Cisco IOS GRE issue Sep 06 2006 04:33PM
FX (fx phenoelit de)
Phenoelit Advisory <wir-haben-auch-mal-was-gefunden #0815 +---->

[ Title ]
Cisco Systems IOS GRE decapsulation fault

[ Authors ]
FX <fx (at) phenoelit (dot) de [email concealed]>

Phenoelit Group (http://www.phenoelit.de)
Advisory http://www.phenoelit.de/stuff/CiscoGRE.txt

[ Affected Products ]
Cisco IOS

Tested on: C3550 IOS 12.1(19)

Cisco Bug ID: CSCuk27655, CSCea22552, CSCei62762
CERT Vu ID: <not assinged>

[ Vendor communication ]
07.07.05 Initial Notification, gaus (at) cisco (dot) com [email concealed]
27.07.05 PSIRT realized that nobody took this bug, Paul Oxman
took over
28.07.05 Paul successfully reproduces the issue
04.08.05 Paul notifies FX about availabe fixes
05.08.05 Paul notifies FX about new side effects discovered
by Cisco
06.09.06 Final advisory going public as coordinated release
*Note-Initial notification by phenoelit
includes a cc to cert (at) cert (dot) org [email concealed] by default

[ Overview ]
Cisco Systems IOS contains a bug when parsing GRE packets
with GRE source routing information. A specially crafter GRE packet
can cause the router to reuse packet packet data from unrelated
ring buffer memory. The resulting packet is reinjected in the routing

[ Description ]
The GRE protocol according to RFC1701 supports source routing
different from the one known in IPv4. An optional header is added to
the GRE header containing Source Route Entries for further routing.

GRE header:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|C|R|K|S|s|Recur| Flags | Ver | Protocol Type |
| Checksum (optional) | Offset (optional) |
| Key (optional) |
| Sequence Number (optional) |
| Routing (optional) |

When a specially crafted GRE packet with routing information is
received by a Cisco IOS device, the offset field is not verified
to point inside the packet but is subtracted from what appears
to be a short integer holding the overall length of the IP packet,
causing an overflow of the same.

This causes other memory contents of the packet ring buffers to
be interpreted as the payload IP packet and reinjected into the
routing queue with fairly large length information:

GRE decapsulated IP> (len=65407, ttl=39)
GRE decapsulated IP> (len=64904, ttl=0)
GRE decapsulated IP> (len=64894, ttl=237)
GRE decapsulated IP> (len=64884, ttl=128)

The outer IP packet must come from the configured tunnel source
and be sent to the configured tunnel destination IP address.

By carefully filling the ring buffers with legitimate traffic like
ICMP, containing an IP header at the right offset, an attacker can
create IP packets with large length values inside IOS. PSIRT
believes this cannot be done, Phenoelit differs on that.

[ Example ]
Internet Protocol,
Src Addr: (,
Dst Addr: (
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00
Total Length: 28
Identification: 0xaffe (45054)
Flags: 0x00
Fragment offset: 0
Time to live: 30
Protocol: GRE (0x2f)
Header checksum: 0xf409 (correct)
Source: (
Destination: (
Generic Routing Encapsulation (IP)
Flags and version: 0x4000
0... .... .... .... = No checksum
.1.. .... .... .... = Routing
..0. .... .... .... = No key
...0 .... .... .... = No sequence number
.... 0... .... .... = No strict source route
.... .000 .... .... = Recursion control: 0
.... .... 0000 0... = Flags: 0
.... .... .... .000 = Version: 0
Protocol Type: IP (0x0800)
Checksum: 0x0000
Offset: 99

[ Notes ]
IOS implements GRE source routing as forwarding of the inner IP
packet. Thus, a Source Route Entry of will cause
IOS to resend the GRE packet to the specified address according
to the routing table (all in this case) on the appropriate
interface (all in this case).
The source address of the new packet will be the router's IP
address, the destination address according to the received packet.
This can be used to circumvent Access Control Lists with GRE.

[ Solution ]
Stop using GRE. There is no way in IOS to turn off source routing
for GRE tunnels.

To correct the parsing issue, try to install an IOS version
containing the fixes CSCuk27655 or CSCea22552 or CSCei62762.

[ end of file ($Revision: 1.3 $) ]

FX <fx (at) phenoelit (dot) de [email concealed]>
Phenoelit (http://www.phenoelit.de)
672D 64B2 DE42 FCF7 8A5E E43B C0C1 A242 6D63 B564

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