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JUNOS too sensitive to fragmented packets which are addressed to the router.

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Article ID: JSA10431 SECURITY_ADVISORIES Last Updated: 09 May 2013Version: 1.0
Legacy Advisory Id:
PSN-2010-04-709
Product Affected:
This is a zero day issue which affects all JUNOS Devices.
Problem:
Fragmented packets sent directly to a JUNOS device will not efficiently clear from route engine's packet memory (mbuf). This exposes the router to additional risk, where mbuf can fill to where no other packets can be accepted. The creates a denial of service against the router's control and management plane traffic. In most cases the fragmented packets will time out and get cleared from mbuf, restoring service. In rare cases, the fragmented packets will not be cleared.

Indications of this issue can be seen by executing the 'show system buffers' command:
lab@m320> show system buffers    
26876/529/27405 mbufs in use (current/cache/total)
26851/425/27276/30000 mbuf clusters in use (current/cache/total/max)
Additionally, system statistics may display the following error:
lab@m320> show system statistics
ttp:               
        819650 input packets couldn't get buffer 


This will only occur when packets are specifically addressed to an interface on the router and is passed to the route engine (RE). Fragmented packets which are forwarded through the router do not pose a threat to the router.
Solution:
All JUNOS software releases built on or after 2010-02-12 have fixed this specific issue. Releases containing the fix specifically include: 8.5S4, 9.0S3, 9.1S6, 9.3R4, 9.6R1, 10.0R1, 10.1R1 and all subsequent releases (i.e. all releases built after 10.1R1).

This issue is being tracked as PR 450448. While this PR is not viewable by customers, it can be used as a reference when discussing the issue with JTAC.

Workaround:
Today's network infrastructure typically will not have fragmented packets destined for the router's control or management plane. In most cases, it is safe to apply packet filters which will prevent fragmented packets from arriving on the router. Usually, fragmented packets received by a router indicate a problem with the network or a DDOS attack against the router. In either case, fragmented packets should be dropped to protect the router's control and management plane.

Below is a sample firewall filter to demonstrate this recommendation:

[edit firewall family inet filter fragment]
user@junos# show
term first-frag {
    from {
        first-fragment;
    }
    then {
        discard;
    }
term next-frag {
    from {
        is-fragment;
    }
    then {
        discard;
    }
Implementation:

How to obtain Service Releases:

Security vulnerabilities are fixed in the next available Maintenance Release of each supported JUNOS version. In some cases, a Maintenance Release is not planned to be available in an appropriate time-frame. For these cases, Service Releases are made available in order to be more timely. Security Advisory notices will indicate which Maintenance and/or Service Releases contain fixes for the issues described. Upon request to JTAC, customers will be provided download instructions for a Service Release. Although Juniper does not provide formal Release Note documentation for a Service Release, a list of "PRs fixed" can be provided on request.
CVSS Score:
7.1 (AV:N/AC:M/Au:N/C:N/I:N/A:C/E:P/RL:W/RC:C/CDP:H/TD:H/CR:ND/IR:ND/AR:ND)
Severity Level:
High
Severity Assessment:
Information for how Juniper Networks uses CVSS can be found at KB 16446: "Common Vulnerability Scoring System (CVSS) and Juniper's Security Advisories."

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