| パッケージ | 影響を受けるバージョン | 修正済みバージョン |
|---|---|---|
| basic-ftp | < 5.3.1 | 5.3.1 |
basic-ftp is vulnerable to client-side denial of service when parsing FTP control-channel multiline responses.
A malicious or compromised FTP server can send an unterminated multiline response during the initial FTP banner phase, before authentication. The client keeps appending attacker-controlled data into FtpContext._partialResponse and repeatedly reparses the accumulated buffer without enforcing a maximum control response size.
As a result, an application using basic-ftp can remain stuck in connect() while memory and CPU usage grow under attacker-controlled input. This can lead to process-level denial of service, container OOM kills, worker restarts, queue backlog, or service degradation in applications that automatically connect to FTP endpoints.
The root cause is that incomplete FTP multiline control responses are buffered without an upper bound.
FtpContext stores incomplete control-channel data in _partialResponse:
Incoming control-channel data is handled in _onControlSocketData. The implementation concatenates the previous incomplete response with the new chunk, parses the entire accumulated string, and stores parsed.rest back into _partialResponse:
The relevant flow is:
completeResponse = this._partialResponse + chunk parsed = parseControlResponse(completeResponse) this._partialResponse = parsed.rest
There is no maximum size check before concatenating, before parsing, or before storing parsed.rest.
The parser accepts incomplete multiline responses and returns the entire unterminated multiline group as rest:
If a server starts a multiline FTP response:
これらの製品をお使いですか？サイトをスキャンして、脆弱なバージョンが公開されていないか確認しましょう。
無料でサイトをスキャン220-malicious banner starts
but never sends the terminating line:
220 ready
then parseControlResponse() treats the accumulated multiline data as incomplete and returns it as rest.
Because _onControlSocketData() feeds _partialResponse + chunk back into the parser on every new data event, the client repeatedly reparses a growing attacker-controlled buffer. This creates both memory growth and increasing parsing work.
The vulnerable component is a client library. The attacker does not need to authenticate to the victim system and does not need valid FTP credentials.
The attack occurs automatically when an application using basic-ftp connects to a malicious or compromised FTP server. The malicious response is sent as the FTP server banner before login. No additional user interaction is required after the application initiates a normal FTP connection.
This is realistic for applications that use FTP for:
In those environments, one malicious or compromised FTP endpoint can cause the Node.js process using basic-ftp to consume excessive memory and CPU or remain stuck in a pending connection state.
The PoC uses a local malicious FTP server that accepts a victim connection and sends an unterminated multiline FTP banner. The banner starts with 220-, but the server never sends the required terminating 220 line.
From the root of the basic-ftp project:
npm ci
npm run buildOnly
CHUNKS=1000 node poc_control_parser_direct.js | tee poc-results/parser_direct_1000.log
Run the end-to-end malicious FTP server PoC:
CHUNK_SIZE=8192 CHUNKS=1000 DELAY_MS=1 node poc_control_multiline_dos.js | tee poc-results/control_multiline_dos_1000.log
[basic-ftp parseControlResponse incomplete multiline DoS]
Input fed: 7.81 MiB
Retained rest: 7.81 MiB
Initial rss/heap: 54.77 MiB 3.69 MiB
Final rss/heap: 141.64 MiB 80.77 MiB
This shows that parseControlResponse() retained the full unterminated multiline response as rest.
The retained buffer grew to 7.81 MiB. Heap usage increased from 3.69 MiB to 80.77 MiB, and RSS increased from 54.77 MiB to 141.64 MiB.
[server] listening on 127.0.0.1:34429
[server] victim connected
[progress] chunks=850 sent=6.6 MiB partialResponse=6.6 MiB heapUsed=227.5 MiB rss=292.4 MiB
[progress] chunks=900 sent=7.0 MiB partialResponse=7.0 MiB heapUsed=213.1 MiB rss=278.0 MiB
[final-before-close] chunks=1000 sent=7.8 MiB partialResponse=7.8 MiB heapUsed=82.1 MiB rss=146.8 MiB
[result] client connect() is still pending because the multiline response never terminated
Only 7.8 MiB of malicious control-channel data was sent. The client retained 7.8 MiB in _partialResponse, showed large memory spikes, and remained pending inside connect() because the multiline response was never terminated.
The client should enforce a maximum size for incomplete FTP control responses. If the accumulated multiline response exceeds a safe limit, the client should close the connection and reject the active task with an error.
The client should not allow a remote FTP server to make _partialResponse grow without bound.
A malicious FTP server can keep the client in a pending connection state by sending an unterminated multiline control response. basic-ftp continues buffering and reparsing the accumulated data without a maximum response size.
A malicious or compromised FTP server can cause denial of service in applications using basic-ftp.
Possible real-world impact includes:
The attacker controls, compromises, or can impersonate an FTP server that a victim application connects to.
Examples:
In each case, the victim application initiates a normal FTP connection. The malicious server sends an unterminated multiline banner before authentication. The vulnerable client then buffers and reparses the response indefinitely.
No FTP credentials are required for exploitation because the attack happens before login.
Introduce a maximum control response buffer size, especially for incomplete multiline responses.
Recommended changes:
maxControlResponseBytes or maxControlResponseLength limit.Example defensive logic:
if (completeResponse.length > maxControlResponseLength) {
closeWithError(new Error("FTP control response exceeded maximum allowed size"))
}
A regression test should verify that a response beginning with 220- and never terminating with 220 is rejected after the configured size limit instead of being retained indefinitely.
A test server should:
220-malicious banner\r\n.220 line._partialResponse does not grow without bound.If you publish an advisory or assign a CVE, please credit me as:
Ali Firas (thesmartshadow) - https://www.smartshadow.dev