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Remote machine 10.10.10.1 has password "asdFGH12" for user named "user". I'm able to log in even if I type in password "asdFGH12dasdkjlkjasdus" or any other characters after the "asdFGH12" string.

$ ssh -v 10.10.10.1
OpenSSH_5.2p1 FreeBSD-20090522, OpenSSL 0.9.8k 25 Mar 2009
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Connecting to 10.10.10.1 [10.10.10.1] port 22.
debug1: Connection established.
debug1: identity file /home/user/.ssh/identity type 0
debug1: identity file /home/user/.ssh/id_rsa type -1
debug1: identity file /home/user/.ssh/id_dsa type 2
debug1: Remote protocol version 1.99, remote software version OpenSSH_4.1
debug1: match: OpenSSH_4.1 pat OpenSSH_4*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.2p1 FreeBSD-20090522
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host '10.10.10.1' is known and matches the RSA host key.
debug1: Found key in /home/user/.ssh/known_hosts:58
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,keyboard-interactive
debug1: Next authentication method: publickey
debug1: Offering public key: /home/user/.ssh/id_dsa
debug1: Authentications that can continue: publickey,keyboard-interactive
debug1: Trying private key: /home/user/.ssh/id_rsa
debug1: Next authentication method: keyboard-interactive
Password:
debug1: Authentication succeeded (keyboard-interactive).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
Warning: untrusted X11 forwarding setup failed: xauth key data not generated
Warning: No xauth data; using fake authentication data for X11 forwarding.
debug1: Requesting X11 forwarding with authentication spoofing.
Last login: Tue Apr 23 14:30:59 2013 from 10.10.10.2
Have a lot of fun...
user@server:~>

Is this a known behavior of (certain) SSH server versions?

Gilles 'SO- stop being evil'
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Martin
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  • What OS is this? – slm Apr 23 '13 at 11:59
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    My ssh daemon ( OpenSSH_5.9p1 Debian-5ubuntu1.1, OpenSSL 1.0.1 14 Mar 2012 ) does **not** allow me to add extra characters after the password. – Anthon Apr 23 '13 at 12:01
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    Problem is definitely with the hashing scheme. DES/traditional crypt hashing truncates or pads all given passwords to eight characters so the hashing algorithm will work. I'd wager you're using a traditional unix variant, most BSD's and Linux distros have at least been at md5 by default for the last decade or so. – Bratchley Apr 23 '13 at 12:14

2 Answers2

26

This is not a limitation on the part of your SSH server, this is a limitation on the part of your server's password hash algorithm.

When hashing passwords on Unix, the crypt() function is called. This may use one of many backends, a possibility is using DES, or another limiting algorithm (for this particular case, I will assume your server is using DES). DES is generally not used by default in modern operating systems because it results in a particularly bad limitation: password strength and validation is limited to 8 bytes.

This means that if your password was set as "foobarbaz", it becomes "foobarba", usually without a warning or notice. The same limitation applies to validation, which means that "foobarbaz", "foobarba", and "foobarbazqux" all validate for this particular case.

Chris Down
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20

I suspect you're OS is using DES password encryption, which only supports a maximum of 8 characters.

https://serverfault.com/questions/361591/ssh-accepts-only-the-half-password

From man crypt(3)

GNU EXTENSION

The glibc2 version of this function has the following additional features. If salt is a character string starting with the three characters "$1$" followed by at most eight characters, and optionally terminated by "$", then instead of using the DES machine, the glibc crypt function uses an MD5-based algorithm, and outputs up to 34 bytes, namely "$1$<string>$", where "<string>" stands for the up to 8 characters following "$1$" in the salt, followed by 22 bytes chosen from the set [a–zA–Z0–9./].
The entire key is significant here (instead of only the first 8 bytes).

You can check your pam setup to see whether you're using MD5 or DES:

% egrep "password.*pam_unix.so" /etc/pam.d/system-auth
password    sufficient    pam_unix.so md5 shadow nis nullok try_first_pass use_authtok

You can also confirm which hashing function your system is using with this command:

% authconfig --test | grep hashing
 password hashing algorithm is md5

And you can see in this systems /etc/shadow file that it's using MD5 as well:

root:$1$<DELETED PASSWORD HASH>:14245:0:99999:7:::

The codes you'll see in the /etc/shadow for each type of hashing:

  • $1 – MD5
  • $2 – blowfish
  • $2a – eksblowfish
  • $5 – SHA-256
  • $6 – SHA-512

You can reconfigure your system with this command:

% authconfig --passalgo=sha512 --update

Any existing passwords will need to be regenerated, you can use this command to force users to reset them the next time they login:

% chage -d 0 userName

References

Community
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slm
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    Checking the PAM config will only tell you what the system is using *right now*, not what was used to encrypt the password that the user is using. It is possible for them to be different if it was ever changed. – Chris Down Apr 23 '13 at 12:20
  • Sorry this was as hot question, I couldn't type the answer as fast as everyone was thinking 8-). – slm Apr 23 '13 at 12:27
  • Note that `authconfig` is generally specific to RHEL (and derivatives). – Chris Down Apr 23 '13 at 12:28
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    if `authconfig` specific then alternate option is `grep ENCRYPT_METHOD /etc/login.defs` – Rahul Patil May 25 '13 at 20:37