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Crafted TCP Packet Can Cause DoS
------------------------------------------------------------------------
SUMMARY
The Cisco IOS Transmission Control Protocol (TCP) listener in certain
versions of Cisco IOS software is vulnerable to a remotely-exploitable
memory leak that may lead to a denial of service condition.
This vulnerability only applies to traffic destined to the Cisco IOS
device. Traffic transiting the Cisco IOS device will not trigger this
vulnerability.
Cisco has made free software available to address this vulnerability for
affected customers.
This issue is documented as Cisco bug ID
<http://www.cisco.com/pcgi-bin/Support/Bugtool/onebug.pl?bugid=CSCek37177>
CSCek37177 ( registered customers only) .
There are workarounds available to mitigate the effects of the
vulnerability.
DETAILS
Affected Products:
Vulnerable Products
This issue affects all Cisco devices running Cisco IOS software. To be
affected, devices must be configured to process Internet Protocol version
4 (IPv4) packets and receive TCP packets. Devices which run only Internet
Protocol version 6 (IPv6) are not affected.
This vulnerability is present in all unfixed versions of Cisco IOS
software, including versions 9.x, 10.x, 11.x and 12.x.
To determine the software running on a Cisco product, log in to the device
and issue the "show version" command to display the system banner. Cisco
IOS software will identify itself as "Internetwork Operating System
Software" or simply "IOS". On the next line of output, the image name will
be displayed between parentheses, followed by "Version" and the IOS
release name. Other Cisco devices will not have the "show version" command
or will give different output.
The following example identifies a Cisco product running Cisco IOS release
12.2(14)S16 with an installed image name of C7200-IS-M:
Cisco Internetwork Operating System Software
IOS (tm) 7200 Software (C7200-IS-M), Version 12.2(14)S16, RELEASE
SOFTWARE (fc1)
The release train label is "12.2".
The next example shows a product running IOS release 12.3(7)T12 with an
image name of C7200-IK9S-M:
Cisco IOS Software, 7200 Software (C7200-IK9S-M), Version 12.3(7)T12,
RELEASE SOFTWARE (fc1)
Additional information about Cisco IOS Banners is available at:
<http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_white_paper09186a008018305e.shtml#3>
http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_white_paper09186a008018305e.shtml#3
Products Confirmed Not Vulnerable
Cisco products that do not run IOS are unaffected by this vulnerability.
Cisco IOS-XR is not affected.
No other Cisco products are currently known to be affected by this
vulnerability.
Details
TCP is the transport layer protocol designed to provide
connection-oriented, reliable delivery of a data stream. To accomplish
this, TCP uses a mixture of flags to indicate state and sequence numbers
to identify the order in which the packets are to be reassembled. TCP also
provides a number, called an acknowledgment number, that is used to
indicate the sequence number of the next packet expected. The full
specification of the TCP protocol can be found at
<http://www.ietf.org/rfc/rfc0793.txt> http://www.ietf.org/rfc/rfc0793.txt.
Cisco IOS devices that are configured to receive TCP packets are exposed
to this issue. This Advisory does not apply to traffic that is transiting
the device.
Certain crafted packets destined to an IPv4 address assigned to a physical
or virtual interface on a Cisco IOS device may cause the device to leak a
small amount of memory. Over time, such a memory leak may lead to memory
exhaustion and potentially degraded service.
Although this is an issue with TCP, it is not required to complete the TCP
3-way handshake in order for the memory leak to be triggered. Therefore,
TCP packets with a spoofed source address may trigger the leak.
The following document contains additional information on how to identify
if your router is suffering from a memory leak in Processor memory:
<http://www.cisco.com/warp/public/63/mallocfail.shtml#tshoot2>
http://www.cisco.com/warp/public/63/mallocfail.shtml#tshoot2
Impact:
Successful exploitation of the vulnerability may result in a small amount
of processor memory to leak, which may lead to degraded service. This
issue will not resolve over time, and will require a device reset to
recover the leaked memory.
This vulnerability only applies to traffic destined to the Cisco IOS
device. Traffic transiting the device will not trigger this issue.
Workarounds:
Additional mitigations that can be deployed on Cisco devices within the
network are available in the Cisco Applied Intelligence companion document
for this advisory:
<http://www.cisco.com/warp/public/707/cisco-air-20070124-crafted-tcp.shtml>
http://www.cisco.com/warp/public/707/cisco-air-20070124-crafted-tcp.shtml
Note: Configuring VTY access-class filters is not an effective mitigation
strategy for this vulnerability.
Infrastructure ACLs (iACL)
Although it is often difficult to block traffic transiting your network,
it is possible to identify traffic which should never be allowed to target
your infrastructure devices and block that traffic at the border of your
network. Infrastructure ACLs are considered a network security best
practice and should be considered as a long-term addition to good network
security as well as a workaround for this specific vulnerability. The ACL
example shown below should be included as part of the deployed
infrastructure access-list which will protect all devices with IP
addresses in the infrastructure IP address range.
A sample access list for devices running Cisco IOS is below:
!--- Permit TCP services from trust hosts destined
!--- to infrastructure addresses.
access-list 150 permit tcp TRUSTED_HOSTS MASK INFRASTRUCTURE_ADDRESSES
MASK
!--- Deny TCP packets from all other sources destined to
infrastructure addresses.
access-list 150 deny tcp any INFRASTRUCTURE_ADDRESSES MASK
!--- Permit all other traffic to transit the device.
access-list 150 permit IP any any
interface serial 2/0
ip access-group 150 in
The white paper entitled "Protecting Your Core: Infrastructure Protection
Access Control Lists" presents guidelines and recommended deployment
techniques for infrastructure protection access lists. This white paper
can be obtained here: <http://www.cisco.com/warp/public/707/iacl.html>
http://www.cisco.com/warp/public/707/iacl.html
Receive ACLs (rACL)
For distributed platforms, Receive ACLs may be an option starting in Cisco
IOS Software Versions 12.0(21)S2 for the 12000 (GSR), 12.0(24)S for the
7500, and 12.0(31)S for the 10720. The Receive ACL protects the device
from harmful traffic before the traffic can impact the route processor.
