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-195 Variante Brouillon
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 non-negative values). If the original signed value is negative,…
Définition
What is CWE-195?
This vulnerability occurs when a signed integer (which can hold negative values) is converted to an unsigned integer (which holds only non-negative values). If the original signed value is negative, the conversion produces a large, unexpected positive number instead of an error, breaking the program's logic.
Implicit conversions between signed and unsigned numbers are a common source of bugs because they happen silently during assignments or function calls. Developers often assume the value remains semantically the same, but a negative signed number becomes a very large unsigned number. This violates program assumptions and can lead to incorrect calculations, infinite loops, or flawed condition checks.
A critical risk emerges when these converted values control memory operations. Many functions return negative numbers to signal errors (like -1). If such a return value is passed directly as a 'size' argument to functions like memcpy() or malloc(), the implicit conversion turns the failure indicator into a massive allocation or copy length. This typically causes a buffer overflow, crashing the program or creating a serious security exploit.
Impact réel
Real-world CVEs caused by CWE-195
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Font rendering library does not properly handle assigning a signed short value to an unsigned long (CWE-195), leading to an integer wraparound (CWE-190), causing too small of a buffer (CWE-131), leading to an out-of-bounds write (CWE-787).
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Chain: integer signedness error (CWE-195) passes signed comparison, leading to heap overflow (CWE-122)
Comment les attaquants l'exploitent
Parcours de l'attaquant étape par étape
- 1
In this example the variable amount can hold a negative value when it is returned. Because the function is declared to return an unsigned int, amount will be implicitly converted to unsigned.
- 2
If the error condition in the code above is met, then the return value of readdata() will be 4,294,967,295 on a system that uses 32-bit integers.
- 3
In this example, depending on the return value of accecssmainframe(), the variable amount can hold a negative value when it is returned. Because the function is declared to return an unsigned value, amount will be implicitly cast to an unsigned number.
- 4
If the return value of accessmainframe() is -1, then the return value of readdata() will be 4,294,967,295 on a system that uses 32-bit integers.
- 5
The following code is intended to read an incoming packet from a socket and extract one or more headers.
Exemple de code vulnérable
Vulnerable C
In this example the variable amount can hold a negative value when it is returned. Because the function is declared to return an unsigned int, amount will be implicitly converted to unsigned.
unsigned int readdata () {
int amount = 0;
...
if (result == ERROR)
amount = -1;
...
return amount;
} Exemple de code sécurisé
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.
Liste de contrôle de prévention
How to prevent CWE-195
- 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.
Signaux de détection
How to detect CWE-195
Plexicus détecte automatiquement CWE-195 et ouvre une PR de correction en moins de 60 secondes.
Codex Remedium analyse chaque commit, identifie cette faiblesse précise et livre une pull request prête à être relue avec le correctif. Pas de tickets. Pas de transferts.
Questions fréquentes Qu'est-ce que CWE-195 ?
Quelle est la gravité de CWE-195 ?
Quels langages ou plateformes sont affectés par CWE-195 ?
Comment puis-je prévenir CWE-195 ?
Comment Plexicus détecte et corrige CWE-195 ?
Où puis-je en savoir plus sur CWE-195 ?
Frequently asked questions
Qu'est-ce que CWE-195 ?
This vulnerability occurs when a signed integer (which can hold negative values) is converted to an unsigned integer (which holds only non-negative values). If the original signed value is negative, the conversion produces a large, unexpected positive number instead of an error, breaking the program's logic.
Quelle est la gravité de CWE-195 ?
MITRE n'a pas publié de note de probabilité d'exploitation pour cette faiblesse. Traitez-la comme un impact moyen jusqu'à ce que votre modèle de menace prouve le contraire.
Quels langages ou plateformes sont affectés par CWE-195 ?
MITRE lists the following affected platforms: C, C++.
Comment puis-je prévenir CWE-195 ?
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.
Comment Plexicus détecte et corrige CWE-195 ?
Le moteur SAST de Plexicus reconnaît la signature de flux de données de CWE-195 à chaque commit. Lorsqu'une correspondance est trouvée, notre agent Codex Remedium ouvre une PR de correction avec le code corrigé, les tests et un résumé d'une ligne pour le relecteur.
Où puis-je en savoir plus sur CWE-195 ?
MITRE publie la définition canonique à https://cwe.mitre.org/data/definitions/195.html. Vous pouvez également consulter la documentation OWASP et NIST pour des conseils adjacents.
Faiblesses associées
Weaknesses related to CWE-195
CWE-681 Parent
Incorrect Conversion between Numeric Types
This vulnerability occurs when a program converts a value from one numeric type to another (like a 64-bit integer to a 32-bit integer) and…
CWE-192 Frère
Integer Coercion Error
An integer coercion error occurs when a program incorrectly converts, extends, or truncates a number between different data types, leading…
CWE-194 Frère
Unexpected Sign Extension
This vulnerability occurs when a signed number from a smaller data type is moved or cast to a larger type, causing its sign bit to be…
CWE-196 Frère
Unsigned to Signed Conversion Error
This vulnerability occurs when a program takes an unsigned integer and converts it directly to a signed integer. If the original unsigned…
CWE-197 Frère
Numeric Truncation Error
A numeric truncation error happens when a program converts a number to a smaller data type, cutting off its higher-order bits and…
CWE-119 Peut précéder
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.…
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