‘OpenBSD DNS Cache Poisoning and Multiple O/S Predictable IP ID Vulnerability’

Summary

A serious weakness has been discovered in OpenBSD’s PRNG, which allows an attacker to predict the next transaction ID (typically up to 8-10 guesses) given a series of consecutive 12-15 transaction IDs.’

Credit:

‘The information has been provided by Amit Klein.
The original article can be found at: http://www.trusteer.com/docs/dnsopenbsd.html


Details

‘Recently Amit’has been looking at the OpenBSD PRNG implementation for DNS transaction ID (OpenBSD ported BIND 9 into their code tree, but rolled their own PRNG for the DNS transaction ID field). Amit discovered a serious weakness in OpenBSD’s PRNG, which allows an attacker to predict the next transaction ID (typically up to 8-10 guesses) given a series of consecutive 12-15 transaction IDs. As you may appreciate, this enables DNS cache poisoning for OpenBSD much like my earlier attacks on BIND 9, BIND 8 and Microsoft Windows DNS server.

Interestingly enough, OpenBSD uses a flavor of this PRNG for another field, this time the IP fragmentation ID, part of the OpenBSD kernel network stack. The analysis carries out quite similarly to show that OpenBSD’s IP ID is predictable as well, which gives way to O/S fingerprinting, idle-scanning, host alias detection, traffic analysis, and in some cases, even to TCP blind data injection.

But it gets more interesting. Several other BSD operating systems copied the OpenBSD code for their own IP ID PRNG, so they’re
vulnerable too. This is particularly so with Apple’s Mac OS X, Mac OS X Server and Darwin, but also with NetBSD, FreeBSD and DragonFlyBSD (the 3 latter O/S however only use this PRNG when the kernel flag net.inet.ip.random_id is set to 1; it is 0 by default, resulting in a sequential counter to be used instead…). OpenBSD, NetBSD and FreeBSD also use this PRNG for IP fragmentation ID normalization feature (e.g. ‘scrub out random-id’) in the packet filter module.

Somewhat more distant flavors are used for various IPv6 fields across many BSD operating systems, which may be affected, and
some other O/S not mentioned here, including possibly non-BSD O/S may be affected, since this code seems to have been extensively copied and modified.

All the above mentioned vendors were contacted November 2007. FreeBSD, NetBSD and DragonFlyBSD committed a fix to their
respective source code trees. OpenBSD decided not to fix, and Apple refused to provide any schedule for such fix.

The impact per O/S is summarized below:
* OpenBSD 3.3-4.2
o DNS server cache poisoning (predictable DNS transaction ID).

* OpenBSD 2.6-4.2
o Idle-scanning, O/S fingerprinting, host alias detection, traffic analysis, TCP blind data injection, etc. (predictable IP fragmentation ID) in ‘regular’ IP packets and raw IP packets.

o Predictable IP fragmentation ID in Ethernet-inside-IP encapsulation, IP-inside-IP encapsulation, the CARP protocol, IP multicast routing, pfsync interface protocol, packet filter (IP packet normalization), and network bridge (ICMP error packets).

* OpenBSD 2.5-4.2
o libc resolver predictable DNS transaction ID (the source UDP port is random though).

* Mac OS X 10.0-10.5.1, Mac OS X Server 10.0-10.5.1, Darwin 1.0-9.1
o Idle-scanning, O/S fingerprinting, host alias detection, traffic analysis, TCP blind data injection, etc. (predictable IP fragmentation ID) in ‘regular’ IP packets and raw IP packets.

o Predictable IP fragmentation ID in DHCP, IP multicast routing and IPsec encapsulation in IP.

* NetBSD 1.6.2-4.0
o Idle-scanning, O/S fingerprinting, host alias detection, traffic analysis, TCP blind data injection, etc. when the packet filter is used to normalize outbound IP packets (predictable IP fragmentation ID).

o Predictable IP fragmentation ID in the CARP protocol.

* NetBSD 1.6.2-4.0 (if the kernel flag net.inet.ip.random_id is 1)
o Idle-scanning, O/S fingerprinting, host alias detection, traffic analysis, TCP blind data injection, etc. (predictable IP fragmentation ID) in ‘regular’ IP packets and raw IP packets.

o Predictable IP fragmentation ID in IP multicast routing, IP-inside-IP encapsulation and IPsec encapsulation in IP.

* FreeBSD 4.4-7.0
o Idle-scanning, O/S fingerprinting, host alias detection, traffic analysis, TCP blind data injection, etc. when the packet filter is used to normalize outbound IP packets (predictable IP fragmentation ID).

o Predictable IP fragmentation ID in the pfsync interface protocol.

* FreeBSD 4.4-7.0 (if the kernel flag net.inet.ip.random_id is 1)
o Idle-scanning, O/S fingerprinting, host alias detection, traffic analysis, TCP blind data injection, etc. (predictable IP fragmentation ID) in ‘regular’ IP packets and raw IP packets.

o Predictable IP fragmentation ID in the CARP protocol, IP multicast routing, SCTP, IP-inside-IP encapsulation and IPv6 over GRE.

* DragonFlyBSD 1.0-1.10.1 (if the kernel flag net.inet.ip.random_id is 1)
o Idle-scanning, O/S fingerprinting, host alias detection, traffic analysis, TCP blind data injection, etc. (predictable IP fragmentation ID) in ‘regular’ IP packets and raw IP packets.

o Predictable IP fragmentation ID in the CARP protocol, IP multicast routing, and IP-inside-IP encapsulation.’

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