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 searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
CWE-191 Base Draft
Integer Underflow (Wrap or Wraparound)
Integer underflow occurs when a subtraction operation results in a value smaller than the data type's minimum limit, causing the value to wrap around to a large, incorrect number.
Definition
What is CWE-191?
Integer underflow occurs when a subtraction operation results in a value smaller than the data type's minimum limit, causing the value to wrap around to a large, incorrect number.
Integer underflow, also known as wraparound, happens when a program subtracts a larger number from a smaller one, pushing the result below the minimum value the integer type can hold. Instead of throwing an error, the value wraps around to the maximum end of the range—for example, an unsigned 8-bit integer going from 0 to 255 after subtracting 1. This creates a logic flaw where the program operates on a wildly incorrect value, often leading to unexpected behavior.
This vulnerability affects both signed and unsigned integers and is a common source of critical security flaws. In signed integers, underflow can flip a positive value to a massive negative one, while in unsigned integers, it creates an unexpectedly large positive value. Developers must validate all subtraction inputs, use safe math libraries, or choose data types with sufficient range to prevent these dangerous wrap-around scenarios.
Real-world impact
Real-world CVEs caused by CWE-191
-
Integer underflow in firewall via malformed packet.
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Integer underflow by packet with invalid length.
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Long input causes incorrect length calculation.
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Malformed icon causes integer underflow in loop counter variable.
How attackers exploit it
Step-by-step attacker path
- 1
The following example subtracts from a 32 bit signed integer.
- 2
The example has an integer underflow. The value of i is already at the lowest negative value possible, so after subtracting 1, the new value of i is 2147483647.
- 3
This code performs a stack allocation based on a length calculation.
- 4
Since a and b are declared as signed ints, the "a - b" subtraction gives a negative result (-1). However, since len is declared to be unsigned, len is cast to an extremely large positive number (on 32-bit systems - 4294967295). As a result, the buffer buf[len] declaration uses an extremely large size to allocate on the stack, very likely more than the entire computer's memory space.
- 5
Miscalculations usually will not be so obvious. The calculation will either be complicated or the result of an attacker's input to attain the negative value.
Vulnerable code example
Vulnerable C
The following example subtracts from a 32 bit signed integer.
#include <stdio.h>
#include <stdbool.h>
main (void)
{
int i;
i = -2147483648;
i = i - 1;
return 0;
} Secure code example
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} 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-191
- Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
- Implementation Validate input at trust boundaries; use allowlists, not denylists.
- Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
- Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
- Operation Monitor logs for the runtime signals listed in the next section.
Detection signals
How to detect CWE-191
Plexicus auto-detects CWE-191 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-191?
How serious is CWE-191?
What languages or platforms are affected by CWE-191?
How can I prevent CWE-191?
How does Plexicus detect and fix CWE-191?
Where can I learn more about CWE-191?
Frequently asked questions
What is CWE-191?
Integer underflow occurs when a subtraction operation results in a value smaller than the data type's minimum limit, causing the value to wrap around to a large, incorrect number.
How serious is CWE-191?
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-191?
MITRE lists the following affected platforms: C, C++, Java, C#.
How can I prevent CWE-191?
Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.
How does Plexicus detect and fix CWE-191?
Plexicus's SAST engine matches the data-flow signature for CWE-191 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-191?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/191.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-191
CWE-682 Parent
Incorrect Calculation
This vulnerability occurs when software performs a calculation that produces wrong or unexpected results, which are then used to make…
CWE-128 Sibling
Wrap-around Error
A wrap-around error happens when a variable exceeds the maximum value its data type can hold, causing it to unexpectedly reset to a very…
CWE-131 Sibling
Incorrect Calculation of Buffer Size
This vulnerability occurs when a program miscalculates the amount of memory needed for a buffer, potentially leading to a buffer overflow…
CWE-1335 Sibling
Incorrect Bitwise Shift of Integer
This vulnerability occurs when a program attempts to shift an integer's bits by an invalid amount—either a negative number or a value…
CWE-1339 Sibling
Insufficient Precision or Accuracy of a Real Number
This vulnerability occurs when a program uses a data type or algorithm that cannot accurately represent or calculate the fractional part…
CWE-135 Sibling
Incorrect Calculation of Multi-Byte String Length
This vulnerability occurs when software incorrectly measures the length of strings containing multi-byte or wide characters, leading to…
CWE-190 Sibling
Integer Overflow or Wraparound
Integer overflow or wraparound occurs when a calculation produces a numeric result that exceeds the maximum value a variable can hold.…
CWE-193 Sibling
Off-by-one Error
An off-by-one error occurs when a program incorrectly calculates a boundary, such as a loop counter or array index, by being one unit too…
CWE-369 Sibling
Divide By Zero
A divide-by-zero error occurs when software attempts to perform a division operation where the denominator is zero.
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