Local, automatic variables assume unexpected values if they are read before they are initialized. The C Standard, 6.7.9, paragraph 10, specifies [ISO/IEC 9899:2011]

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

Function

Initialization

aligned_alloc()

Does not perform initialization

calloc()

Zero-initializes allocated memory

malloc()

Does not perform initialization

realloc()

Copies contents from original pointer; may not initialize all memory

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 > 0) {
    *sign_flag = 1;
  } else if (number < 0) {
    *sign_flag = -1;
  }
}

int is_negative(int number) {
  int sign;
  set_flag(number, &sign);
  return sign < 0;
}

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.

This defect results from a failure to consider all possible data states. (See MSC01-C. Strive for logical completeness for more information.)

Compliant Solution (Return-by-Reference)

This compliant solution trivially repairs the problem by accounting for the possibility that number 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) {
  if (NULL == sign_flag) {
    return;
  }

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

int is_negative(int number) {
  int sign = 0; /* Initialize for defense-in-depth */
  set_flag(number, &sign);
  return sign < 0;
}

Noncompliant Code Example (Uninitialized Local)

In this noncompliant code example, the programmer mistakenly fails to set the local variable error_log to the msg argument in the report_error() function [Mercy 2006]. Because error_log has not been 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 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.) However, 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.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

EXP33-C

High

Probable

Medium

P12

L1

Automated Detection

ToolVersionCheckerDescription
Astrée

uninitialized-local-read

uninitialized-variable-use

Fully checked
Axivion Bauhaus Suite

CertC-EXP33
CodeSonar
LANG.MEM.UVARUninitialized variable
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 uninitialized variables as arguments. It does catch the second noncompliant code example, and can be extended to catch the first as well

Coverity

UNINIT

Implemented
Cppcheck

uninitvar
uninitdata
uninitstring
uninitMemberVar
uninitStructMember

Detects uninitialized variables, uninitialized pointers, uninitialized struct members, and uninitialized array elements (However, if one element is initialized, then cppcheck assumes the array is initialized.)
There are FN compared to some other tools because Cppcheck tries to avoid FP in impossible paths.

GCC4.3.5

Can detect some violations of this rule when the -Wuninitialized flag is used

Klocwork

UNINIT.HEAP.MIGHT
UNINIT.HEAP.MUST
UNINIT.STACK.ARRAY.MIGHT
UNINIT.STACK.ARRAY.MUST UNINIT.STACK.ARRAY.PARTIAL.MUST
UNINIT.STACK.MIGHT
UNINIT.STACK.MUST


LDRA tool suite

53 D, 69 D, 631 S, 652 S

Fully implemented

Parasoft C/C++test

CERT_C-EXP33-a

Avoid use before initialization

Parasoft Insure++


Runtime analysis
Polyspace Bug Finder

CERT C: Rule EXP33-C


Checks for:

  • Non-initialized variable
  • Non-initialized pointer

Rule partially covered

PRQA QA-C

2726, 2727, 2728, 2961, 2962, 2963, 2966,

2967, 2968, 2971, 2972, 2973, 2976, 2977,

2978

Fully implemented
PRQA QA-C++

2961, 2962, 2963, 2966, 2967, 2968, 2971,

2972, 2973, 2976, 2977, 2978


PVS-Studio

V573, V614, V670, V679
RuleChecker

uninitialized-local-read

Partially checked
Splint3.1.1

TrustInSoft Analyzer

initialisation
Exhaustively verified (see one compliant and one non-compliant example).

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)

Taxonomy

Taxonomy item

Relationship

CERT C Secure Coding StandardMSC00-C. Compile cleanly at high warning levelsPrior to 2018-01-12: CERT: Unspecified Relationship
CERT C Secure Coding StandardMSC01-C. Strive for logical completenessPrior to 2018-01-12: CERT: Unspecified Relationship
CERT CEXP53-CPP. Do not read uninitialized memoryPrior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013Initialization of Variables [LAV]Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961Referencing uninitialized memory [uninitref]Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-4562017-07-05: CERT: Exact
CWE 2.11CWE-4572017-07-05: CERT: Exact
CWE 2.11CWE-7582017-07-05: CERT: Rule subset of CWE
CWE 2.11CWE-9082017-07-05: CERT: Rule subset of CWE

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-119 and EXP33-C





CWE-676 and EXP33-C





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 =



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 =



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 =



Bibliography

[Flake 2006]
[ISO/IEC 9899:2011]Subclause 6.7.9, "Initialization"
Subclause 6.2.6.1, "General"
Subclause 6.3.2.1, "Lvalues, Arrays, and Function Designators"
[Mercy 2006]
[VU#925211]
[Wang 2012]"More Randomness or Less"
[xorl 2009]"CVE-2009-1888: SAMBA ACLs Uninitialized Memory Read"