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Transcript
Auditing binaries for security
vulnerabilities
Speech outline (I)
• Legal considerations concerning reverse
engineering
• Introduction to the topic: The different
approaches to auditing binaries
• Review of C/C++ programming mistakes
• Spotting these mistakes in the binary
• Demonstration of finding a vulnerability
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--- Break ---
© 2001 Halvar Flake
Auditing binaries for security
vulnerabilities
Speech outline (II)
• Patching the problem away
• Dealing with Run-time-encrypted binaries
• Automated scanning for suspicious constructs:
sprintf() and strncpy()
• Automating the process of reconstructing
structures
• Extending structure reconstruction to C++ OOP
class reconstruction
• Free time to answer questions and discuss the
topic
© 2001 Halvar Flake
Legal considerations
Technically, the reverse engineer breaks the license
agreement between him and the software vendor, as
he is forced to accept upon installation that he will not
reverse engineer the program.
The vendor could theoretically sue the reverse engineer
and revoke the license.
Depending on your local law, there are different ways
to defend your situation:
© 2001 Halvar Flake
Legal considerations (EU)
EU Law:
1991 EC Directive on the Legal Protection of
Computer Programs
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Section 6 grants the right to decompilation for
interoperability purposes
Section 5.3 grants the right to decompilation for
error correction purposes
Under EU Law, these rights cannot be contracted away
© 2001 Halvar Flake
Legal considerations (USA)
US Law:
Final form of DMCA includes exceptions to
copyright for:
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Reverse engineering for interoperability
Encryption research
Security testing
One should ask his lawyer if these rights can be
contracted away.
© 2001 Halvar Flake
Approach A: Stress testing
Overly long (or malformed) strings are automatically
generated and supplied to the program
Pro‘s:
• The process is largely automatic
• No specially skilled personnel is needed
• The stress-testing tool is re-usable
Con‘s:
• The protocol has to be known
• Complex conditions will be missed
• Exception handling will hide problems
• malloc() overwrites will not be found
© 2001 Halvar Flake
Approach B: Tracing Input
A reverse engineer reads the program from the
point where it receives input on and analyzes the
code to find possible weaknesses
Pro‘s:
• Even very complex conditions are found
Con‘s:
• Auditor needs to be highly skilled
• Nearly infeasible for large applications
• Very time consuming since one will be
reading a lot of irrelevant `tentacles´
© 2001 Halvar Flake
Approach C: Finding suspicious
constructs and reading backwards
Certain constructs which appear suspicious are
detected, and a reverse engineer then manually
analyzes the threat they pose
Pro‘s:
• A lot less time consuming than approach B
• The process of detecting suspicious
constructs can be automated
• Fairly complex conditions can be found
Con‘s:
• Some vulnerabilities will be missed
• Needs highly specialized auditor
© 2001 Halvar Flake
Blackhat vs Whitehat auditing
Blackhat:
• Wants the fastest way to find a vulnerability
• Doesn‘t care if he misses some problems
• Only needs to repeat the process if the
vulnerability was fixed
Whitehat:
• Wants security, so he needs to read all code
• Has to repeat the process with every upgrade
• Has to continue after he has found something
The Blackhat is at an advantage here
© 2001 Halvar Flake
Tools the auditor needs
IDA Pro by Ilfak Guilfanov
www.datarescue.com
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Can disassemble x86, SPARC, IA64, MIPS and much more ...
Includes a powerful scripting language
Can recognize statically linked library calls
Features a powerful plug-in interface
Features CPU Module SDK for self-developed CPU modules
Automatically reconstructs arguments to standard calls via
type libraries, allows parsing of C-headers for adding new
standard calls & types
Great technical support
... much more ...
© 2001 Halvar Flake
C/C++ auditing recap
strcpy() and strcat()
Old news:
strcpy() and strcat() copying dynamic data into any
kind of fixed-size buffer are always suspicious
© 2001 Halvar Flake
C/C++ auditing recap
sprintf() and vsprintf()
Old news:
Since sprintf() can expand an arbitrary string using
the `%s` format character, any call to sprintf() or
vsprintf() which expands dynamic data into a fixedsize buffer has to be considered suspicious.
© 2001 Halvar Flake
C/C++ auditing recap
The *scanf() function family
As *scanf() parses data of dynamic origin into
fixed buffers by using the ´%s` format character,
any *scanf() call which targets a fixed-size buffer
with a `%s` format character is suspicious
© 2001 Halvar Flake
C/C++ auditing recap
The strncpy()-pitfall (I)
While strncpy supports size checking, it does not
guarantee NUL-termination of the destination buffer.
So in cases where the code includes something like
strncpy(destbuff, srcbuff, sizeof(destbuff));
problems will arise.
© 2001 Halvar Flake
C/C++ auditing recap
The strncpy()-pitfall (II)
Source string
\x0
data
After copying the source into a smaller buffer, the
destination string is not properly terminated any more.
