Hello BugTraq
Recently I've 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). I
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 full paper is available at the following URL:
http://www.trusteer.com/docs/dnsopenbsd.html
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.
Thanks,
-Amit
Amit Klein
CTO
Trusteer