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SSL/TLS Alert Protocol & the Alert Codes

  • Articolo

There have been many occasions where a event corresponding to SChannel is logged in the System event logs which indicates a problem with the SSL/TLS handshake and many a times depicts a number. The logging mechanism is a part of the SSL/TLS Alert Protocol. These alerts are used to notify peers of the normal and error conditions. The numbers especially, play a trivial role in understanding the problem/failure with the SSL/TLS handshake.

SChannel logging may have to be enabled on the windows machines to get detailed SChannel messages. Please refer the following article to do so: https://support.microsoft.com/kb/260729

Below is an example of one such event:

Log Name: System Source: Schannel Date: x/xx/xxxx x:xx:xx Event ID: 36887 Task Category: None Level: Error Keywords: User: SYSTEM Computer: xxxxxxx Description: The following fatal alert was received: 47.

These warnings sometimes are very helpful in troubleshooting SSL related issues and provide important clues. However, there is not much documentation available on the description of the alert codes.

These alert codes have been defined precisely in TLS/SSL RFC’s for all the existing protocol versions. For example lets consider the RFC 5246 (TLS 1.2). This RFC corresponds to the latest protocol version and it defines the alert messages.

Follow this link: https://tools.ietf.org/html/rfc5246#appendix-A.3

Below is a snippet from the above RFC describing the various alert messages:

A.3 . Alert Messages

   enum { warning(1), fatal(2), (255) } AlertLevel; enum { close_notify(0), unexpected_message(10), bad_record_mac(20), decryption_failed_RESERVED(21), record_overflow(22), decompression_failure(30), handshake_failure(40), no_certificate_RESERVED(41), bad_certificate(42), unsupported_certificate(43), certificate_revoked(44), certificate_expired(45), certificate_unknown(46), illegal_parameter(47), unknown_ca(48), access_denied(49), decode_error(50), decrypt_error(51), export_restriction_RESERVED(60), protocol_version(70), insufficient_security(71), internal_error(80), user_canceled(90), no_renegotiation(100), unsupported_extension(110), /* new */ (255) } AlertDescription; struct { AlertLevel level; AlertDescription description; } Alert;

There is MSDN article which describes these messages more briefly. Here is the link: https://technet.microsoft.com/en-us/library/cc783349%28v=ws.10%29.aspx.

However, the article never mentions the alert codes while explaining the messages. For simplicity, I have created a simpler table combining both the MSDN documentation and the RFC for usability. Below is the table:

Alert Code

Alert Message

Description

0

close_notify

Notifies the recipient that the sender will not send any more messages on this connection.

10

unexpected_message

Received an inappropriate message This alert should never be observed in communication between proper implementations. This message is always fatal.

20

bad_record_mac

Received a record with an incorrect MAC. This message is always fatal.

21

decryption_failed

Decryption of a TLSCiphertext record is decrypted in an invalid way: either it was not an even multiple of the block length or its padding values, when checked, were not correct. This message is always fatal.

22

record_overflow

Received a TLSCiphertext record which had a length more than 2^14+2048 bytes, or a record decrypted to a TLSCompressed record with more than 2^14+1024 bytes. This message is always fatal.

30

decompression_failure

Received improper input, such as data that would expand to excessive length, from the decompression function. This message is always fatal.

40

handshake_failure

Indicates that the sender was unable to negotiate an acceptable set of security parameters given the options available. This is a fatal error.

42

bad_certificate

There is a problem with the certificate, for example, a certificate is corrupt, or a certificate contains signatures that cannot be verified.

43

unsupported_certificate

Received an unsupported certificate type.

44

certificate_revoked

Received a certificate that was revoked by its signer.

45

certificate_expired

Received a certificate has expired or is not currently valid.

46

certificate_unknown

An unspecified issue took place while processing the certificate that made it unacceptable.

47

illegal_parameter

Violated security parameters, such as a field in the handshake was out of range or inconsistent with other fields. This is always fatal.

48

unknown_ca

Received a valid certificate chain or partial chain, but the certificate was not accepted because the CA certificate could not be located or could not be matched with a known, trusted CA. This message is always fatal.

49

access_denied

Received a valid certificate, but when access control was applied, the sender did not proceed with negotiation. This message is always fatal.

