Authentication Mechanisms



This mechanism does not require the client to authenticate or provide any information.

Defined in RFC 2245


This mechanism avoids sending the users password over the network in plain text by hashing the password with a server provided random value (known as a nonce). A disadvantage of this mechanism is that the server must maintain a database of plaintext passwords for comparison.

CRAM-MD5 does not provide adequate security services for use on the Internet, it does not protect the user’s authentication identifier from eavesdroppers and is subject to a number of passive and active attacks.

Defined in RFC 2195

Documented in a RFC Draft: draft-ietf-sasl-crammd5


The CRAM-MD5 SASL mechanism is obsolete. It has been moved to Historic in draft-ietf-sasl-crammd5-to-historic


This mechanism improves upon the CRAM-MD5 mechanism by avoiding the need for the server to store plaintext passwords. With digest authentication the server needs to store the MD5 digest of the users password which helps to make the system more secure. As in CRAM-MD5 the password is hashed with a server nonce and other data before being transmitted across the network.

Defined in RFC 2831


EXTERNAL is a SASL Mechanism that allows a client to request the server to use credentials established by means external to the mechanism to authenticate the client.

SASL EXTERNAL means may be, for instance, IP Security (RFC 4301) or TLS services. In absence of a prior agreement between the client and the server, the client cannot make any assumption as to what SASL EXTERNAL means the server has used to obtain the client’s credentials, nor make an assumption as to the form of credentials. For example, the client cannot assume that the server will use the credentials the client has established via TLS.


The server will not offer EXTERNAL unless other credentials are already available in the session, such as a client certificate used in establishing a TLS connection.


Generic Security Service Application Program Interface (GSS-API). The GS2 mechanism family offers a number of improvements over the previous Configuring GSSAPI and Cyrus SASL mechanism.

Defined in RFC 5801


Not sure how to get GSSAPI going? Check out our GSSAPI configuration guide.


This is a Microsoft specific customization of GSSAPI.

Described in the Microsoft documentation and RFC 4178


Documented in a RFC Draft: draft-murchison-sasl-login


The LOGIN SASL mechanism is obsoleted in favor of the PLAIN SASL mechanism.

The LOGIN SASL mechanism does not provide a security layer. This mechanism must not be used without adequate security protection as the mechanism affords no integrity nor confidentiality protection itself.


OTP is the One-Time Password system described in RFC 2289. This mechanism is secure against replay attacks and also avoids storing password or password equivalents on the server. Only a digest of a seed and a passphrase is ever transmitted across the network.

  • OTP-MD4
  • OTP-MD5
  • OTP-SHA1


DSS Secured Password Authentication Mechanism (PASSDSS)

Documented in a RFC Draft: draft-newman-sasl-passdss



Defined in RFC 4616

This is the simplest mechanism. The users authentication details are transmitted in plain text. This mechanism should not be provided unless an encrypted link is in use - typically after TLS has been negotiated.


Salted Challenge Response Authentication Mechanism (SCRAM) is a family of modern, password-based challenge–response authentication mechanisms providing authentication of a user to a server.

Defined in RFC 5802

  • SCRAM-SHA-224(-PLUS)
  • SCRAM-SHA-256(-PLUS) (RFC 7677)
  • SCRAM-SHA-384(-PLUS)
  • SCRAM-SHA-512(-PLUS)


The Secure Remote Password (SRP) is a password-based, zero-knowledge, authentication and key-exchange protocol. It has good performance, is not plaintext-equivalent and maintains perfect forward secrecy. It provides authentication (optionally mutual authentication) and the negotiation of a shared context key.

Documented in a RFC Draft: draft-burdis-cat-srp-sasl

  • mda=sha1,rmd160,md5
  • confidentiality=des-ofb,des-ede-ofb,aes-128-ofb,bf-ofb,cast5-ofb,idea-ofb

Non-SASL Authentication


This table shows what security flags and features are supported by each of the mechanisms provided by the Cyrus SASL Library.

ANONYMOUS 0 X             X          
CRAM-MD5 0 X       X       X        
DIGEST-MD5 128 X       X   X reauth initial auth X X   X
EXTERNAL 0 X   X   X     X     X    
GS2 56 X X     X   X X   X X X  
GSSAPI 56 X X     X X X X     X X  
GSS-SPNEGO 56 X X     X X X X     X   X
LOGIN 0         X X     X        
OTP 0 X     X X     X     X    
PASSDSS 112 X X X X X X X X     X    
PLAIN 0         X X   X     X    
SCRAM 0 X X     X   X X   X X X X
SRP 128 X X X X X   X X   X X    

Understanding this table:

Security Properties:

  • MAX SSF - The maximum Security Strength Factor supported by the mechanism (roughly the number of bits of encryption provided, but may have other meanings, for example an SSF of 1 indicates integrity protection only, no encryption).
  • NOPLAIN - Mechanism is not susceptible to simple passive (eavesdropping) attack.
  • NOACTIVE - Protection from active (non-dictionary) attacks during authentication exchange. (Implies MUTUAL).
  • NODICT - Not susceptible to passive dictionary attack.
  • NOFORWARD - Breaking one session won’t help break the next.
  • NOANON - Don’t permit anonymous logins.
  • CRED - Mechanism can pass client credentials.
  • MUTUAL - Supports mutual authentication (authenticates the server to the client)


  • CLTFIRST - The client should send first in this mechanism.
  • SRVFIRST - The server must send first in this mechanism.
  • SRVLAST - This mechanism supports server-send-last configurations.
  • PROXY - This mechanism supports proxy authentication.
  • BIND - This mechanism supports channel binding.
  • HTTP - This mechanism has a profile for HTTP.