The following proof of concept uses fake credentials. A real SearXNG server is not required.

Requirements

Node.js 20 or newer
npm
mcp-searxng 1.11.0 source code

Build the application

unzip mcp-searxng-main.zip
cd mcp-searxng-main

npm ci
npm run build

Test 1: Credential disclosure through MCP logging

Create an MCP initialization request:

cat > /tmp/mcp-init.jsonl <<'EOF'
{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"credential-leak-poc","version":"1.0.0"}}}
EOF

Start the server with fake credentials embedded in a valid HTTP URL:

SEARXNG_URL='http://MCP_POC_USER_7391:[email protected]:9' \
timeout 8s node dist/cli.js \
< /tmp/mcp-init.jsonl \
2>&1 | tee credential-log-leak.txt

Search the output for the credentials:

grep -nE \
'MCP_POC_USER_7391|MCP_POC_PASS_7391' \
credential-log-leak.txt

Observed result

The complete credential-bearing URL is exposed:

SearXNG URLs: http://MCP_POC_USER_7391:[email protected]:9

It is also delivered to the MCP client:

{
  "method": "notifications/message",
  "params": {
    "level": "info",
    "data": {
      "message": "SearXNG URLs: http://MCP_POC_USER_7391:[email protected]:9"
    }
  },
  "jsonrpc": "2.0"
}

This confirms that a connected MCP client can recover the configured username and password without accessing the host environment.

Test 2: Credential disclosure through JSON-RPC errors

Create initialization and tool-call requests:

cat > /tmp/mcp-error-poc.jsonl <<'EOF'
{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"credential-error-poc","version":"1.0.0"}}}
{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"searxng_web_search","arguments":{"query":"credential leak test"}}}
EOF

Start the server with a credential-bearing URL that uses an unsupported protocol:

SEARXNG_URL='ftp://MCP_POC_USER_7391:[email protected]' \
timeout 8s node dist/cli.js \
< /tmp/mcp-error-poc.jsonl \
2>&1 | tee credential-error-leak.txt

Search the response:

grep -nE \
'MCP_POC_USER_7391|MCP_POC_PASS_7391' \
credential-error-leak.txt

Observed result

The complete URL is exposed in the MCP logging notification:

Tool execution error: Configuration Issues: SEARXNG_URL invalid protocol for "ftp://MCP_POC_USER_7391:[email protected]": ftp:

It is also returned directly in the JSON-RPC error:

{
  "jsonrpc": "2.0",
  "id": 2,
  "error": {
    "code": -32603,
    "message": "Configuration Issues: SEARXNG_URL invalid protocol for \"ftp://MCP_POC_USER_7391:[email protected]\": ftp:"
  }
}

The raw username and password are therefore exposed through both logging and protocol responses.

Impact

This is a sensitive credential disclosure vulnerability.

The following parties may obtain the credentials:

1. A connected MCP client receiving logging notifications.
2. A client capable of invoking a tool and receiving JSON-RPC errors.
3. A user or process with access to captured stderr output.
4. A centralized logging or monitoring system collecting application logs.
5. Other users with access to shared log files or container logs.

The exposed credentials may allow an attacker to authenticate directly to the configured SearXNG instance.

Depending on the SearXNG deployment and the permissions associated with the account, this may allow:

1. Unauthorized use of a private SearXNG service.
2. Access to functionality restricted through Basic Authentication.
3. Consumption of private server resources.
4. Exposure of information available only to authenticated users.
5. Further account compromise where the credentials have been reused.

The default STDIO transport limits the exposure to the connected parent MCP client and local logging environment. However, MCP clients should not receive upstream service credentials, and the project security documentation explicitly treats credentials embedded in SEARXNG_URL as secrets that must be redacted.

Suggested mitigation

Apply redactSearxngInstanceUrl() before including any SearXNG URL in console or MCP logging:

const redactedInstances = getSearxngInstances()
  .map(redactSearxngInstanceUrl);

logMessage(
  mcpServer,
  "info",
  `SearXNG URLs: ${
    redactedInstances.length > 0
      ? redactedInstances.join("; ")
      : "not configured"
  }`
);

Do not include raw configuration values in validation errors. A generic error can be returned instead:

return `SEARXNG_URL entry has an unsupported protocol: ${url.protocol}`;

For malformed URLs:

return "SEARXNG_URL contains an invalid URL";

The following additional changes are recommended:

1. Redact URLs before writing them to stderr.
2. Redact secrets before sending MCP logging notifications.
3. Avoid including raw environment-variable values in exceptions.
4. Avoid returning detailed stack traces containing secrets to MCP clients.
5. Mark SEARXNG_URL as secret in .mcp/server.json:

"isSecret": true

6. Add regression tests that assert usernames and passwords never appear in:

   • stderr output
   • MCP logging notifications
   • JSON-RPC error responses
   • stack traces
   • configuration resources