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Wiki MarkupLocal, automatic variables can assume unexpected values if they are used read before they are initialized. C99 specifies, "If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate" \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]. In the common case, on architectures that make use of a program stack, this value defaults to whichever values are currently stored in stack memory. While uninitialized memory often contains zeroes, this is not guaranteed. Consequently, uninitialized memory can cause a program to behave in an unpredictable or unplanned manner and may provide an avenue for attack.

In most cases, compilers warn about uninitialized variables. These warnings should be resolved as recommended by MSC00-A. Compile cleanly at high warning levels.

Additionally, memory allocated by functions such as malloc() should not be used before being initialized as its contents are indeterminate.

Non-Compliant Code Example

In this non-compliant code example, the set_flag() function is intended to set the variable sign to 1 if number is positive and -1 if number is negative. However, the programmer neglected to account for number being 0. If number is 0, then sign remains uninitialized. Because sign is uninitialized, and again assuming that the architecture makes use of a program stack, it uses whatever value is at that location in the program stack. This may lead to unexpected or otherwise incorrect program behavior.

The C Standard, 6.7.11, paragraph 11, specifies [ISO/IEC 9899:2024]

If an object that has automatic storage duration is not initialized explicitly, its representation is indeterminate.

See undefined behavior 11.

When local, automatic variables are stored on the program stack, for example, their values default to whichever values are currently stored in stack memory.

Additionally, some dynamic memory allocation functions do not initialize the contents of the memory they allocate.

Uninitialized automatic variables or dynamically allocated memory has indeterminate values, which for objects of some types, can be a trap representation. Reading such trap representations is undefined behavior; it can cause a program to behave in an unexpected manner and provide an avenue for attack. (See undefined behavior 10 and undefined behavior 12.)  In many cases, compilers issue a warning diagnostic message when reading uninitialized variables. (See MSC00-C. Compile cleanly at high warning levels for more information.)

Noncompliant Code Example (Return-by-Reference)

In this noncompliant code example, the set_flag() function is intended to set the parameter, sign_flag, to the sign of number. However, the programmer neglected to account for the case where number is equal to 0. Because the local variable sign is uninitialized when calling set_flag() and is never written to by set_flag(), the comparison operation exhibits undefined behavior when reading sign.

void set_flag(int number, int *sign_flag) {
  if (NULL == sign_flag) {
    return;
  }

  if (number >

void set_flag(int number, int *sign_flag) {
  if (sign_flag == NULL) {
    return;
  }
  if (number > 0) {
    *sign_flag = 1;
  }
  else if (number < 0) {
    *sign_flag = 1;
  } else if (number < 0) {
    *sign_flag = -1;
  }
}

voidint funcis_negative(int number) {
  int sign;

  set_flag(number, &sign);
  /*return use sign */< 0;
}

Compilers Some compilers assume that when the address of an uninitialized variable is passed to a function, the variable is initialized within that function. Because compilers frequently fail to diagnose any resulting failure to initialize the variable, the programmer must apply additional scrutiny to ensure the correctness of the code.

Implementation Details

Microsoft Visual Studio 2005, Visual Studio 2008, GCC version 3.4.4, and GCC version 4.1.3 fail to diagnose this error.

Compliant Solution

This defect results from a This defect results from a failure to consider all possible data states. (see See MSC01-AC. Strive for logical completeness). Once the problem is identified, it can be trivially repaired  for more information.)

Compliant Solution (Return-by-Reference)

This compliant solution trivially repairs the problem by accounting for the possibility that that number can  can be equal to 0.

Although compilers and static analysis tools often detect uses of uninitialized variables when they have access to the source code, diagnosing the problem is difficult or impossible when either the initialization or the use takes place in object code for which the source code is inaccessible. Unless doing so is prohibitive for performance reasons, an additional defense-in-depth practice worth considering is to initialize local variables immediately after declaration.

void set_flag(int number, int *sign_flag) {

void set_flag(int number, int *sign_flag) {
  if (sign_flag == NULL) {
    return;
  }
  if (numberNULL >== 0sign_flag) {
    return;
  }

  /* accountAccount for number being 0 */
  if (number >= 0) { 
    *sign_flag = 1;
  } else {
    assert(number < 0);
    *sign_flag = -1;
  }
}

voidint funcis_negative(int number) {
  int sign;

  set = 0; /* Initialize for defense-in-depth */
  set_flag(number, &sign);
  /* usereturn sign */< 0;
}

...

Noncompliant Code Example

...

