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CWE-839: Numeric Range Comparison Without Minimum Check

 
Numeric Range Comparison Without Minimum Check
Weakness ID: 839 (Weakness Base)Status: Incomplete
+ Description

Description Summary

The program checks a value to ensure that it does not exceed a maximum, but it does not verify that the value exceeds the minimum.

Extended Description

Some programs use signed integers or floats even when their values are only expected to be positive or 0. An input validation check might assume that the value is positive, and only check for the maximum value. If the value is negative, but the code assumes that the value is positive, this can produce an error. The error may have security consequences if the negative value is used for memory allocation, array access, buffer access, etc. Ultimately, the error could lead to a buffer overflow or other type of memory corruption.

The use of a negative number in a positive-only context could have security implications for other types of resources. For example, a shopping cart might check that the user is not requesting more than 10 items, but a request for -3 items could cause the application to calculate a negative price and credit the attacker's account.

+ Alternate Terms
Signed comparison:

The "signed comparison" term is often used to describe when the program uses a signed variable and checks it to ensure that it is less than a maximum value (typically a maximum buffer size), but does not verify that it is greater than 0.

+ Applicable Platforms

Languages

C: (Often)

C++: (Often)

+ Common Consequences
ScopeEffect

Technical Impact: Modify application data; Execute unauthorized code or commands

An attacker could modify the structure of the message or data being sent to the downstream component, possibly injecting commands.

Technical Impact: DoS: resource consumption (other)

in some contexts, a negative value could lead to resource consumption.

Technical Impact: Modify memory; Read memory

If a negative value is used to access memory, buffers, or other indexable structures, it could access memory outside the bounds of the buffer.

+ Demonstrative Examples

Example 1

The following code is intended to read an incoming packet from a socket and extract one or more headers.

(Bad Code)
Example Language:
DataPacket *packet;
int numHeaders;
PacketHeader *headers;

sock=AcceptSocketConnection();
ReadPacket(packet, sock);
numHeaders =packet->headers;

if (numHeaders > 100) {
ExitError("too many headers!");
}
headers = malloc(numHeaders * sizeof(PacketHeader);
ParsePacketHeaders(packet, headers);

The code performs a check to make sure that the packet does not contain too many headers. However, numHeaders is defined as a signed int, so it could be negative. If the incoming packet specifies a value such as -3, then the malloc calculation will generate a negative number (say, -300 if each header can be a maximum of 100 bytes). When this result is provided to malloc(), it is first converted to a size_t type. This conversion then produces a large value such as 4294966996, which may cause malloc() to fail or to allocate an extremely large amount of memory (CWE-195). With the appropriate negative numbers, an attacker could trick malloc() into using a very small positive number, which then allocates a buffer that is much smaller than expected, potentially leading to a buffer overflow.

Example 2

The following code reads a maximum size and performs a sanity check on that size. It then performs a strncpy, assuming it will not exceed the boundaries of the array. While the use of "short s" is forced in this particular example, short int's are frequently used within real-world code, such as code that processes structured data.

(Bad Code)
Example Language:
int GetUntrustedInt () {
return(0x0000FFFF);
}

void main (int argc, char **argv) {
char path[256];
char *input;
int i;
short s;
unsigned int sz;

i = GetUntrustedInt();
s = i;
/* s is -1 so it passes the safety check - CWE-697 */
if (s > 256) {
DiePainfully("go away!\n");
}

/* s is sign-extended and saved in sz */
sz = s;

/* output: i=65535, s=-1, sz=4294967295 - your mileage may vary */
printf("i=%d, s=%d, sz=%u\n", i, s, sz);

input = GetUserInput("Enter pathname:");

/* strncpy interprets s as unsigned int, so it's treated as MAX_INT
(CWE-195), enabling buffer overflow (CWE-119) */
strncpy(path, input, s);
path[255] = '\0'; /* don't want CWE-170 */
printf("Path is: %s\n", path);
}

This code first exhibits an example of CWE-839, allowing "s" to be a negative number. When the negative short "s" is converted to an unsigned integer, it becomes an extremely large positive integer. When this converted integer is used by strncpy() it will lead to a buffer overflow (CWE-119).

Example 3

In the following code, the method retrieves a value from an array at a specific array index location that is given as an input parameter to the method

(Bad Code)
Example Language:
int getValueFromArray(int *array, int len, int index) {

int value;

// check that the array index is less than the maximum
// length of the array
if (index < len) {

// get the value at the specified index of the array
value = array[index];
}
// if array index is invalid then output error message
// and return value indicating error
else {
printf("Value is: %d\n", array[index]);
value = -1;
}

return value;
}

However, this method only verifies that the given array index is less than the maximum length of the array but does not check for the minimum value (CWE-839). This will allow a negative value to be accepted as the input array index, which will result in a out of bounds read (CWE-125) and may allow access to sensitive memory. The input array index should be checked to verify that is within the maximum and minimum range required for the array (CWE-129). In this example the if statement should be modified to include a minimum range check, as shown below.

