Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-839 Base Incomplete
Numeric Range Comparison Without Minimum Check
This vulnerability occurs when software validates that a number is within an acceptable range by only checking that it's less than or equal to a maximum value, but fails to also verify that it is…
Definition
What is CWE-839?
This vulnerability occurs when software validates that a number is within an acceptable range by only checking that it's less than or equal to a maximum value, but fails to also verify that it is greater than or equal to a required minimum. This oversight can allow negative or otherwise invalid low values to pass the check, leading to unexpected behavior.
Developers often use signed data types like integers or floats for values that should logically only be positive or zero. When input validation only enforces an upper limit, a negative value can slip through. This becomes dangerous when that negative number is used in operations like memory allocation, array indexing, or buffer calculations, potentially causing buffer overflows, memory corruption, or application crashes.
Beyond memory issues, this flaw can impact application logic in surprising ways. For instance, an e-commerce system that checks 'item count <= 10' but not 'item count >= 0' might process an order for -3 items. This could trigger faulty calculations, like crediting money to an attacker's account instead of charging it. Always validate both the lower and upper bounds to ensure data integrity and security.
Real-world impact
Real-world CVEs caused by CWE-839
-
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).
-
Chain: 16-bit counter can be interpreted as a negative value, compared to a 32-bit maximum value, leading to buffer under-write.
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Chain: kernel's lack of a check for a negative value leads to memory corruption.
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Chain: parser uses atoi() but does not check for a negative value, which can happen on some platforms, leading to buffer under-write.
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Chain: Negative value stored in an int bypasses a size check and causes allocation of large amounts of memory.
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Chain: negative offset value to IOCTL bypasses check for maximum index, then used as an array index for buffer under-read.
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chain: file transfer client performs signed comparison, leading to integer overflow and heap-based buffer overflow.
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chain: negative ID in media player bypasses check for maximum index, then used as an array index for buffer under-read.
How attackers exploit it
Step-by-step attacker path
- 1
The following code is intended to read an incoming packet from a socket and extract one or more headers.
- 2
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.
- 3
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.
- 4
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).
- 5
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
Vulnerable code example
Vulnerable C
The following code is intended to read an incoming packet from a socket and extract one or more headers.
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); Secure code example
Secure C
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.
...
```
// check that the array index is within the correct*
*// range of values for the array*
if (index >= 0 && index < len) {
... What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist
How to prevent CWE-839
- 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 signals
How to detect CWE-839
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
Plexicus auto-detects CWE-839 and opens a fix PR in under 60 seconds.
Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.
Frequently asked questions What is CWE-839?
How serious is CWE-839?
What languages or platforms are affected by CWE-839?
How can I prevent CWE-839?
How does Plexicus detect and fix CWE-839?
Where can I learn more about CWE-839?
Frequently asked questions
What is CWE-839?
This vulnerability occurs when software validates that a number is within an acceptable range by only checking that it's less than or equal to a maximum value, but fails to also verify that it is greater than or equal to a required minimum. This oversight can allow negative or otherwise invalid low values to pass the check, leading to unexpected behavior.
How serious is CWE-839?
MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.
What languages or platforms are affected by CWE-839?
MITRE lists the following affected platforms: C, C++.
How can I prevent CWE-839?
If the number to be used is always expected to be positive, change the variable type from signed to unsigned or size_t. 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.
How does Plexicus detect and fix CWE-839?
Plexicus's SAST engine matches the data-flow signature for CWE-839 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.
Where can I learn more about CWE-839?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/839.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-839
CWE-1023 Parent
Incomplete Comparison with Missing Factors
This weakness occurs when a program compares two items but fails to check all the necessary attributes that define their true…
CWE-184 Sibling
Incomplete List of Disallowed Inputs
This vulnerability occurs when a security filter or validation mechanism relies on a 'denylist'—a predefined list of forbidden inputs—but…
CWE-187 Sibling
Partial String Comparison
This weakness occurs when software checks only part of a string or token to determine a match, instead of comparing the entire value. This…
CWE-478 Sibling
Missing Default Case in Multiple Condition Expression
This vulnerability occurs when code with multiple conditional branches, like a switch statement, lacks a default case to handle unexpected…
CWE-195 Can precede
Signed to Unsigned Conversion Error
This vulnerability occurs when a signed integer (which can hold negative values) is converted to an unsigned integer (which holds only…
CWE-682 Can precede
Incorrect Calculation
This vulnerability occurs when software performs a calculation that produces wrong or unexpected results, which are then used to make…
CWE-119 Can precede
Improper Restriction of Operations within the Bounds of a Memory Buffer
This vulnerability occurs when software accesses a memory buffer but reads from or writes to a location outside its allocated boundary.…
CWE-124 Can precede
Buffer Underwrite ('Buffer Underflow')
A buffer underwrite, also known as buffer underflow, happens when a program writes data to a memory location before the official start of…
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