Destination string
data with a \x0 somewhere
Any subsequent operations which expect the string to
be terminated will work on the data behind our original
string as well.
© 2001 Halvar Flake
C/C++ auditing recap
The strncat()-pitfall (I)
As with strncpy(), strncat() supports size checking,
but guarantees the proper termination of the string
after the last byte has been written.
If the buffer that is targeted is the first one which
was declared in the offending function, it is possible
to overwrite the frame pointer and gaining control
one function layer outwards.
© 2001 Halvar Flake
C/C++ auditing recap
The strncat()-pitfall (II)
Buffer to which
we append
saved_EBP‘s lowest byte is set to 0x00
Function epilogue:
mov
esp, ebp
saved_EBP
saved_EIP
© 2001 Halvar Flake
C/C++ auditing recap
The strncat()-pitfall (III)
saved_EBP
Function epilogue:
pop
ebp
saved_EIP
© 2001 Halvar Flake
C/C++ auditing recap
The strncat()-pitfall (IV)
The value in EBP (the frame pointer) is now our modified value !
saved_EIP
Function epilogue:
ret
© 2001 Halvar Flake
C/C++ auditing recap
The strncat()-pitfall (V)
Next function epilogue:
mov esp, ebp
ESP slides upwards (as its lowest order byte was
overwritten) into the user-supplied data. We can
now supply a new return address to gain control
User-supplied data
saved_EBP
.. but it lands lands here ...
ESP should be here ...
saved_EIP
© 2001 Halvar Flake
C/C++ auditing recap
The strncat()-pitfall (VI)
Furthermore, the fact that strncat() has to deal with
dynamic values for the len parameter increases the
danger of signedness misconceptions:
strncpy(buff, userdata, sizeof(buff));
strncat(buff, userdata2, sizeof(buff)-strlen(buff)-1);
Fills buff so that strlen(buff) = sizeof(buff)
len is pushed to –1 which is 0xFFFFFFF
© 2001 Halvar Flake
C/C++ auditing recap
Cast screwups (I)
void func(char *dnslabel)
{
char buffer[256];
char *indx = dnslabel;
int
count;
count = *indx;
buffer[0] = '\x00';
First byte at *dnslabel is 0x80 = -128
Gets expanded to 0xFFFFF80
signed comparison passes
while (count != 0 && (count + strlen (buffer)) < sizeof (buffer) - 1)
{
strncat (buffer, indx, count);
indx += count;
arbitrary length string is appended
count = *indx;
}
}
© 2001 Halvar Flake
C/C++ auditing recap
Format string vulnerabilities
Any call that passes user-supplied input directly to a
*printf()-family function is dangerous. These calls can
Also be identified by their argument deficiency.
Consider this code:
printf(“%s“, userdata);
printf(userdata);
Argument deficiency
© 2001 Halvar Flake
C/C++ auditing recap
Exploitation Details (I)
Stack layout during regular printf()-call :
printf(“%lx -- %s -- %d“, var1, buf, var2);
arbitrary local data
Return address
Pointer to the format string
© 2001 Halvar Flake
C/C++ auditing recap
Exploitation Details (II)
Stack layout during malicious printf()-call :
printf(stuff); // Stuff is set to contain
// “%.200lx%n%.40lx%n“
attacker-supplied malicious data
Return address
Pointer to the format string
© 2001 Halvar Flake
-- x86 assembly recap -void *memcpy(void *dest, void *src, size_t n);
Assembly representation:
push
mov
push
lea
push
call
4
eax, unkn_40D278
eax
eax, [ebp+var_458]
eax
_memcpy
© 2001 Halvar Flake
Disassembly: strcpy()/strcat()
The source is variable, not a static string
This call targets a stack buffer
© 2001 Halvar Flake
Disassembly: sprintf()/vsprintf()
Target buffer is a stack buffer
Format string containing „%s“
Expanded strings are not static and not fixed in length
© 2001 Halvar Flake
Disassembly: The *scanf()
function family
Format string contains „%s“
Data is parsed into stack buffers
© 2001 Halvar Flake
Disassembly: The
strncpy()/strncat() pitfall (I)
Copying data into a stack buffer again ...
If the source is larger than n (4000 bytes),
no NULL will be appended
© 2001 Halvar Flake
Disassembly: The
strncpy()/strncat() pitfall (II)
The target buffer is only n bytes long
© 2001 Halvar Flake
Disassembly: The strncat()
pitfall
Dangerous handling of len parameter
© 2001 Halvar Flake
Disassembly: Cast screwups
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Does the function accepts a size_t parameter for copying
data into a buffer ? (e.g. strncpy(), strncat(), fgets())
Is the size_t parameter a dynamic value and not hardcoded
into the binary ?
Is the size_t parameter at any point loaded using the
instruction movsx ?
Is anything substracted from the size_t parameter before it
gets passed to the function ?