(Good Code)
Example Language:

...

// check that the array index is within the correct
// range of values for the array
if (index <= 0 && index < len) {

...

Example 4

The following code shows a simple BankAccount class with deposit and withdraw methods.

(Bad Code)
Example Language: Java 
public class BankAccount {

public final int MAXIMUM_WITHDRAWAL_LIMIT = 350;

// variable for bank account balance
private double accountBalance;

// constructor for BankAccount
public BankAccount() {
accountBalance = 0;
}

// method to deposit amount into BankAccount
public void deposit(double depositAmount) {...}

// method to withdraw amount from BankAccount
public void withdraw(double withdrawAmount) {

if (withdrawAmount < MAXIMUM_WITHDRAWAL_LIMIT) {

double newBalance = accountBalance - withdrawAmount;
accountBalance = newBalance;
}
else {
System.err.println("Withdrawal amount exceeds the maximum limit allowed, please try again...");
...
}
}

// other methods for accessing the BankAccount object
...
}

The withdraw method includes a check to ensure that the withdrawal amount does not exceed the maximum limit allowed, however the method does not check to ensure that the withdrawal amount is greater than a minimum value (CWE-129). Performing a range check on a value that does not include a minimum check can have significant security implications, in this case not including a minimum range check can allow a negative value to be used which would cause the financial application using this class to deposit money into the user account rather than withdrawing. In this example the if statement should the modified to include a minimum range check, as shown below.

(Good Code)
Example Language: Java 
public class BankAccount {

public final int MINIMUM_WITHDRAWAL_LIMIT = 0;
public final int MAXIMUM_WITHDRAWAL_LIMIT = 350;

...

// method to withdraw amount from BankAccount
public void withdraw(double withdrawAmount) {

if (withdrawAmount < MAXIMUM_WITHDRAWAL_LIMIT &&
withdrawAmount > MINIMUM_WITHDRAWAL_LIMIT) {

...

Note that this example does not protect against concurrent access to the BankAccount balance variable, see CWE-413 and CWE-362.

While it is out of scope for this example, note that the use of doubles or floats in financial calculations may be subject to certain kinds of attacks where attackers use rounding errors to steal money.

+ Observed Examples
ReferenceDescription
Chain: integer overflow causes a negative signed value, which later bypasses a maximum-only check, leading to heap-based buffer overflow.
Chain: 16-bit counter can be interpreted as a negative value, compared to a 32-bit maximum value, leading to buffer under-write.
Chain: kernel's lack of a check for a negative value leads to memory corruption.
Chain: parser uses atoi() but does not check for a negative value, which can happen on some platforms, leading to buffer under-write.
Chain: Negative value stored in an int bypasses a size check and causes allocation of large amounts of memory.
Chain: negative offset value to IOCTL bypasses check for maximum index, then used as an array index for buffer under-read.
chain: file transfer client performs signed comparison, leading to integer overflow and heap-based buffer overflow.
chain: negative ID in media player bypasses check for maximum index, then used as an array index for buffer under-read.
+ Potential Mitigations

Phase: Implementation

Strategy: Enforcement by Conversion

If the number to be used is always expected to be positive, change the variable type from signed to unsigned or size_t.

Phase: Implementation

Strategy: Input Validation

If the number to be used could have a negative value based on the specification (thus requiring a signed value), but the number should only be positive to preserve code correctness, then include a check to ensure that the value is positive.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness BaseWeakness Base187Partial Comparison
Research Concepts (primary)1000
ChildOfCategoryCategory189Numeric Errors
Development Concepts (primary)699
CanPrecedeWeakness ClassWeakness Class119Improper Restriction of Operations within the Bounds of a Memory Buffer
Research Concepts1000
CanPrecedeWeakness BaseWeakness Base124Buffer Underwrite ('Buffer Underflow')
Research Concepts1000
CanPrecedeWeakness VariantWeakness Variant195Signed to Unsigned Conversion Error
Research Concepts1000
CanPrecedeWeakness ClassWeakness Class682Incorrect Calculation
Research Concepts1000
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
+ References
[REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 6, "Type Conversion Vulnerabilities" Page 246.. 1st Edition. Addison Wesley. 2006.
[REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 6, "Comparisons", Page 265.. 1st Edition. Addison Wesley. 2006.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
2011-03-24MITREInternal CWE Team
Modifications
Modification DateModifierOrganizationSource
2011-06-01MITREInternal
updated Common_Consequences
2012-05-11MITREInternal
updated Demonstrative_Examples, References, Relationships
2014-02-18MITREInternal
updated Relationships
Page Last Updated: June 23, 2014