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The manner in which sensitive information can be properly cleared varies depending on the resource type and platform.

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Noncompliant Code Example: free()

Dynamic memory managers are not required to clear freed memory and generally do not because of the additional runtime overhead. Furthermore, dynamic memory managers are free to reallocate this same memory. As a result, it is possible to accidentally leak sensitive information if it is not cleared before calling a function that frees dynamic memory. Programmers also cannot rely on memory being cleared during allocation (see MEM09-AC. Do not assume memory allocation routines initialize memory).

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char *secret;

/* initialize secret */

char *new_secret;
size_t size = strlen(secret);
if (size == SIZE_MAX) {
  /* Handle Error */
}

new_secret = (char *)malloc(size+1);
if (!new_secret) {
  /* Handle Error */
}
strcpy(new_secret, secret);

/* Process new_secret... */

free(new_secret);
new_secret = NULL;

Compliant Solution

To prevent information leakage, dynamic memory containing sensitive information should be sanitized before being freed. This is commonly accomplished by clearing the allocated space (that is, filling the space with '\0' characters).

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The calloc() function ensures that the newly allocated memory has also been cleared. Because sizeof(char) is guaranteed to be 1, this solution does not need to check for a numeric overflow as a result of using calloc() (see MEM07-AC. Ensure that the arguments to calloc() when multiplied can be represented as a size_t).

NOTE: It is possible that the call to memset() in this example will be optimized out (see MSC06-AC. Be aware of compiler optimization when dealing with sensitive data). Be very careful to ensure that any sensitive data is actually cleared from memory.

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Noncompliant Code Example: realloc()

Reallocating memory using the realloc() function is a regenerative case of freeing memory. The realloc() function deallocates the old object and returns a pointer to a new object.

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The secret_size is tested to ensure that the integer multiplication (secret_size * 2) does not result in an integer overflow (see INT32-C. Ensure that operations on signed integers do not result in overflow).

Compliant Solution

A compliant program cannot rely on realloc() because it is not possible to clear the memory prior to the call. Instead, a custom function must be used that operates similar to realloc() but sanitizes sensitive information as heap-based buffers are resized. Again, this is done by overwriting the space to be deallocated with '\0' characters.

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The calloc() function ensures that the newly allocated memory has also been cleared. Because sizeof(char) is guaranteed to be 1, this solution does not need to check for a numeric overflow as a result of using calloc() (see MEM07-AC. Ensure that the arguments to calloc() when multiplied can be represented as a size_t).

Risk Assessment

In practice, this type of [security flaw|BB. Definitions#security flaw] can expose sensitive information to unintended parties. The Sun tarball vulnerability discussed in _Secure Coding Principles & Practices: Designing and Implementing Secure Applications_ \[[Graf 03|AA. C References#Graf 03]\] and Sun Security Bulletin #00122 \[[Sun|AA. C References#Sun]\] shows a violation of this recommendation, leading to sensitive data being leaked. Attackers may also be able to leverage this defect to retrieve sensitive information using techniques such as _heap inspection_.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[Fortify 06|AA. C References#Fortify 06]\]
\[[Graff 03|AA. C References#Graf 03]\]
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.20.3, "Memory management functions"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "XZK Sensitive Information Uncleared Before Use"
\[[NIST 06b|AA. C References#NIST 06b]\]

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      08. Memory Management (MEM)       MEM04-AC. Do not perform zero length allocations