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CWE-124 Base Incomplet
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 a buffer.
Définition
What is CWE-124?
A buffer underwrite, also known as buffer underflow, happens when a program writes data to a memory location before the official start of a buffer.
This vulnerability occurs when a program miscalculates a write position, placing it outside the allocated buffer's bounds in the 'backward' direction. Common causes include decrementing a pointer or index too many times, performing incorrect pointer arithmetic that results in a negative offset, or directly using a negative index to access an array. The result is a write operation to an unintended, and often critical, memory location.
Writing to memory before a buffer can corrupt other program data, function pointers, or critical control structures. This can lead to system crashes, unpredictable behavior, or, if exploited, allow an attacker to hijack the program's execution flow. Developers should rigorously validate all pointer arithmetic and array indices, ensuring they remain within the valid, zero-based range of the allocated buffer.
Impact réel
Real-world CVEs caused by CWE-124
-
buffer underwrite in firmware verification routine allows code execution via a crafted firmware image
-
Unchecked length of SSLv2 challenge value leads to buffer underflow.
-
Buffer underflow from a small size value with a large buffer (length parameter inconsistency, CWE-130)
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Buffer underflow from an all-whitespace string, which causes a counter to be decremented before the buffer while looking for a non-whitespace character.
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Buffer underflow resultant from encoded data that triggers an integer overflow.
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Product sets an incorrect buffer size limit, leading to "off-by-two" buffer underflow.
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Negative value is used in a memcpy() operation, leading to buffer underflow.
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Buffer underflow due to mishandled special characters
Comment les attaquants l'exploitent
Parcours de l'attaquant étape par étape
- 1
In the following C/C++ example, a utility function is used to trim trailing whitespace from a character string. The function copies the input string to a local character string and uses a while statement to remove the trailing whitespace by moving backward through the string and overwriting whitespace with a NUL character.
- 2
However, this function can cause a buffer underwrite if the input character string contains all whitespace. On some systems the while statement will move backwards past the beginning of a character string and will call the isspace() function on an address outside of the bounds of the local buffer.
- 3
The following is an example of code that may result in a buffer underwrite. This code is attempting to replace the substring "Replace Me" in destBuf with the string stored in srcBuf. It does so by using the function strstr(), which returns a pointer to the found substring in destBuf. Using pointer arithmetic, the starting index of the substring is found.
- 4
In the case where the substring is not found in destBuf, strstr() will return NULL, causing the pointer arithmetic to be undefined, potentially setting the value of idx to a negative number. If idx is negative, this will result in a buffer underwrite of destBuf.
Exemple de code vulnérable
Vulnerable C
In the following C/C++ example, a utility function is used to trim trailing whitespace from a character string. The function copies the input string to a local character string and uses a while statement to remove the trailing whitespace by moving backward through the string and overwriting whitespace with a NUL character.
char* trimTrailingWhitespace(char *strMessage, int length) {
char *retMessage;
char *message = malloc(sizeof(char)*(length+1));
```
// copy input string to a temporary string*
char message[length+1];
int index;
for (index = 0; index < length; index++) {
```
message[index] = strMessage[index];
}
message[index] = '\0';
```
// trim trailing whitespace*
int len = index-1;
while (isspace(message[len])) {
```
message[len] = '\0';
len--;
}
```
// return string without trailing whitespace*
retMessage = message;
return retMessage;} 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-124
- Requirements Choose a language that is not susceptible to these issues.
- Implementation All calculated values that are used as index or for pointer arithmetic should be validated to ensure that they are within an expected range.
Signaux de détection
How to detect CWE-124
Exécuter des tests de sécurité applicative dynamique (DAST) contre le point de terminaison en ligne.
Surveiller les journaux runtime pour détecter des traces d'exception inhabituelles, des entrées malformées ou des tentatives de contournement d'autorisation.
Revue de code : signaler tout nouveau code qui traite les entrées de cette surface sans utiliser les helpers du framework validés.
Plexicus détecte automatiquement CWE-124 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-124 ?
Quelle est la gravité de CWE-124 ?
Quels langages ou plateformes sont affectés par CWE-124 ?
Comment puis-je prévenir CWE-124 ?
Comment Plexicus détecte et corrige CWE-124 ?
Où puis-je en savoir plus sur CWE-124 ?
Frequently asked questions
Qu'est-ce que CWE-124 ?
A buffer underwrite, also known as buffer underflow, happens when a program writes data to a memory location before the official start of a buffer.
Quelle est la gravité de CWE-124 ?
MITRE évalue la probabilité d'exploitation comme Moyenne — l'exploitation est réaliste mais nécessite généralement des conditions spécifiques.
Quels langages ou plateformes sont affectés par CWE-124 ?
MITRE lists the following affected platforms: C, C++.
Comment puis-je prévenir CWE-124 ?
Choose a language that is not susceptible to these issues. All calculated values that are used as index or for pointer arithmetic should be validated to ensure that they are within an expected range.
Comment Plexicus détecte et corrige CWE-124 ?
Le moteur SAST de Plexicus reconnaît la signature de flux de données de CWE-124 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-124 ?
MITRE publie la définition canonique à https://cwe.mitre.org/data/definitions/124.html. Vous pouvez également consulter la documentation OWASP et NIST pour des conseils adjacents.
Faiblesses associées
Weaknesses related to CWE-124
CWE-786 Parent
Access of Memory Location Before Start of Buffer
This vulnerability occurs when software attempts to read from or write to a memory location positioned before the official start of a…
CWE-127 Frère
Buffer Under-read
A buffer under-read occurs when a program attempts to read data from a memory location positioned before the start of an allocated buffer.
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