(Uninitialized Local)

In this non-compliant noncompliant code example, the programmer mistakenly fails to set the local variable {{error_log}} to the {{msg}} argument in the {{report_error()}} function \[[mercy 06|AA. C References#mercy 06]\]. Because {{[Mercy 2006]. Because error_log}} has not been initialized, on architectures making use of a program stack, it assumes the value already on the stack at this location, which is a pointer to the stack memory allocated to the {{password}} array. The {{sprintf()}} call copies data in {{password}} until a null byte is reached. If the length of the string stored in the {{password}} array is greater than the size of the {{buffer}} array, then a buffer overflow occurs.

#include <stdio.h>
#include <ctype.h>
#include <string.h>

int do_auth(void) {
  char *username;
  char *password;

  /* Get username and password from user, return -1 if invalid */
}

void report_error(char const *msg) {
  char const *error_log;
  char buffer[24];

  sprintf(buffer, "Error: %s", error_log);
  printf("%s\n", buffer);
}

int main(void) {
  if (do_auth() == -1) {
    report_error("Unable to login");
  }
  return 0;
}

Non-Compliant Code Example

In this non-compliant code example, the report_error() function has been modified so that error_log is properly initialized.

void report_error(char const *msg) {
  char const *error_log = msg;
  char buffer[24];

  sprintf(buffer, "Error: %s", error_log);

  printf("%s\n", buffer);
}

This solution is still problematic in that a buffer overflow will occur if the null-terminated byte string referenced by msg is greater than 17 bytes, including the NULL terminator. The solution also makes use of a "magic number," which should be avoided (see DCL06-A. Use meaningful symbolic constants to represent literal values in program logic).

Compliant Solution

In this solution, the magic number is abstracted and the buffer overflow is eliminated.

enum {max_buffer = 24};

void report_error(char const *msg) {
  char const *error_log = msg;
  char buffer[max_buffer];

  snprintf(buffer, sizeof( buffer), "Error: %s", error_log);
  printf("%s\n", buffer);
}

Compliant Solution

A much simpler, less error prone, and better performing compliant solution is shown below.

void report_error(char const *msg) {
  printf("Error: %s\n", msg);
}

Risk Assessment

Accessing uninitialized variables generally leads to unexpected program behavior. In some cases these types of flaws may allow the execution of arbitrary code.

VU#925211 in the OpenSSL package for Debian Linux, and other distributions derived from Debian, is said to reference uninitialized memory. One might say that uninitialized memory caused the vulnerability, but not directly. The original OpenSSL code used uninitialized memory as an additional source of randomness to an already-randomly-generated key. This generated good keys, but caused the code-auditing tools Valgrind and Purify to issue warnings. Debian tried to fix the warnings with two changes. One actually eliminated the uninitialized memory access, but the other weakened the randomness of the keys.

Automated Detection

The LDRA tool suite V 7.6.0 is able to detect violations of this rule.

Fortify SCA Version 5.0 is able to detect violations of this rule, but will return false positives if the initialization was done in another function.

Compass/Rose automatically detects simple violations of this rule, although it may return some false positives. It may not catch more complex violations, such as initialization within functions taking arguments to uninitialized variables. It does catch the second non-compliant code example, and could be extended to catch the first as well.

The Coverity Prevent UNINIT checker can find cases of an uninitialized variable being used before it is initialized, although it cannot detect cases of uninitialized members of a struct. Because Coverity Prevent cannot discover all violations of this rule further verification is necessary.

Related Vulnerabilities

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

References

\[[Flake 06|AA. C References#Flake 06]\]
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.8, "Initialization"
\[[mercy 06|AA. C References#mercy 06]\]

initialized, an indeterminate value is read. The sprintf() call copies data from the arbitrary location pointed to by the indeterminate error_log variable until a null byte is reached, which can result in a buffer overflow.

#include <stdio.h>

/* Get username and password from user, return -1 on error */
extern int do_auth(void);
enum { BUFFERSIZE = 24 }; 
void report_error(const char *msg) {
  const char *error_log;
  char buffer[BUFFERSIZE];

  sprintf(buffer, "Error: %s", error_log);
  printf("%s\n", buffer);
}

int main(void) {
  if (do_auth() == -1) {
    report_error("Unable to login");
  }
  return 0;
}

Noncompliant Code Example (Uninitialized Local)

In this noncompliant code example, the report_error() function has been modified so that error_log is properly initialized:

#include <stdio.h>
enum { BUFFERSIZE = 24 }; 
void report_error(const char *msg) {
  const char *error_log = msg;
  char buffer[BUFFERSIZE];

  sprintf(buffer, "Error: %s", error_log);
  printf("%s\n", buffer);
}

This example remains problematic because a buffer overflow will occur if the null-terminated byte string referenced by msg is greater than 17 characters, including the null terminator. (See STR31-C. Guarantee that storage for strings has sufficient space for character data and the null terminator for more information.)

