Date: Wed, 27 Feb 2013 11:04:57 -0700 From: Kurt Seifried <kseifried@...hat.com> To: oss-security@...ts.openwall.com CC: "Todd C. Miller" <Todd.Miller@...rtesan.com> Subject: Re: CVE request: potential bypass of sudo tty_tickets constraints -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 02/27/2013 09:23 AM, Todd C. Miller wrote: > Sudo 1.8.6p7 and 1.7.10p6 are now available which include a fix > for the following bug: > > Potential bypass of sudo tty_tickets constraints > > Summary: When a user successfully authenticates with sudo, a time > stamp file is updated to allow that user to continue running sudo > without requiring a password for a preset time period (five minutes > by default). > > This time stamp file can either be common to all of a user's > terminals, or it can be specific to the particular terminal the > user authenticated themselves on. The terminal-specific time stamp > file behavior can be controlled using the "tty_tickets" option in > the sudoers file. This option has been enabled by default since > sudo 1.7.4. Prior to sudo 1.7.4, the default was to use a single > time stamp for all the user's sessions. > > A vulnerability exists because the user can control which terminal > the standard input, output and error file descriptors (0-2) refer > to. A malicious user could use this to run commands via sudo > without authenticating, so long as there exists a terminal the user > has access to where a sudo command was successfully run by that > same user within the password timeout period (usually five > minutes). > > The vulnerability does not permit a user to run commands other than > those allowed by the sudoers policy. > > Sudo versions affected: Sudo 1.3.5 through 1.7.10p6 and sudo 1.8.0 > through 1.8.6p7 when the "tty_tickets" option is enabled. This > option is enabled by default in sudo 1.7.4 and above. > > Details: The vulnerability can be triggered when the standard > input, output and error file descriptors (0-2) of a process are > closed and a different terminal device is opened and connected to > those descriptors. When sudo tries to determine the terminal > device via the ttyname() function, it will get the name of the > other terminal instead. The core problem is that while ttyname() > can be used to determine the name of the terminal device connected > to a specific file descriptor, there is no portable way to > determine the name of the terminal associated with the session the > process belongs to. However, on many systems it is possible to > determine this by using the /proc file system or the sysctl() > function. > > Most operating systems that have the /proc file system provide a > way to determine the controlling terminal device number for a > process; this information is used by the ps command for example. > On Linux, this is the tty_nr field in /proc/self/stat (the seventh > entry). On systems with an SVR4-style /proc, this is the pr_ttydev > member of struct psinfo, which comes from /proc/self/psinfo. Most > BSD systems that support the sysctl() function also provide a way > to get the terminal device number via the KERN_PROC_PID sysctl. By > mapping this device number to a file name, it is possible to get > the name of the terminal file without resorting to ttyname(). Sudo > began using this method to determine the process's terminal > starting with version 1.8.5 and 1.7.10. > > However, sudo still used the ttyname() function as a fall back when > no controlling terminal was found via /proc or sysctl(). This > allowed a malicious process to cause sudo to use ttyname() simply > by creating a new session without a controlling tty before > executing sudo. In sudo 1.8.6p6 and 1.7.10p5, this fall back > behavior was removed. This fixed the vulnerability for systems > where the process's controlling terminal could be determined via > /proc or sysctl(). > > Sudo 1.8.6p7 and 1.7.10p6 contain an additional fix for systems > without /proc or sysctl() that stores the POSIX session ID in the > time stamp file itself. The controlling terminal is specific to > the POSIX session it is associated with. It is not possible for > two processes in different sessions to have the same controlling > terminal. Sudo will now compare the current session ID with the > one in the time stamp file and ignore the time stamp file if the > session ID does not match. This has the additional benefit of > making it much less likely that a user will be able to reuse the > time stamp file after logging out and back in again on the same > terminal. > > Impact: A (potentially malicious) program run by a user with sudo > access may be able to bypass the "tty_ticket" constraints. In > order for this to succeed there must exist on the machine a > terminal device that the user has previously authenticated > themselves on via sudo within the last time stamp timeout (5 > minutes by default). > > This program may use sudo's -n flag to "probe" the terminals in > question to see if there is an active time stamp file for the user. > Prior to sudo 1.8.6 and 1.7.10, if a password was required when the > -n flag was specified the failure would not be logged, allowing the > program to perform such probes without being detected. The > successful command (if any), would still be logged. > > Fix: The bug is fixed in sudo 1.8.6p7 and 1.7.10p6. > > Credit: Ryan Castellucci brought the initial ttyname() issue to my > attention. Subsequently, James Ogden discovered that using > setsid() to create a new session would cause sudo to fall back to > using ttyname(). > > Other shortcomings in sudo's "tty_tickets" functionality have been > known and discussed openly for some time. There is a long > discussion about them at: > https://bugs.launchpad.net/ubuntu/+source/sudo/+bug/87023 > Please use CVE-2013-1776 for this issue. - -- Kurt Seifried Red Hat Security Response Team (SRT) PGP: 0x5E267993 A90B F995 7350 148F 66BF 7554 160D 4553 5E26 7993 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.13 (GNU/Linux) iQIcBAEBAgAGBQJRLkrJAAoJEBYNRVNeJnmTMZ0P/1MWqbRiIoSjVZnhY0d7D1WJ wLEuWu0pGLXaM55m5CNZWP8tKO675JMI9wUekD7ea5GnJ1wIyX73+1S+j6wHvPOz r2f3fcsV73rjfDAwqysY8RHn9YLoLbvZFGZXEOtE4S5zHcvrTqbiQDFKbA5wYSnD 61WCJOBj5flrvKXUZKCcNUnyI4g49KkZ+HCuCxpD0alfsz6krxH00aKzQLZwFvtN 5tYKoMY+m0ekVjV/fIkS0KcBvyp7nUa3blLlo/sI+RC9h8APwexpX78syngqCJmf Y2mfl/ZgBp/td0OIWJuoUqwpaigPY0Xns2EQGF3NrkAdKOKxaBR83/c+Lmv1CiEm WpniXQvBshs+ejkTxTTTAAZLZ9WlY/WHhHsxz/lYqgReU3P2eM2GoduAOFSIm7Hd ciSRESdM+zz4jX/tF97YExf5oufAsvaCkATJs1oy9BHeVdJ0ebf4mLAAXXCeYjLI djldhvm8rLEvSmMpTv/q1202Hg1+2M9Z6/gN0OQITbG7dpCsCfzBpokgsEVm0zS0 bLbj44F4UO6MgEo6fgUxQI4FA/FyrBAk1GzTCmamB0xMK+AChYlGHBZ6ccO2doXd GzhugXIZS0NrH1Dof2gYYfTYbwd6TF/K0BTLZgsauBIbkWD2B/Lfckaaj/wI9IKn X9OMaTqpq4SI2eEQySK0 =XUiR -----END PGP SIGNATURE-----
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