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.
Auswirkungen in der Praxis

Real-world CVEs caused by CWE-191

  • CVE-2004-0816

    Integer underflow in firewall via malformed packet.

  • CVE-2004-1002

    Integer underflow by packet with invalid length.

  • CVE-2005-0199

    Long input causes incorrect length calculation.

  • CVE-2005-1891

    Malformed icon causes integer underflow in loop counter variable.

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

  1. 1

    The following example subtracts from a 32 bit signed integer.

  2. 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. 3

    This code performs a stack allocation based on a length calculation.

  4. 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. 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.

Verwundbares Codebeispiel

Vulnerable C

The following example subtracts from a 32 bit signed integer.

Verwundbar C 
#include <stdio.h>
  #include <stdbool.h>
  main (void)
  {
  	int i;
  	i = -2147483648;
  	i = i - 1;
  	return 0;
  }
Sicheres Codebeispiel

Secure pseudo

Sicher 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.
Präventions-Checkliste

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.
Erkennungssignale

How to detect CWE-191

Automated Static Analysis High

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.)

Plexicus Auto-Fix

Plexicus erkennt CWE-191 automatisch und öffnet in unter 60 Sekunden einen Fix-PR.

Codex Remedium scannt jeden Commit, identifiziert genau diese Schwachstelle und liefert einen reviewer-ready Pull Request mit dem Patch. Keine Tickets. Keine Hand-offs.

Demo anfordern Plexicus kostenlos testen
Häufig gestellte Fragen

Frequently asked questions

Was ist 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.

Wie gravierend ist CWE-191?

MITRE hat für diese Schwachstelle keine Exploit-Wahrscheinlichkeit veröffentlicht. Behandle sie als mittlere Auswirkung, bis dein Threat Model anderes belegt.

Welche Sprachen oder Plattformen sind von CWE-191 betroffen?

MITRE lists the following affected platforms: C, C++, Java, C#.

Wie kann ich CWE-191 verhindern?

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.

Wie erkennt und behebt Plexicus CWE-191?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-191 bei jedem Commit. Bei einem Treffer öffnet unser Codex-Remedium-Agent einen Fix-PR mit korrigiertem Code, Tests und einer einzeiligen Zusammenfassung für den Reviewer.

Wo erfahre ich mehr über CWE-191?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/191.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

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.

Ressourcen

Further reading

Bereit, wenn du es bist

Schluss mit dem Bezahlen pro Entwickler.
Schließ den Kreislauf.

Plexicus ist die KI-native ASPM, die scannt, filtert, fixt, pentestet und erklärt — autonom. Unbegrenzte Entwickler, unbegrenzte Repos, Fair-Use-KI-Aktionen. Echter kostenloser Tarif, €269/mo jährlich, wenn du bereit bist.

Kostenlos starten Demo buchen