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 Rascunho
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.
Definição
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.
Impacto no mundo real
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.
Como os atacantes a exploram
Trajeto do atacante passo a passo
- 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.
Exemplo de código vulnerável
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;
} Exemplo de código seguro
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.
Lista de verificação de prevenção
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.
Sinais de deteção
How to detect CWE-191
O Plexicus deteta automaticamente o CWE-191 e abre um PR de correção em menos de 60 segundos.
O Codex Remedium analisa cada commit, identifica esta fraqueza exata e entrega um pull request pronto para revisão com o patch. Sem tickets. Sem transferências.
Perguntas frequentes O que é o CWE-191?
Qual a gravidade do CWE-191?
Que linguagens ou plataformas são afetadas pelo CWE-191?
Como posso prevenir o CWE-191?
Como é que o Plexicus deteta e corrige o CWE-191?
Onde posso saber mais sobre o CWE-191?
Frequently asked questions
O que é o 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.
Qual a gravidade do CWE-191?
A MITRE não publicou uma classificação de probabilidade de exploração para esta fraqueza. Trate-a como impacto médio até o seu modelo de ameaças provar o contrário.
Que linguagens ou plataformas são afetadas pelo CWE-191?
MITRE lists the following affected platforms: C, C++, Java, C#.
Como posso prevenir o 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.
Como é que o Plexicus deteta e corrige o CWE-191?
O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-191 em cada commit. Quando é encontrada uma correspondência, o nosso agente Codex Remedium abre um PR de correção com o código corrigido, testes e um resumo de uma linha para o revisor.
Onde posso saber mais sobre o CWE-191?
A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/191.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.
Fraquezas relacionadas
Weaknesses related to CWE-191
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CWE-190 Irmão
Integer Overflow or Wraparound
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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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