projectdiscovery/nuclei · error

failed to send MS-TNAP login packet: %w

Error message

failed to send MS-TNAP login packet: %w

What it means

Thrown by telnet.DetectNTLM when writing the crafted MS-TNAP login packet to the TCP connection fails. The dial itself succeeded (protocolstate dialer connected, a 10-second deadline was set), but the subsequent conn.Write of telnetmini.CreateTNAPLoginPacket() returned an error — the connection was broken between dial and write, typically a reset by the server or a middlebox.

Source

Thrown at pkg/js/libs/telnet/telnet.go:253

	}()

	// Create telnet client using the telnetmini library
	client := telnetmini.New(conn)
	defer func() {
		_ = client.Close()
	}()

	// Set timeout
	_ = conn.SetDeadline(time.Now().Add(10 * time.Second))

	// Use the MS-TNAP packet crafting functions from our telnetmini library
	// Create MS-TNAP Login Packet (Option Command IS) as per Nmap script
	tnapLoginPacket := telnetmini.CreateTNAPLoginPacket()

	// Send the MS-TNAP login packet
	_, err = conn.Write(tnapLoginPacket)
	if err != nil {
		return nil, fmt.Errorf("failed to send MS-TNAP login packet: %w", err)
	}

	// Read response data
	buffer := make([]byte, 4096)
	n, err := conn.Read(buffer)
	if err != nil {
		return nil, fmt.Errorf("failed to read response: %w", err)
	}

	if n == 0 {
		return nil, fmt.Errorf("no response received")
	}

	// Parse NTLM response using our telnetmini library functions
	response := buffer[:n]

	// Use the parsing functions from our library instead of reimplementing
	// This should use the NTLM parsing functions we added to telnetmini

View on GitHub (pinned to 265b3a3dec)

Solutions

1. Verify the target actually speaks telnet on that port before calling DetectNTLM
2. Retry once — transient resets are common during scanning
3. Check local egress/firewall rules if every host fails at the write step

// confirm the port actually speaks telnet before the NTLM probe
// (e.g. banner check), then call DetectNTLM

let r = null; for (let i = 0; i < 2 && !r; i++) { try { r = client.DetectNTLM(host, port); } catch (e) { if (!String(e).includes('send MS-TNAP')) throw e; } }

When it happens

Trigger: Target accepts the TCP handshake but immediately resets non-telnet or suspicious traffic; a firewall/IPS dropping the MS-TNAP packet; the service closing the connection during protocol negotiation before this step; transient network failure mid-handshake.

Common situations: Scanning hosts where port 23 is forwarded to a non-telnet service; security devices resetting anomalous payloads; flaky links to distant targets.

Related errors

• no response received
• failed to read response: %w
• failed to parse NTLM response: %w
• authentication failed
• prompt not found (read cap reached)

AI-assisted analysis of projectdiscovery/nuclei@265b3a3dec (2026-08-15). Data as JSON: /api/errors/60be5ba4486d13d4. Report an issue: GitHub.