Compliant Solution (Uninitialized Local)

In this compliant solution, the buffer overflow is eliminated by calling the snprintf() function:

#include <stdio.h>
enum { BUFFERSIZE = 24 };
void report_error(const char *msg) {
  char buffer[BUFFERSIZE];

  if (0 < snprintf(buffer, BUFFERSIZE, "Error: %s", msg))
    printf("%s\n", buffer);
  else
    puts("Unknown error");
}

Compliant Solution (Uninitialized Local)

A less error-prone compliant solution is to simply print the error message directly instead of using an intermediate buffer:

#include <stdio.h>
 
void report_error(const char *msg) {
  printf("Error: %s\n", msg);
}

Noncompliant Code Example (mbstate_t)

In this noncompliant code example, the function mbrlen() is passed the address of an automatic mbstate_t object that has not been properly initialized. This is undefined behavior 200 because mbrlen() dereferences and reads its third argument.

#include <string.h> 
#include <wchar.h>
 
void func(const char *mbs) {
  size_t len;
  mbstate_t state;

  len = mbrlen(mbs, strlen(mbs), &state);
}

Compliant Solution (mbstate_t)

Before being passed to a multibyte conversion function, an mbstate_t object must be either initialized to the initial conversion state or set to a value that corresponds to the most recent shift state by a prior call to a multibyte conversion function. This compliant solution sets the mbstate_t object to the initial conversion state by setting it to all zeros:

#include <string.h> 
#include <wchar.h>
 
void func(const char *mbs) {
  size_t len;
  mbstate_t state;

  memset(&state, 0, sizeof(state));
  len = mbrlen(mbs, strlen(mbs), &state);
}

Noncompliant Code Example (POSIX, Entropy)

In this noncompliant code example described in "More Randomness or Less" [Wang 2012], the process ID, time of day, and uninitialized memory junk is used to seed a random number generator. This behavior is characteristic of some distributions derived from Debian Linux that use uninitialized memory as a source of entropy because the value stored in junk is indeterminate. However, because accessing an indeterminate value is undefined behavior, compilers may optimize out the uninitialized variable access completely, leaving only the time and process ID and resulting in a loss of desired entropy.

#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/time.h>
  
void func(void) {
  struct timeval tv;
  unsigned long junk;

  gettimeofday(&tv, NULL);
  srandom((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ junk);
}

In security protocols that rely on unpredictability, such as RSA encryption, a loss in entropy results in a less secure system.

Compliant Solution (POSIX, Entropy)

This compliant solution seeds the random number generator by using the CPU clock and the real-time clock instead of reading uninitialized memory:

#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/time.h>

void func(void) {     
  double cpu_time;
  struct timeval tv;

  cpu_time = ((double) clock()) / CLOCKS_PER_SEC;
  gettimeofday(&tv, NULL);
  srandom((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ cpu_time);
}

Noncompliant Code Example (realloc())

The realloc() function changes the size of a dynamically allocated memory object. The initial size bytes of the returned memory object are unchanged, but any newly added space is uninitialized, and its value is indeterminate. As in the case of malloc(), accessing memory beyond the size of the original object is undefined behavior 181.

It is the programmer's responsibility to ensure that any memory allocated with malloc() and realloc() is properly initialized before it is used.

In this noncompliant code example, an array is allocated with malloc() and properly initialized. At a later point, the array is grown to a larger size but not initialized beyond what the original array contained. Subsequently accessing the uninitialized bytes in the new array is undefined behavior.

