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Hacking Exposed 6: Network Security Secrets & Solutions
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is requested, it is very likely that the attacker can launch a brute force attack without being locked out and in many cases go unnoticed by the owner of the device Account Enumeration via Telnet As you re learning in this chapter, services, daemons, and all other types of client-facing applications can provide us valuable information if we just know how to ask for it and what response to look for One perfect example of this is account enumeration, which is the process of attempting to login with a particular username and observing the error messages returned by the server One instance of account enumeration via telnet was demonstrated by Shalom Carmel at Black Hat Europe during his presentation AS/400 for Pentesters Shalom showed that the AS/400 will allow for username enumeration during telnet authentication (and POP3) For instance, if an attacker attempted to log in with a valid username but an invalid password, the system would respond with CPF1107 Password not correct for user profile If an attacker attempted to log in with an invalid username, the system would respond CPF 1120 User X does not exit By harvesting the responses from the server for particular usernames the attacker can begin to build a list of valid accounts for brute forcing Shalom also provided a list of other common but useful AS/400 error messages provided during authentication, shown in Table 3-1
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Generally speaking, the insecure nature of telnet should be cause enough to discontinue its use and seek alternate means of remote management Secure shell (SSH) is a widely deployed alternative that should be used as a replacement in all possible cases In situations where telnet must be used, mitigating controls to restrict access to the service
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Error CPF1107 CPF1109 CPF1110 CPF1116 CPF1118 CPF1120 CPF1133 CPF1392 CPF1394
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Message Password not correct for user pro le Not authorized to subsystem Not authorized to work station Next not valid sign-on attempt varies off device No password associated with user X User X does not exist Value X is not a valid name Next not valid sign-on disables user pro le User pro le X cannot sign in
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Table 3-1
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Enumeration
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on a host/segment basis should be deployed Banner information can be modified in most cases, so be sure to consult your vendor for more information In regards to the specific AS/400 telnet enumeration issue, these error messages can be modified to be generalized using the CHMSGD command, and it is recommended you require users to reconnect between failed login attempts
Enumerating SMTP, TCP 25
Popularity: Simplicity: Impact: Risk Rating: 5 9 1 5
One of the most classic enumeration techniques takes advantage of the lingua franca of Internet mail delivery, the Simple Mail Transfer Protocol (SMTP), which typically runs on TCP port 25 SMTP provides two built-in commands that allow for the enumeration of users: VRFY, which confirms names of valid users, and EXPN, which reveals the actual delivery addresses of aliases and mailing lists Although most companies give out e-mail addresses quite freely these days, allowing this activity on your mail server raises the possibility of forged e-mail and, more importantly, can provide intruders with the names of local user accounts on the server We use telnet in the next example to illustrate SMTP enumeration, but you can use netcat as well:
[root$]telnet 102191001 25 Trying 102191001 Connected to 102191001 Escape character is '^]' 220 mailexamplecom ESMTP Sendmail Tue, 15 Jul 2008 11:41:57 vrfy root 250 root <root@mailexamplecom> expn test 250 test <test@mailexamplecom> expn non-existent 550 511 non-existent User unknown quit 221 mailexamplecom closing connection
To speed up this process is a tool called vrfypl, which an attacker can use to specify the target SMTP server and a list of usernames to test vrfypl will then run through the username file and report back on which users the server has identified as valid
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