F-Secure Security Bulletin FSC-2002-2
AllowedAuthentications can be overridden by client

Date issued	2002-05-21
Revision history	FSC-2002-2.1 - 2002-05-21
Risk factor	Low (Low/Medium/High/Critical)
Brief description	In server configurations where the "AllowedAuthentications" configuration does not include "Password" as an authentication option, some secure shell protocol version 2 based clients may be capable of overriding the configuration and still allow password authentication despite of it.
Affected software	F-Secure SSH Server, protocol SSH2
Affected versions	3.0.x and 3.1.0
Affected platforms	All UNIX/Linux platforms
Bulletin location	http://www.F-Secure.com/support/security/fsc-2002-2.shtml

F-Secure advices those users who use F-Secure SSH Server for Unix versions 3.0.1 and 3.1 and have denied password authentication to ensure the security of their systems.

Other versions of F-Secure SSH software are NOT affected by this vulnerability, nor any SSH server that allows password authentication.

Issue:

In server configurations where the "AllowedAuthentications" configuration does not include "Password" as an authentication option, some secure shell protocol version 2 based clients may be capable of overriding the configuration and still allow password authentication despite of it.

When other stronger authentication methods, such as SecurID or digital certificates, are being enforced, weak passwords may be defined by a system administrator for a system due to the fact that password authentication is not expected at all. With some secure shell protocol 2 based clients being capable to override this system configuration, a possibility to exploit these weak passwords may occur.

Solutions and patch availability:

1) Workaround by using "RequiredAuthentications", see below.

2) Upgrading to F-Secure SSH Server for Unix 3.1.0 build 9

F-Secure SSH Server for Unix 3.1.0 build 9 is available at F-Secure Web Club pages for download:

http://www.f-secure.com/webclub

The new build will also be available on the next product CD which will be available later.

3) Recompiling with the patch.

The patch for the source code is available at:

Patch for version 3.0.1

Patch for version 3.1.0

Short instructions for applying patches:

# ssh /path/to/ssh/source
# make clean
# patch -p 1 < /path/to/patch
# ./configure
# make

We apologize for any inconvenience this may cause.

F-secure takes security issues seriously and a CERT advisory and notification to customers regarding this issue have been distributed. Please make every effort to ensure that your systems are protected using one of the above methods as quickly as possible. As this information becomes widely known, your systems could be at even greater risk if appropriate measures are not taken immediately.

Technical Description of the Vulnerability:

Server configuration variable "AllowedAuthentications" can be overridden by a client, ignoring servers' list of allowed authentication methods.

For example if server configuration sshd2_config specifies:

AllowedAuthentications hostbased, publickey

It is possible to login using password authentication with for example old PuTTY client versions.

A workaround is to use "RequiredAuthentications" keyword instead of "AllowedAuthentications" in sshd2_config:

RequiredAuthentications hostbased, publickey

This will require both hostbased and publickey authentication to succeed before user is granted access to the system. The RequiredAuthentications will be enforced even if the client attempts to force a disallowed authentication method.

FAQ:

Is this a serious vulnerability?

No, no-one can gain system access by using this vulnerability alone as there is still password authentication protecting the connection.

Technically, as this bug does only allow password authentication when only public key authentication is in place, this bug is considered a rather theoretical one - to exploit this, it would  require user to guess possible passwords with modified ssh client in an environment where this particular configuration option is used.

As the password authentication is most widely used with SSH, this functionality does not pose real threat. The fact that this configuration option is not widely used, makes it even more unlikely for the exploit to be applicable.

Can you give me example when my server is in danger?

The server is in danger, if you have weak or known passwords in your server.

Common scenario for SSH usage is to replace an automated FTP script with SSH using public keys. If the password used in this old FTP script is in still in place, this password can be used for logins.

Also this kind of situation may occur if you have been using a plain-text connection method with widely known passwords. If you have not changed your password after transferring your system to use of SSH.

Do you have any reports on systems that were hacked because of this vulnerability?

No, we don't have any reports on anyone been cracked or attacked by using this vulnerability.

Revision history:

FSC-2002-2.1 - 2002-05-21

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