#include <stdlib.h>
#include <stdio.h>
enum { OLD_SIZE = 10, NEW_SIZE = 20 };
 
int *resize_array(int *array, size_t count) {
  if (0 == count) {
    return 0;
  }
 
  int *ret = (int *)realloc(array, count * sizeof(int));
  if (!ret) {
    free(array);
    return 0;
  }
 
  return ret;
}
 
void func(void) {
 
  int *array = (int *)malloc(OLD_SIZE * sizeof(int));
  if (0 == array) {
    /* Handle error */
  }
 
  for (size_t i = 0; i < OLD_SIZE; ++i) {
    array[i] = i;
  }
 
  array = resize_array(array, NEW_SIZE);
  if (0 == array) {
    /* Handle error */
  }
 
  for (size_t i = 0; i < NEW_SIZE; ++i) {
    printf("%d ", array[i]);
  }
}

Compliant Solution (realloc())

In this compliant solution, the resize_array() helper function takes a second parameter for the old size of the array so that it can initialize any newly allocated elements:

#include <stdlib.h>
#include <stdio.h> 
#include <string.h>

enum { OLD_SIZE = 10, NEW_SIZE = 20 };
 
int *resize_array(int *array, size_t old_count, size_t new_count) {
  if (0 == new_count) {
    return 0;
  }
 
  int *ret = (int *)realloc(array, new_count * sizeof(int));
  if (!ret) {
    free(array);
    return 0;
  }
 
  if (new_count > old_count) {
    memset(ret + old_count, 0, (new_count - old_count) * sizeof(int));
  }
 
  return ret;
}
 
void func(void) {
 
  int *array = (int *)malloc(OLD_SIZE * sizeof(int));
  if (0 == array) {
    /* Handle error */
  }
 
  for (size_t i = 0; i < OLD_SIZE; ++i) {
    array[i] = i;
  }
 
  array = resize_array(array, OLD_SIZE, NEW_SIZE);
  if (0 == array) {
    /* Handle error */
  }
 
  for (size_t i = 0; i < NEW_SIZE; ++i) {
    printf("%d ", array[i]);
  }
}

Exceptions

EXP33-C-EX1: Reading uninitialized memory by an lvalue of type unsigned char that could not have been declared with the register storage class does not trigger undefined behavior. The unsigned char type is defined to not have a trap representation, which allows for moving bytes without knowing if they are initialized. (See the C Standard, 6.2.6.1, paragraph 3.) The requirement that register could not have been used (not merely that it was not used) is because on some architectures, such as the Intel Itanium, registers have a bit to indicate whether or not they have been initialized. The C Standard, 6.3.2.1, paragraph 2, allows such implementations to cause a trap for an object that never had its address taken and is stored in a register if such an object is referred to in any way.

Risk Assessment

Reading uninitialized variables is undefined behavior and can result in unexpected program behavior. In some cases, these security flaws may allow the execution of arbitrary code.

Reading uninitialized variables for creating entropy is problematic because these memory accesses can be removed by compiler optimization. VU#925211 is an example of a vulnerability caused by this coding error.

Automated Detection

Related Vulnerabilities

CVE-2009-1888 results from a violation of this rule. Some versions of SAMBA (up to 3.3.5) call a function that takes in two potentially uninitialized variables involving access rights. An attacker can exploit these coding errors to bypass the access control list and gain access to protected files [xorl 2009].

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-119 and EXP33-C

• Intersection( CWE-119, EXP33-C) = Ø

• EXP33-C is about reading uninitialized memory, but this memory is considered part of a valid buffer (on the stack, or returned by a heap function). No buffer overflow is involved.

CWE-676 and EXP33-C

• Intersection( CWE-676, EXP33-C) = Ø

• EXP33-C implies that memory allocation functions (e.g., malloc()) are dangerous because they do not initialize the memory they reserve. However, the danger is not in their invocation, but rather reading their returned memory without initializing it.

CWE-758 and EXP33-C

Independent( INT34-C, INT36-C, MSC37-C, FLP32-C, EXP33-C, EXP30-C, ERR34-C, ARR32-C)

CWE-758 = Union( EXP33-C, list) where list =

• Undefined behavior that results from anything other than reading uninitialized memory

CWE-665 and EXP33-C

Intersection( CWE-665, EXP33-C) = Ø

CWE-665 is about correctly initializing items (usually objects), not reading them later. EXP33-C is about reading memory later (that has not been initialized).

CWE-908 and EXP33-C

CWE-908 = Union( EXP33-C, list) where list =

• Use of uninitialized items besides raw memory (objects, disk space, etc)

New CWE-CERT mappings:

CWE-123 and EXP33-C

Intersection( CWE-123, EXP33-C) = Ø

EXP33-C is only about reading uninitialized memory, not writing, whereas CWE-123 is about writing.

CWE-824 and EXP33-C

EXP33-C = Union( CWE-824, list) where list =

• Read of uninitialized memory that does not represent a pointer

Bibliography

...

Image Added Image Added Image AddedEXP32-C. Do not cast away a volatile qualification      03. Expressions (EXP)       EXP34-C. Ensure a null pointer is not dereferenced