50

decode_error

A message could not be decoded because some field was out of the specified range or the length of the message was incorrect. This message is always fatal.

51

decrypt_error

Failed handshake cryptographic operation, including being unable to correctly verify a signature, decrypt a key exchange, or validate a finished message.

60

export_restriction

Detected a negotiation that was not in compliance with export restrictions; for example, attempting to transfer a 1024 bit ephemeral RSA key for the RSA_EXPORT handshake method. This message is always fatal.

70

protocol_version

The protocol version the client attempted to negotiate is recognized, but not supported. For example, old protocol versions might be avoided for security reasons. This message is always fatal.

71

insufficient_security

Failed negotiation specifically because the server requires ciphers more secure than those supported by the client. Returned instead of handshake_failure. This message is always fatal.

80

internal_error

An internal error unrelated to the peer or the correctness of the protocol makes it impossible to continue, such as a memory allocation failure. The error is not related to protocol. This message is always fatal.

90

user_cancelled

Cancelled handshake for a reason that is unrelated to a protocol failure. If the user cancels an operation after the handshake is complete, just closing the connection by sending a close_notify is more appropriate. This alert should be followed by a close_notify. This message is generally a warning.

100

no_renegotiation

Sent by the client in response to a hello request or sent by the server in response to a client hello after initial handshaking. Either of these would normally lead to renegotiation; when that is not appropriate, the recipient should respond with this alert; at that point, the original requester can decide whether to proceed with the connection. One case where this would be appropriate would be where a server has spawned a process to satisfy a request; the process might receive security parameters (key length, authentication, and so on) at start-up and it might be difficult to communicate changes to these parameters after that point. This message is always a warning.

255

unsupported_extension

 

There were few articles that I found while searching that contain additional alert codes. However, I don’t find these to be part of the RFC. Here is one: https://botan.randombit.net/doxygen/classBotan_1_1TLS_1_1Alert.html

It includes additional alerts like 110, 111, 112, 113, 114, 115. You can browse the above link for further reading.

Hope someone finds the above table useful. It may not help you in solving any issue but would provide useful pointers.

Until then, Ciao! Smile

Comments

  • Anonymous
    April 03, 2013
    Thanks for combing those together. A very useful table for understanding what the specific codes really mean! :-)

  • Anonymous
    January 19, 2014
    How do you fix #48? I have a CA key installed.

  • Anonymous
    January 19, 2014
    Could you provide more detail? what is the error? #48 is not about having the CA key installed. Its about verifying the CA, a simple comparison of the CA cert part of  the certificate chain to the CA cert present in the corresponding certificate stores.

  • Anonymous
    March 06, 2014
    I'm actually receiving error 42 and 43 while the user is connected via usb ethernet. When i disable his usb ethernet and connect through wifi he is fine. Any ideas?

  • Anonymous
    March 23, 2014
    Good information, but still unanswered questions. When this event is logged why does it not include the name of the cert with the issue and the client presenting the cert?

  • Anonymous
    March 27, 2014
    @Mike I'm assuming that there was an issue with the users certificate. However, this is a guess based on the response. I'm not entirely sure. You may wanna take up this question in the Certificates discussion forums of Microsoft. @Craig I couldn't agree with you more. One reason I can think of is that most of the times when there is a schannel failure the certificate information is not available and hence it is not logged. Also parsing through a certificate requires certain amount of processing which will incurr compute cost on the logging per say. I guess the developers did a trade off here to save processing time.

  • Anonymous
    July 02, 2014
    We have a problem with error #46 on a IIS server. Any idea how to troubleshoot this issue? (Our IIS serversseems to have a problem with receiving requests from another server, since no requests are logged and error #46 is logged in the event log).

  • Anonymous
    July 02, 2014
    @Anders The description is really not helping me to provide you any suggestions. The site on your server to which the other server connects to should have a SSL cert I believe. Is the SSL cert installed on the site issued by a internal CA or a public CA. Is the cert trusted which implies is the issuer of the cert trusted by both your server and the client (in this case the other server) This is typically seen in Client Certificate Authentication. Are you using client certificate Auth for the site?

