Specially crafted IPv6 packets can cause a kernel memory 
leak and eventually a reboot in ALL VERSIONS of JUNOS released before May 10 2006
From https://www.juniper.net/support/security/alerts/IPv6_bug.txt

"Issue:
This issue affects all releases of JUNOS Internet Software running
on M-series, T-series, and J-series routers and built prior to May 10, 2006. 
Affected JUNOS routers, when receiving certain IPv6 packets, 
do not release the memory buffer occupied by the IPv6 packet.� 
Repeated reception of such packets can eventually consume all kernel packet 
memory and cause the router to crash.

Solution:
The JUNOS IPv6 code has been corrected to release the memory occupied
by the invalid packet in all cases.� All releases of JUNOS software
built on or after May 10, 2006 include the corrected code.� Corrective
software is available for JUNOS releases 6.4 through 8.0 inclusive."�

You can get updated software at http://www.juniper.net. 
If your not already a registered juniper customer you will need to register first.
"Customers without a JUNOS support or maintenance contract can gain
access to corrective software by requesting a Juniper user account at
the following link: http://www.juniper.net/entitlement/setupAccountInfo.do
The account must be set up with Authorization Code: JNPRIPV6.� After
receiving the user account information via email, customers can then
contact Juniper Support at 1-800-638-8296 (US and Canada) or
+1-408-745-9500 (worldwide) in order to obtain the
links to the appropriate software image."�

Workaround: 
If you do not need to process IPv6 packets remove family inet6 from the interface configurations.

Detection:
After an attack:
You will see a kernel crash  and you might see an "out of mbufs" message
in the sylog if the kernel had a  chance to write that to syslog before it crashes.
During an attack:
Show system buffers will show the mbuf count getting smaller

Reference numbers:
JTAC bulletin PSN-2006-06-017
CVE# VU#294036

Additional references:
http://www.niscc.gov.uk/niscc/docs/br-20060711-00471.html?lang=en
http://www.kb.cert.org/vuls/id/294036