Microsoft Windows 2000 Kerberos Change Password and Set Password Protocols
Windows 2000 protocols for Kerberos password changes and administrative password setting.
Swift, M., Trostle, J., and J. Brezak, "Microsoft Windows 2000 Kerberos Change Password and Set Password Protocols", RFC 3244, DOI 10.17487/RFC3244, February 2002, <https://www.rfc-editor.org/info/rfc3244>.
@misc{rfc3244,
series = {Request for Comments},
number = 3244,
howpublished = {RFC 3244},
publisher = {RFC Editor},
doi = {10.17487/RFC3244},
url = {https://www.rfc-editor.org/info/rfc3244},
author = {M. Swift and J. Trostle and J. Brezak},
title = {{Microsoft Windows 2000 Kerberos Change Password and Set Password Protocols}},
pagetotal = 7,
year = 2002,
month = feb,
} RFC text
Contents
Network Working Group M. Swift
Request for Comments: 3244 University of Washington
Category: Informational J. Trostle
Cisco Systems
J. Brezak
Microsoft
February 2002
Microsoft Windows 2000 Kerberos Change Password
and Set Password Protocols
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This memo specifies Microsoft's Windows 2000 Kerberos change password
and set password protocols. The Windows 2000 Kerberos change
password protocol interoperates with the original Kerberos change
password protocol. Change password is a request reply protocol that
includes a KRB_PRIV message that contains the new password for the
user.
1. Introduction
Microsoft's Windows 2000 Kerberos change password protocol
interoperates with the original Kerberos change password protocol.
Change password is a request reply protocol that includes a KRB_PRIV
message that contains the new password for the user. The original
change password protocol does not allow an administrator to set a
password for a new user. This functionality is useful in some
environments, and this proposal extends the change password protocol
to allow password setting. The changes are: adding new fields to the
request message to indicate the principal which is having its
password set, not requiring the initial flag in the service ticket,
using a new protocol version number, and adding three new result
codes.
2. The Protocol
The service accepts requests on UDP port 464 and TCP port 464 as
well. The protocol consists of a single request message followed by
a single reply message. For UDP transport, each message must be
fully contained in a single UDP packet.
For TCP transport, there is a 4 octet header in network byte order
that precedes the message and specifies the length of the message.
Request Message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message length | protocol version number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AP_REQ length | AP_REQ data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ KRB-PRIV message /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All 16 bit fields are in big-endian order.
message length field: contains the number of bytes in the message
including this field.
protocol version number: contains the hex constant 0xff80 (big-endian
integer).
AP-REQ length: length of AP-REQ data, in bytes. If the length is
zero, then the last field contains a KRB-ERROR message instead of a
KRB-PRIV message.
AP-REQ data: (see [1]) The AP-REQ message must be for the service
principal kadmin/changepw@REALM, where REALM is the REALM of the user
who wishes to change/set his password. The authenticator in the AP-
REQ must include a subsession key. (NOTE: The subsession key must be
pseudo-randomly generated and must never be reused for multiple
authenticators.) To enable setting of passwords, it is not required
that the initial flag be set in the Kerberos service ticket.
KRB-PRIV message (see [1]) This user-data field in the KRB-PRIV
message is encrypted using the subkey from the authenticator in the
AP-REQ data. The usec and seq-number fields of the KRB_PRIV message
are present and have the same value as the seq-number field in the
authenticator from the AP_REQ message (the seq-number in the
authenticator will be present). The server ignores the optional
r-address field in the KRB_PRIV message, if it is present.
The user-data component of the message consists of the following
ASN.1 structure encoded as an OCTET STRING:
ChangePasswdData ::= SEQUENCE {
newpasswd[0] OCTET STRING,
targname[1] PrincipalName OPTIONAL,
targrealm[2] Realm OPTIONAL
}
The server must verify the AP-REQ message, check whether the client
principal in the ticket is authorized to set/change the password
(either for that principal, or for the principal in the targname
field if present), and decrypt the new password. The server also
checks whether the initial flag is required for this request,
replying with status 0x0007 if it is not set and should be. An
authorization failure is cause to respond with status 0x0005. For
forward compatibility, the server should be prepared to ignore fields
after targrealm in the structure that it does not understand.
The newpasswd field contains the cleartext password, and the server
will apply any local policy checks including password policy checks.
The server then generates the appropriate keytypes from the password
and stores them in the KDC database. If all goes well, status 0x0000
is returned to the client in the reply message (see below).
Reply Message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message length | protocol version number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AP_REP length | AP-REP data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ KRB-PRIV message /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All 16 bit fields are in big-endian order.
message length field: contains the number of bytes in the message
including this field.
protocol version number: contains the hex constant 0x0001 (big-endian
integer). (The reply message has the same format as the original
change password protocol.)
AP-REP length: length of AP-REP data, in bytes. If the length is
zero, then the last field contains a KRB-ERROR message instead of a
KRB-PRIV message.
AP-REP data: the AP-REP is the response to the AP-REQ in the request
packet.
KRB-PRIV message: This KRB-PRIV message must be encrypted with the
subsession key from the authenticator in the AP-REQ data.
The server will respond with a KRB-PRIV message unless it cannot
decode the client AP-REQ or KRB-PRIV message, in which case it will
respond with a KRB-ERROR message. NOTE: Unlike change password
version 1, the KRB-ERROR message will be sent back without any
encapsulation.
