[NEWS] Cisco VPN3000 Gateway MTU Overflow
From: support@securiteam.comDate: 07/11/02
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From: support@securiteam.com To: list@securiteam.com Date: Thu, 11 Jul 2002 12:07:21 +0200 (CEST)
The following security advisory is sent to the securiteam mailing list, and can be found at the SecuriTeam web site: http://www.securiteam.com
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Cisco VPN3000 Gateway MTU Overflow
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SUMMARY
The Cisco <http://www.cisco.com/univercd/cc/td/doc/pcat/3000.htm> VPN3000
gateway lets remote client dictate which maximum MTU to use when sending
back ESP frames, regardless of the transmitting capabilities of the
physical medium (thus going over the frame buffer's size).
DETAILS
Vulnerable systems:
* Cisco/VPN 3000 Concentrator with software vpn3000-3.5.Rel-k9.bin
Impact:
* Oversized frames get silently discarded by equipments linked to the
gateway's public
interface and retransmissions occur.
* Other disturbances or DoS against neighboring equipments may occur,
especially as many IP stacks on routers, sniffers, etc are poorly
implemented.
Technical details:
When a target sends back Ethernet frames with size close to the max
Ethernet MTU (1500), the gateway encrypts the frames adding ESP headers
and then tries to send a 1580-bytes (which is well over the 1500+- frame
size limit) frame back to the client.
Workaround:
For Windows-based OS (likely UNIX and Linux-based OS too), Cisco released
a tool called
<http://www.cisco.com/univercd/cc/td/doc/product/vpn/client/3_1/admin/vcach5.htm> setMTU.exe that can prevent ill MTU negotiation from happening.
ADDITIONAL INFORMATION
The information has been provided by <mailto:porte10@free.fr> porte10.
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