Receive ACLs are designed to only protect the device on which it is
configured. On the 12000, transit traffic is never affected by a receive
ACL. Because of this, the destination IP address "any" used in the example
ACL entries below only refer to the router's own physical or virtual IP
addresses. On the 7500 and 10720, transit traffic with IP options set will
be subject to the Receive ACL and permitted or denied accordingly. Receive
ACLs are considered a network security best practice, and should be
considered as a long-term addition to good network security, as well as a
workaround for this specific vulnerability. The white paper entitled "GSR:
Receive Access Control Lists" will help you identify and allow legitimate
traffic to your device and deny all unwanted packets:
http://www.cisco.com/warp/public/707/racl.html
The following is the receive path ACL written to permit this type of
traffic from trusted hosts:
!--- Permit tcp services from trusted hosts allowed to the RP.
access-list 151 permit tcp TRUSTED_ADDRESSES MASK any
!--- Deny tcp services from all other sources to the RP.
access-list 151 deny tcp any any
!--- Permit all other traffic to the RP.
access-list 151 permit ip any any
!--- Apply this access list to the 'receive' path.
ip receive access-list 151
Control Plane Policing (CoPP)
The Control Plane Policing (CoPP) feature may be used to mitigate this
vulnerability. In the following example, only TCP traffic from trusted
hosts and with 'receive' destination IP addresses is permitted to reach
the route processor (RP). All other 'transit' IP traffic is unaffected.
It should be noted that dropping traffic from unknown or untrusted IP
addresses may affect hosts with dynamically assigned IP addresses from
connecting to the Cisco IOS device.
access-list 152 deny tcp TRUSTED_ADDRESSES MASK any
access-list 152 permit tcp any any
access-list 152 deny ip any any
!
class-map match-all permit-tcp-class
match access-group 152
!
!
policy-map permit-tcp-policy
class permit-tcp-class
drop
!
control-plane
service-policy input permit-tcp-policy
In the above CoPP example, the ACL entries that match the exploit packets
with the "permit" action result in these packets being discarded by the
policy-map "drop" function, while packets that match the "deny" action are
not affected by the policy-map drop function.
Please note that in the 12.2S and 12.0S Cisco IOS trains the policy-map
syntax is different:
policy-map permit-tcp-policy
class class permit-tcp-class
police 32000 1500 1500 conform-action drop exceed-action drop
CoPP is available in Cisco IOS release trains 12.0S, 12.2SX, 12.2S, 12.3T,
12.4, and 12.4T.
Additional information on the configuration and use of the CoPP feature
can be found at the following URL:
<http://www.cisco.com/en/US/products/sw/iosswrel/ps1838/products_white_paper09186a0080211f39.shtml>
http://www.cisco.com/en/US/products/sw/iosswrel/ps1838/products_white_paper09186a0080211f39.shtml
Anti-spoofing
The Unicast Reverse Path Forwarding (Unicast RPF or uRPF) feature helps to
mitigate problems that are caused by spoofed IP source addresses. It is
available on Cisco routers and firewalls. For further details, please
refer to:
<http://www.cisco.com/en/US/partner/products/ps6441/products_command_reference_chapter09186a00804ae49f.html#wp1229984>
http://www.cisco.com/en/US/partner/products/ps6441/products_command_reference_chapter09186a00804ae49f.html#wp1229984
By enabling Unicast Reverse Path Forwarding (uRPF), all spoofed packets
will be dropped at the first device. To enable uRPF, use the following
commands.
router(config)# ip cef
router(config)# interface interface #
router(config-if)# ip verify unicast source reachable-via rx
BGP and BTSH/GTSM
Depending on your release of software, it may be possible to protect your
BGP sessions from this memory leak. With the introduction of CSCee73956 (
registered customers only) , Cisco IOS has improved support for BTSH (BGP
TTL Security Hack) to reduce, if not eliminate a risk of a memory leak due
to this vulnerability. This functionality is also known as GTSM
(Generalized TTL Security Mechanism) and documented in RFC 3682. This
section refers to GTSM as applied to eBGP sessions only.
Releases of Cisco IOS that contain CSCee73956 are protected from this
attack against the BGP port (TCP port 179) only. Other ports should be
protected accordingly.
BTSH is not supported for iBGP sessions. BTSH was first introduced in
Cisco IOS in 12.0(27)S, 12.3(7)T and 12.2(25)S. Note that the BTSH feature
prior to CSCee73956 will not protect against this vulnerability.
For more information on BTSH, please see:
<http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123newft/123t/123t_7/gt_btsh.htm>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123newft/123t/123t_7/gt_btsh.htm
ADDITIONAL INFORMATION
sThe information has been provided by <mailto:psirt@cisco.com> Cisco
Systems Product Security Incident Response Team.
The original article can be found at:
<http://www.cisco.com/warp/public/707/cisco-sa-20070124-crafted-tcp.shtml>
http://www.cisco.com/warp/public/707/cisco-sa-20070124-crafted-tcp.shtml
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