Clearly, write-what-where conditions can be used to write data to
areas of memory outside the scope of a policy. Also, they almost
invariably can be used to execute arbitrary code, which is usually
outside the scope of a program's implicit security policy.
If the attacker can overwrite a pointer's worth of memory (usually 32
or 64 bits), he can redirect a function pointer to his own malicious
code. Even when the attacker can only modify a single byte arbitrary
code execution can be possible. Sometimes this is because the same
problem can be exploited repeatedly to the same effect. Other times it
is because the attacker can overwrite security-critical
application-specific data -- such as a flag indicating whether the user
is an administrator.
Many memory accesses can lead to program termination, such as when
writing to addresses that are invalid for the current process.
Access Control
Other
Technical Impact: Bypass protection
mechanism; Other
When the consequence is arbitrary code execution, this can often be
used to subvert any other security service.
Likelihood of Exploit
High
Demonstrative Examples
Example 1
The classic example of a write-what-where condition occurs when the
accounting information for memory allocations is overwritten in a particular
fashion. Here is an example of potentially vulnerable code:
(Bad Code)
Example
Language: C
#define BUFSIZE 256
int main(int argc, char **argv) {
char *buf1 = (char *) malloc(BUFSIZE);
char *buf2 = (char *) malloc(BUFSIZE);
strcpy(buf1, argv[1]);
free(buf2);
}
Vulnerability in this case is dependent on memory layout. The call to
strcpy() can be used to write past the end of buf1, and, with a typical
layout, can overwrite the accounting information that the system keeps
for buf2 when it is allocated. Note that if the allocation header for
buf2 can be overwritten, buf2 itself can be overwritten as well.
The allocation header will generally keep a linked list of memory
“chunks”. Particularly, there may be a “previous” chunk and a “next”
chunk. Here, the previous chunk for buf2 will probably be buf1, and the
next chunk may be null. When the free() occurs, most memory allocators
will rewrite the linked list using data from buf2. Particularly, the
“next” chunk for buf1 will be updated and the “previous” chunk for any
subsequent chunk will be updated. The attacker can insert a memory
address for the “next” chunk and a value to write into that memory
address for the “previous” chunk.
This could be used to overwrite a function pointer that gets
dereferenced later, replacing it with a memory address that the attacker
has legitimate access to, where he has placed malicious code, resulting
in arbitrary code execution.
Potential Mitigations
Phase: Architecture and Design
Strategy: Language Selection
Use a language that provides appropriate memory abstractions.
Phase: Operation
Use OS-level preventative functionality integrated after the fact. Not
a complete solution.
Weakness Ordinalities
Ordinality
Description
Resultant
(where
the weakness is typically related to the presence of some other
weaknesses)
Description
