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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 void MEM09-C. Do not assume memory allocation functions initialize memory.)

To prevent information leakage, sensitive information must be cleared from dynamically allocated buffers before they are freed. Calling free() on a block of dynamic memory causes the space to be deallocated; that is, the memory block is made available for future allocation. However, the data stored in the block of memory to be recycled may be preserved. If this memory block contains sensitive information, that information may be unintentionally exposed.

In this noncompliant example, sensitive information stored in the dynamically allocated memory referenced by secret is copied to the dynamically allocated buffer, new_secret, which is processed and eventually deallocated by a call to free(). Because the memory is not cleared, it may be reallocated to another section of the program where the information stored in new_secret may be unintentionally leaked.

char *secret;

/* Initialize secret to a null-terminated byte string, 
   of less than SIZE_MAX chars */

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

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

/* Process new_secret... */

free(new_secret);
new_secret = NULL;

...

char *secret;

/* Initialize secret to a null-terminated byte string, 
   of less than SIZE_MAX chars */

char *new_secret;
size_t size = strlen(secret);
if (size == SIZE_MAX) {
  /* Handle error */
}
char *new_secret;
/* Use calloc() to zero-out allocated space */
new_secret = (char *)calloc(size+1, sizeof(char));
if (!new_secret) {
  /* Handle error */
}
strcpy(new_secret, secret);

/* Process new_secret... */

/* Sanitize memory  */
memset_s(new_secret, '\0', size);
free(new_secret);
new_secret = NULL;

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In practice, this type of 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 2003] and Sun Security Bulletin #00122 [Sun 1993] 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.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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ISO/IEC TR 24772:2013	Sensitive Information Uncleared Before Use [XZK]
MITRE CWE	CWE-226, Sensitive information uncleared before release CWE-244, Failure to clear heap memory before release ("heap inspection")

Bibliography

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