© 2001 Halvar Flake
Disassembly: Format String
vulnerabilities
Argument deficiency
Format string is a dynamic variable
© 2001 Halvar Flake
Demonstration of finding
vulnerabilities by manually
auditing binaries
© 2001 Halvar Flake
-- BREAK --
© 2001 Halvar Flake
Patching the problem away (I)
PE File Header
.text section
containing code
so-called `Cave`
other sections
containing data
Zero-padded to
the file alignment
(usually 0x200)
so-called ´Cave´
...
© 2001 Halvar Flake
Patching the problem away (II)
jmp‘ing into our code
passing control back
.text section
containing code
`Cave` where we have
put our new code
© 2001 Halvar Flake
Dealing with runtime
encryption (I)
PE File Header
.text section
containing code
1.
.data section
containing data
The de-scrambling code is added
to the end of the executable
2.
The entry point is moved to the
descrambler
3.
The contents of the file are
scrambled
.rsrc section
containing code
descrambling code
Entry point
Entry point
© 2001 Halvar Flake
Dealing with runtime
encryption (II)
Steps to undertake:
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Trace through the descrambler until it passes control
back to the application
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Alternate approach: Hijack known compiler startup
signatures instead of tracing
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Repair the damage done to the executable structure by
the scrambler/descrambler/executable loader
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Dump the memory to disk
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Very time consuming !
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Automated tools exist to do this for many scramblers
(e.g. IceDump)
© 2001 Halvar Flake
Automating the scanning for
suspicious sprintf()-calls
Criteria for suspicious sprintf() calls:
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Does the call expand data using a `%s`format
character without size checking ?
Does the call expand a non-static string
through the ´%s´ ?
Does the call suffer from an argument
deficiency ?
If so, is the format string dynamic or static ?
Demonstration script:
sprintf.idc
© 2001 Halvar Flake
Automating the scanning for
suspicious strncpy()-calls
Criteria for suspicious strncpy() calls:
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Is the size_t parameter smaller or equal to the
size of the target buffer ?
Does the call copy dynamic data into a stack
buffer ?
Demonstration script:
strncpy.idc
© 2001 Halvar Flake
Automating the scanning for
format string vulnerabilities (I)
As we will frequently encounter wrapper functions that
implement printf() – like functionality using either
vsprintf() or vsnprintf(), it is desirable to have a script that
can be used for all functions. The data it needs to get from
the auditor is:
1. The address of the function that gets analyzed
2. The proper minimum stack correction of that function
3. The argument number of the format string
© 2001 Halvar Flake
Automating the scanning for
format string vulnerabilities (II)
The criteria the script should then apply are:
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Is the stack correction smaller than our supplied
minimum value ?
Is the format string dynamic or static ?
Demonstration script:
format.idc
© 2001 Halvar Flake
Finding some bugs (I)
Let‘s imagine the following situation:
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A hypothetical company sells a product on which
many companies rely as their only perimeter defense
This company (I will call them PeckJoint) has gained
a marketleading position – meaning a lot if confidential
data passes through their product (which I will call
BireFall-1 for simplicities sake)
So let us see what our script could turn up on this.
© 2001 Halvar Flake
Reasons why we need to
reconstruct structures
Many applications store data in large structures which are
passed around between functions. The information about
the layout of these structures is lost during the compilation.
This is bad for the reverse engineer for a variety of
reasons:
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Without knowing how large target/source buffers are, it
becomes very hard to evaluate the danger posed by a
suspicious construct
Many overflows happen within structures. Without
knowing what we‘re overwriting, it becomes hard to
see if a condition is exploitable at all
© 2001 Halvar Flake
Demonstration of manual
structure reconstruction
While the manual reconstruction of structures using IDA‘s
built-in capabilities is great for `real` reverse engineering,
it takes too much time when only looking for suspicious
constructs.
Automated ways to at least reconstruct the structure
member sizes is desirable.
© 2001 Halvar Flake
Automated structure
reconstruction
Frequently, we have a pointer to a structure as a local variable
in a function. What we want the script to do is:
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Trace through the entire function and find all places
where this pointer is loaded into a register
Each time the pointer is loaded, trace the code until the
register is overwritten. Each time anything is referenced
relative to the register, retrieve that value
Use the retrieved values to add members to a structure,
thus reconstructing accesses to it
Demonstration script:
bas_objrec.idc
© 2001 Halvar Flake
Considerations concerning
class reconstruction
vTable
Method1(...)
Method2(...)
Method3(...)
Method4(...)
Method5(...)
Every vTable entry points to
a function which accesses the
same structure via the this –
pointer. The vTable therefore
gives us a list of functions
we can use to reconstruct the
class data layout.
© 2001 Halvar Flake
Why is this interesting when
auditing IIS ?
Because it consists mostly of OOP code, and OOP code is
notoriously annoying to read in the disassembly.
Now, automated structure reconstruction can be of great interest
when auditing OOP code:
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The more functions we can analyze which access the
same structure, the more exact our reconstruction of that
structure will be
A class is nothing but a collection of functions which all
work with the same structure
© 2001 Halvar Flake
Any Questions ?
© 2001 Halvar Flake