  • Anonymous
    August 15, 2014
    Verry good article. Now you only need to read this Post for Server 2008 & 2011: www.techieshelp.com/how-to-enable-ssl-3-0-server-2008-sbs-2008 Greets Carsten

  • Anonymous
    September 01, 2014
    Hi Carsten, Thanks for your comment. I would suggest you to go through this support.microsoft.com/.../245030 It is more in detail than the link you shared. Another things is that disabling and enabling of protocols is done via registry which remains consistent throughout different version of windows. You may copy the registry from a older version to a newer version. NOTE: I'm only referring to the registry keys corresponding to enabling and disabling of SSL protocols. The registry keys do change from an older version to a newer version. Please be careful while migrating registry keys, as it can put the OS in an unstable state.

  • Anonymous
    November 11, 2014
    This article is useless without an explanation of what to do when the errors occur!!

  • Anonymous
    November 19, 2014
    I like the article, but it does not mention any further RFCs. Alert 115 for example is used for PSK key exchange, which is described in RFC 4279 ( www.ietf.org/.../rfc4279.txt ). Note, that RFC 5246 does not cover all possibilities of TLS 1.2.

  • Anonymous
    March 11, 2015
    Hi, How do you fix #40 (Handshake fatal error)? Thanks.

  • Anonymous
    July 01, 2015
    How do you fix #20 (A fatal alert was generated and sent to the remote endpoint. This may result in termination of the connection. The TLS protocol defined fatal error code is 20. The Windows SChannel error state is 960.) thank in advance

  • Anonymous
    July 06, 2015
    @George In this case I will gather a TCP dump from client and server and enable logging on either client/server depending on where the message was seen. In case of Windows Server I will enable SChannel logging.

  • Anonymous
    July 08, 2015
    I have #49 repeatedly filling logs, no idea what the problem is

  • Anonymous
    July 08, 2015
    Server is Windows Server 2012 R2 and keeps registering the schannel error 49 I am not running IIS it's just a file server, how can I determine cause and resolve this issue?

  • Anonymous
    July 13, 2015
    The comment has been removed

  • Anonymous
    July 26, 2015
    @Mark, 49 for a File server sounds tricky. I would suggest you to get a support ticket logged for this and have Microsoft support take a look at this. @Jivach Looks there are compatibility issues with one of the clients trying to connect to the server. If you have access to the client, then capture a end to end network trace and review it to see what parameters are being passed in client hello by that client.

  • Anonymous
    September 27, 2015
    I am getting lots of 49 errors on my workstation, and wonder if there is a way to find out what machine is causing the error.  Are you able to find what machine the communication is with?

  • Anonymous
    November 05, 2015
    Hi, anyone ever seen lots of 70 errors on an Exchange 2010 CAS? getting loads of these throughout the day, we have also noticed that Outlook Anywhere users get randomly disconnected, but dont know if this is just a red herring

  • Anonymous
    January 03, 2016
    I am getting message type id 128. i dont find any description for 128.

  • Anonymous
    April 01, 2016
    THank you very much received error 45, and your article helped me to solve this

  • Anonymous
    May 23, 2016
    Only get the SChannel 36887 error when I remove a service account from the Administrator group. Otherwise the ssl/tls handshake works. Problem is, I cannot have the service account in the Administrator group for Security reasons.

  • Anonymous
    August 22, 2016
    Thanks for this great collection in human friendly form.

  • Anonymous
    October 05, 2016
    Kaushal,I originally had a DC (DC02) configured as the root CA in my environment. I forgot about this DC role and I recently configured my second DC (DC01) as another root CA using a different key SHA256. When I noticed that my DC02 has already the root CA configured, I uninstalled the CA role on that DC and re-installed that role but as a subordinate CA requesting the certificate from the parent DC01.Since this change, I'm receiving the TLS fatal error #48 and I don't know what I have to do to fix it. It's probably related to the old original root Ca that I didn't properly uninstalled...any Idea?Thanks

  • Anonymous
    October 09, 2016
    Awesome man! thanks a lot for your deep info!

  • Anonymous
    April 20, 2017
    UDTS 36887 Error Schannel System 20/04/2017 18:47:55how to remove this error please suggest

  • Anonymous
    June 20, 2017
    40 how to fix

    • Anonymous
      June 26, 2017
      @Rakesh. You will have to debug this further to determine what caused the handshake to fail. You can start by gather network traces via Wireshark.