1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192using MailSharp.Common;
using MailSharp.SMTP.Extensions;
using System.Security.Cryptography;
using System.Text;
using System.Text.Json;

namespace MailSharp.SMTP.Session;

public partial class SmtpSession
{
	// Handle AUTH command
	private async Task HandleAuthAsync(string[] parts, string line, CancellationToken ct)
	{
		bool tlsActive = security == SecurityEnum.Tls || state == SmtpState.TlsStarted;
		if (ipGroup == null || (ipGroup.Access.RequireSslTlsForAuth && !tlsActive))
		{
			await writer.WriteLineAsync(configuration["SmtpResponses:CommandNotRecognized"], ct);
			return;
		}

		if (state != SmtpState.HeloReceived && state != SmtpState.TlsStarted)
		{
			await writer.WriteLineAsync(configuration["SmtpResponses:BadSequence"], ct);
			return;
		}

		if (security == SecurityEnum.StartTls && state != SmtpState.TlsStarted)
		{
			await writer.WriteLineAsync(configuration["SmtpResponses:TlsRequired"], ct);
			return;
		}

		if (parts.Length < 2)
		{
			await writer.WriteLineAsync(configuration["SmtpResponses:SyntaxError"], ct);
			return;
		}

		string mechanism = parts[1].ToUpper();
		if (mechanism == "PLAIN")
		{
			// Handle AUTH PLAIN
			string? credentials = parts.Length > 2 ? parts[2] : await reader.ReadLineAsync(ct);
			if (credentials == null)
			{
				await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
				metrics.IncrementAuthFailed();
				return;
			}

			try
			{
				string decoded = Encoding.UTF8.GetString(Convert.FromBase64String(credentials));
				string[] credentialParts = decoded.Split('\0');
				if (credentialParts.Length != 3 || !await ValidateCredentialsAsync(credentialParts[1], credentialParts[2]))
				{
					await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
					metrics.IncrementAuthFailed();
					return;
				}
				await writer.WriteLineAsync(configuration["SmtpResponses:AuthSuccess"], ct);
				metrics.IncrementAuthSuccess();
			}
			catch
			{
				await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
				metrics.IncrementAuthFailed();
			}
		}
		else if (mechanism == "CRAM-MD5")
		{
			// Handle AUTH CRAM-MD5
			string challenge = $"<{Guid.NewGuid()}.{DateTime.UtcNow.Ticks}@{configuration["SmtpSettings:Host"]}>";
			await writer.WriteLineAsync(string.Format(configuration["SmtpResponses:CramMd5Challenge"]!, Convert.ToBase64String(Encoding.UTF8.GetBytes(challenge))), ct);

			string? response = await reader.ReadLineAsync(ct);
			if (response == null)
			{
				await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
				metrics.IncrementAuthFailed();
				return;
			}

			try
			{
				string decodedResponse = Encoding.UTF8.GetString(Convert.FromBase64String(response));
				string[] responseParts = decodedResponse.Split(' ');
				if (responseParts.Length != 2)
				{
					await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
					metrics.IncrementAuthFailed();
					return;
				}

				string username = responseParts[0];
				string clientDigest = responseParts[1];
				string? password = configuration[$"SmtpSettings:Credentials:{username}"];
				if (password == null || !ValidateCramMd5(challenge, password, clientDigest))
				{
					await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
					metrics.IncrementAuthFailed();
					return;
				}
				await writer.WriteLineAsync(configuration["SmtpResponses:AuthSuccess"], ct);
				metrics.IncrementAuthSuccess();
			}
			catch
			{
				await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
				metrics.IncrementAuthFailed();
			}
		}
		else if (mechanism == "LOGIN")
		{
			// Handle AUTH LOGIN
			await writer.WriteLineAsync(configuration["SmtpResponses:AuthLoginUsernamePrompt"], ct);
			string? usernameBase64 = await reader.ReadLineAsync(ct);
			if (usernameBase64 == null)
			{
				await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
				metrics.IncrementAuthFailed();
				return;
			}

			await writer.WriteLineAsync(configuration["SmtpResponses:AuthLoginPasswordPrompt"], ct);
			string? passwordBase64 = await reader.ReadLineAsync(ct);
			if (passwordBase64 == null)
			{
				await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
				metrics.IncrementAuthFailed();
				return;
			}

			try
			{
				string username = Encoding.UTF8.GetString(Convert.FromBase64String(usernameBase64));
				string password = Encoding.UTF8.GetString(Convert.FromBase64String(passwordBase64));
				if (!await ValidateCredentialsAsync(username, password))
				{
					await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
					metrics.IncrementAuthFailed();
					return;
				}
				await writer.WriteLineAsync(configuration["SmtpResponses:AuthSuccess"], ct);
				metrics.IncrementAuthSuccess();
			}
			catch
			{
				await writer.WriteLineAsync(configuration["SmtpResponses:AuthFailed"], ct);
				metrics.IncrementAuthFailed();
			}
		}
		else
		{
			await writer.WriteLineAsync(configuration["SmtpResponses:SyntaxError"], ct);
		}
	}

	// Validate PLAIN and LOGIN credentials
	private async Task<bool> ValidateCredentialsAsync(string username, string password)
	{
		try
		{
			string userStorePath = configuration["SmtpSettings:UserStorePath"] ?? throw new InvalidOperationException("UserStorePath not configured");
			if (!File.Exists(userStorePath))
			{
				logger.LogWarning("User store file {0} not found", userStorePath);
				return false;
			}

			string json = await File.ReadAllTextAsync(userStorePath);
			var users = JsonSerializer.Deserialize<List<UserConfig>>(json) ?? throw new InvalidOperationException("Invalid user store format");
			var user = users.FirstOrDefault(u => u.Username == username && u.Password == password);

			return user != null;
		}
		catch (Exception ex)
		{
			logger.LogError("Error validating credentials for user {0}: {1}", username, ex.Message);
			return false;
		}
	}

	// Validate CRAM-MD5 response
	private static bool ValidateCramMd5(string challenge, string password, string clientDigest)
	{
		using HMACMD5 hmac = new(Encoding.UTF8.GetBytes(password));
		byte[] hash = hmac.ComputeHash(Encoding.UTF8.GetBytes(challenge));
		string expectedDigest = BitConverter.ToString(hash).Replace("-", "").ToLower();
		return clientDigest.Equals(expectedDigest, StringComparison.OrdinalIgnoreCase);
	}
}