The user-data component of the KRB-PRIV message, or e-data component
of the KRB-ERROR message, consists of the following data.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| result code | result string /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
result code (16 bits) (result codes 0-4 are from the original change
password protocol):
The result code must have one of the following values
(big-endian integer):
KRB5_KPASSWD_SUCCESS 0 request succeeds (This value
is not allowed in a KRB-ERROR
message)
KRB5_KPASSWD_MALFORMED 1 request fails due to being
malformed
KRB5_KPASSWD_HARDERROR 2 request fails due to "hard"
error in processing the
request (for example, there
is a resource or other
problem causing the request
to fail)
KRB5_KPASSWD_AUTHERROR 3 request fails due to an error
in authentication processing
KRB5_KPASSWD_SOFTERROR 4 request fails due to a
"soft" error in processing
the request
KRB5_KPASSWD_ACCESSDENIED 5 requestor not authorized
KRB5_KPASSWD_BAD_VERSION 6 protocol version unsupported
KRB5_KPASSWD_INITIAL_FLAG_NEEDED 7 initial flag required
0xFFFF is returned if the request fails for some other reason.
Although only a few non-zero result codes are specified here, the
client should accept any non-zero result code as indicating
failure.
result string:
This field contains information which might be useful to the user,
such as feedback about policy failures. The string is encoded in
UTF-8. It may be omitted if the server does not wish to include
it. If it is present, the client will display the string to the
user.
3. Security Considerations
Password policies should be enforced to make sure that users do not
pick passwords (for change password) that are vulnerable to brute
force password guessing attacks. An administrator who is authorized
to set other principal's passwords is highly trusted and must also
carefully protect his/her own credentials.
4. References
[1] Kohl, J. and C. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
5. Authors' Addresses
Mike Swift
University of Washington
Seattle, WA
EMail: mikesw@cs.washington.edu
Jonathan Trostle
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
EMail: john3725@world.std.com
John Brezak
Microsoft
1 Microsoft Way
Redmond, WA 98052
EMail: jbrezak@microsoft.com
6. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1Kerberos Password Management
Kerberos is a network authentication system where a server (the Key Distribution Center, or KDC) stores user passwords and issues credentials to prove identity. When a user logs in or changes their password, the KDC is involved.
RFC 3244 documents how Microsoft Windows 2000 implements password change and set operations. The change-password protocol is compatible with the original Kerberos protocol but extends it with a new feature: password setting, where an administrator can assign a password to a newly created account without the user having to authenticate first.
Kerberos is a login system. Users need to change their passwords sometimes. This RFC explains how Windows does it so other systems can understand the same language.
§2Request and Reply Message Structure
The protocol uses UDP port 464 and TCP port 464. Messages have a 4-octet header for TCP showing message length. All 16-bit fields are big-endian.
A request message contains: a 2-byte message length, a 2-byte protocol version (hex 0xff80), a 2-byte AP-REQ length, the AP-REQ data (Kerberos authentication), and a KRB-PRIV message (encrypted with a subsession key from the AP-REQ authenticator) holding the new password in an ASN.1 structure with fields for the new password, an optional target principal name, and optional target realm.
A reply message has: message length, protocol version (0x0001), AP-REP length, the AP-REP response, and a KRB-PRIV message (or KRB-ERROR if decoding failed) with a result code and optional result string in UTF-8.
A request says 'change my password' and proves who you are with Kerberos magic, then whispers the new password in an encrypted box. The reply says 'done' or explains what went wrong.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
MSG-LEN | Message Length | 2 bytes | Total bytes in message including this field. | |
VERSION | Protocol Version | 2 bytes | Hex 0xff80 for request. | |
AP-REQ-LEN | AP-REQ Length | 2 bytes | Bytes of AP-REQ data. | |
AP-REQ-DATA | AP-REQ Data | 2 bytes | Kerberos authentication message. |
§3Protocol Operation and Result Codes
When changing a password, the client sends a request with the new password encrypted inside the KRB-PRIV message using a subsession key. The server verifies the AP-REQ message, checks if the client is authorized to change the password (either their own or another principal's if specified in targname), and applies local password policy checks.
The server responds with result codes: 0x0000 for success, 0x0005 for authorization denied, 0x0006 for unsupported protocol version, 0x0007 if the initial flag is required but not set, and others for various errors. The reply may include a UTF-8 text message describing the result, such as password policy feedback.
The server checks your permission, follows password rules, and tells you 'okay' or why it refused—like 'that password is too weak'.
§4Password Setting for New Accounts
The Windows 2000 extension allows administrators to set a password for a newly created account. The client includes a targname field in the request to specify which principal's password to set. The server checks if the client is authorized as an administrator.
Unlike the original change-password protocol, password setting does not require the initial flag to be set in the Kerberos service ticket. This lets an administrator (who may not have the user's credentials) set a password that the new user can use to log in. The server generates appropriate key types from the password and stores them in the KDC database.
Admins can use this to give new employees their first password without needing to log in as the employee first.
§5Password Policies and Credential Protection
The RFC requires that password policies be enforced to prevent weak passwords vulnerable to guessing attacks. The new password string in the request is encrypted under the subsession key derived from mutual Kerberos authentication, so it is protected in transit.
Administrators authorized to set other users' passwords are highly trusted and must carefully protect their own credentials. The RFC also notes that applications using Kerberos failover must be aware of security state (authentication, cached ciphertext for block ciphers): when a user's connection fails over to another server, that security state may need to be re-established. This is an application design concern, not handled by the protocol itself.
Passwords must be hard to guess. The secret is encrypted when sent. Admins are powerful and must keep their own passwords safe.