Portability is a concern when using the fread() and fwrite() functions across multiple, heterogeneous systems. In particular, it is never guaranteed that reading or writing of scalar data types such as integers, let alone aggregate types such as arrays or structures, will preserve the representation or value of the data. Implementations may differ in structure padding, floating point model, number of bits per byte, endianness, and other attributes that cause binary data formats to be incompatible.
Noncompliant Code Example
This noncompliant code example reads data from a file stream into a data structure.
struct myData {
char c;
long l;
};
/* ... */
FILE *file;
struct myData data;
/* initialize file */
if (fread(&data, sizeof(struct myData), 1, file) < sizeof(struct myData)) {
/* Handle error */
}
However, the code makes assumptions about the layout of myData, which may be represented differently on a different platform.
Compliant Solution
The best solution is to use either a text representation or a special library that will ensures data integrity.
struct myData {
char c;
long l;
};
/* ... */
FILE *file;
struct myData data;
char buf[25];
char *end_ptr;
/* initialize file */
if (fgets(buf, 1, file) == NULL) {
/* Handle error */
}
data.c = buf[0];
if (fgets(buf, sizeof(buf), file) == NULL) {
/* Handle Error */
}
data.l = strtol(buf, &end_ptr, 10);
if ((ERANGE == errno)
|| (end_ptr == buf)
|| ('\n' != *end_ptr && '\0' != *end_ptr)) {
/* Handle Error */
}
Risk Assessment
Reading binary data that has a different format than expected may result in unintended program behavior.
Recommendation |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
FIO09-C |
medium |
probable |
high |
P4 |
L3 |
Automated Detection
Compass/ROSE could flag possible violations of this rule by noting any pointer to struct that is passed to fread(), as the NCCE demonstrates.
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Other Languages
This rule appears in the C++ Secure Coding Standard as FIO09-CPP. Be careful with binary data when transferring data across systems.
