basic-ftp Client-Side Denial of Service via Malicious FTP Server

The basic-ftp library, versions 5.3.0 and earlier, is susceptible to a client-side denial-of-service (DoS) attack. A malicious or compromised FTP server can exploit this vulnerability by sending an unterminated multiline response during the initial FTP banner exchange. This occurs before authentication, allowing the attacker to control the data being buffered by the client. The vulnerable client continuously appends attacker-controlled data to FtpContext._partialResponse and repeatedly reparses the growing buffer without enforcing a maximum size limit. This can lead to excessive memory consumption and CPU usage on the client-side, ultimately resulting in process-level DoS, container OOM kills, worker restarts, queue backlogs, or service degradation in applications that rely on automated FTP connections. The vulnerability was reported in May 2026.

Attack Chain

1. A victim application initiates an FTP connection to a server using the basic-ftp library.
2. The attacker, controlling the FTP server, sends an initial FTP banner that starts a multiline response (e.g., “220-malicious banner starts”).
3. The attacker intentionally omits the terminating line of the multiline response (e.g., “220 ready”).
4. The basic-ftp library’s _onControlSocketData function receives the initial chunk of data.
5. The _onControlSocketData function concatenates the received chunk with the existing _partialResponse.
6. The parseControlResponse function parses the complete response, identifies it as an incomplete multiline response, and returns the entire accumulated data as rest.
7. The _partialResponse is updated with the rest value, storing the unterminated multiline data.
8. The process repeats indefinitely with each new chunk of data, causing the _partialResponse to grow without bound, leading to memory exhaustion and DoS.

Impact

Successful exploitation of this vulnerability can result in significant disruptions to applications that utilize the basic-ftp library. Observed damage includes Node.js process memory exhaustion, container OOM kills, worker crashes or restart loops, event loop CPU pressure due to repeated parsing, stuck FTP jobs, queue backlogs in scheduled import/export systems, and degraded availability of services relying on automated FTP ingestion. This can affect a wide range of applications including SaaS applications, backend jobs, document ingestion pipelines, legacy integrations, and build/deployment pipelines.

Recommendation

• Implement the provided Sigma rule Detect Basic-ftp Unbounded Buffer DoS to detect connections to FTP servers sending excessive data before authentication.
• Upgrade to a patched version of basic-ftp that includes a maximum control response buffer size to address CVE-2026-44240.
• Configure network monitoring to detect unusually large FTP banner responses based on the network_connection log source, which may indicate a malicious FTP server.
• Implement application-level monitoring to track the memory usage of Node.js processes using basic-ftp to identify potential memory exhaustion issues.