{"record":{"id":"201a78f53e8ccc69","repo":"jstedfast/MailKit","slug":"missingchallenge-201a78","errorCode":"MissingChallenge","errorMessage":"Server response did not contain any authentication data.","messagePattern":"Server response did not contain any authentication data\\.","errorType":"error_code","errorClass":"SaslException","httpStatus":null,"severity":"error","filePath":"MailKit/Security/SaslMechanismScramBase.cs","lineNumber":337,"sourceCode":"\t\t\t\t//\n\t\t\t\t// Based on this, we attempt to use \"tls-server-end-point\" instead of \"tls-unique\" when available.\n\t\t\t\tif (SupportsChannelBinding) {\n\t\t\t\t\tif (TryGetChannelBindingToken (ChannelBindingKind.Endpoint, out channelBindingToken)) {\n\t\t\t\t\t\tchannelBindingKind = ChannelBindingKind.Endpoint;\n\t\t\t\t\t} else if (TryGetChannelBindingToken (ChannelBindingKind.Unique, out channelBindingToken)) {\n\t\t\t\t\t\tchannelBindingKind = ChannelBindingKind.Unique;\n\t\t\t\t\t} else {\n\t\t\t\t\t\tchannelBindingKind = ChannelBindingKind.Unknown;\n\t\t\t\t\t}\n\t\t\t\t}\n\n\t\t\t\tinput = GetChannelBindingInput (channelBindingKind, AuthorizationId);\n\t\t\t\tresponse = Encoding.UTF8.GetBytes (input + client);\n\t\t\t\tstate = LoginState.Final;\n\t\t\t\tbreak;\n\t\t\tcase LoginState.Final:\n\t\t\t\tif (token == null)\n\t\t\t\t\tthrow new SaslException (MechanismName, SaslErrorCode.MissingChallenge, \"Server response did not contain any authentication data.\");\n\n\t\t\t\tserver = Encoding.UTF8.GetString (token, startIndex, length);\n\t\t\t\tvar tokens = ParseServerChallenge (server);\n\t\t\t\tstring? salt, nonce, iterations;\n\t\t\t\tint count;\n\n\t\t\t\tif (!tokens.TryGetValue ('s', out salt))\n\t\t\t\t\tthrow new SaslException (MechanismName, SaslErrorCode.IncompleteChallenge, \"Challenge did not contain a salt.\");\n\n\t\t\t\tif (!tokens.TryGetValue ('r', out nonce))\n\t\t\t\t\tthrow new SaslException (MechanismName, SaslErrorCode.IncompleteChallenge, \"Challenge did not contain a nonce.\");\n\n\t\t\t\tif (!tokens.TryGetValue ('i', out iterations))\n\t\t\t\t\tthrow new SaslException (MechanismName, SaslErrorCode.IncompleteChallenge, \"Challenge did not contain an iteration count.\");\n\n\t\t\t\tif (!nonce.StartsWith (cnonce!, StringComparison.Ordinal))\n\t\t\t\t\tthrow new SaslException (MechanismName, SaslErrorCode.InvalidChallenge, \"Challenge contained an invalid nonce.\");\n","sourceCodeStart":319,"sourceCodeEnd":355,"githubUrl":"https://github.com/jstedfast/MailKit/blob/9d3859a7855e3e17582c07fd01972b8e262bf176/MailKit/Security/SaslMechanismScramBase.cs#L319-L355","documentation":"SCRAM (SCRAM-SHA-1/256/512) is a multi-step exchange: after the client-first message the server must return a server-first message containing salt, nonce, and iteration count. If Challenge() is entered in the Final state with a null token, SaslException with SaslErrorCode.MissingChallenge is thrown because no server-first data arrived.","triggerScenarios":"Calling Challenge() on a SaslMechanismScramSha1/Sha256/Sha512 when state == LoginState.Final and token is null — the server sent no data in response to the client-first message.","commonSituations":"Servers that advertise SCRAM but fail mid-exchange, protocol layers delivering an empty continuation, or IMAP/SMTP servers rejecting the mechanism with an empty continuation instead of an error tag.","solutions":["Log the server's exact reply to the client-first message; an empty reply usually means the server rejected the mechanism","Verify the server supports the specific SCRAM hash (SCRAM-SHA-1 vs SCRAM-SHA-256) via its advertised AUTH mechanisms","Authenticate over TLS with PLAIN if the server's SCRAM support is broken","Catch SaslException with ErrorCode == SaslErrorCode.MissingChallenge and fall back to another mechanism"],"exampleFix":"// before\nclient.Authenticate (new SaslMechanismScramSha256 (\"user\", \"pass\"));\n// after\ntry {\n    client.Authenticate (new SaslMechanismScramSha256 (\"user\", \"pass\"));\n} catch (SaslException ex) when (ex.ErrorCode == SaslErrorCode.MissingChallenge) {\n    client.Authenticate (new SaslMechanismPlain (\"user\", \"pass\")); // over TLS\n}","handlingStrategy":"try-catch","validationCode":"// Check the mechanism is advertised before use:\nif (!client.AuthenticationMechanisms.Contains (\"SCRAM-SHA-256\"))\n    useFallback = true;","typeGuard":null,"tryCatchPattern":"try {\n    client.Authenticate (new SaslMechanismScramSha256 (\"user\", \"pass\"));\n} catch (SaslException ex) when (ex.ErrorCode == SaslErrorCode.MissingChallenge) {\n    // empty server-first message: fall back to PLAIN over TLS\n}","preventionTips":["Verify SCRAM support in the server's advertised mechanisms","Use TLS so fallback mechanisms are safe","Log raw SASL exchanges when integrating with a new server","Keep MailKit updated for SCRAM fixes"],"tags":["sasl","scram","authentication","mailkit"],"backgroundTag":"unexpected-response-shape","analyzedSha":"9d3859a7855e3e17582c07fd01972b8e262bf176","analyzedAt":"2026-09-15T15:46:11.592Z","contentChangedAt":"2026-09-15T15:46:11.592Z","schemaVersion":2},"datasetVersion":"2026-09-16T04:17:20.429Z"}