CWE-839: Numeric Range Comparison Without Minimum Check

Description

The product checks a value to ensure that it is less than or equal to a maximum, but it does not also verify that the value is greater than or equal to the minimum.

Some products 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.

Potential Impact

Integrity, Confidentiality, Availability

Modify Application Data, Execute Unauthorized Code or Commands

Availability

DoS: Resource Consumption (Other)

Confidentiality, Integrity

Modify Memory, Read Memory

Demonstrative Examples

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

Bad

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.

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

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).

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

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 reading data before the beginning of the buffer (CWE-127) 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

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

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

Bad

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

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.

Mitigations & Prevention

Implementation

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

Implementation

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.

Detection Methods

Automated Static Analysis — Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then sea

Real-World CVE Examples

CVE ID	Description
CVE-2010-1866	Chain: integer overflow (CWE-190) causes a negative signed value, which later bypasses a maximum-only check (CWE-839), leading to heap-based buffer overflow (CWE-122).
CVE-2009-1099	Chain: 16-bit counter can be interpreted as a negative value, compared to a 32-bit maximum value, leading to buffer under-write.
CVE-2011-0521	Chain: kernel's lack of a check for a negative value leads to memory corruption.
CVE-2010-3704	Chain: parser uses atoi() but does not check for a negative value, which can happen on some platforms, leading to buffer under-write.
CVE-2010-2530	Chain: Negative value stored in an int bypasses a size check and causes allocation of large amounts of memory.
CVE-2009-3080	Chain: negative offset value to IOCTL bypasses check for maximum index, then used as an array index for buffer under-read.
CVE-2008-6393	chain: file transfer client performs signed comparison, leading to integer overflow and heap-based buffer overflow.
CVE-2008-4558	chain: negative ID in media player bypasses check for maximum index, then used as an array index for buffer under-read.

Frequently Asked Questions

What is CWE-839?

CWE-839 (Numeric Range Comparison Without Minimum Check) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. The product checks a value to ensure that it is less than or equal to a maximum, but it does not also verify that the value is greater than or equal to the minimum.

How can CWE-839 be exploited?

Attackers can exploit CWE-839 (Numeric Range Comparison Without Minimum Check) to modify application data, execute unauthorized code or commands. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-839?

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

What is the severity of CWE-839?

CWE-839 is classified as a Base-level weakness (Medium abstraction). It has been observed in 8 real-world CVEs.

Description

Potential Impact

Integrity, Confidentiality, Availability

Availability

Confidentiality, Integrity

Demonstrative Examples

Mitigations & Prevention

Detection Methods

Real-World CVE Examples

Related Weaknesses

CWE-1023: Incomplete Comparison with Missing Factors

CWE-195: Signed to Unsigned Conversion Error

CWE-682: Incorrect Calculation

CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer

CWE-124: Buffer Underwrite ('Buffer Underflow')

Frequently Asked Questions