Fuzz testing (fuzzing) is a powerful technique for generating large numbers of diverse inputs - either randomly or algorithmically - and dynamically invoking the code with those inputs. Even with random inputs, it is often capable of generating unexpected results such as crashes, memory corruption, or resource consumption. Fuzzing effectively produces repeatable test cases that clearly indicate bugs, which helps developers to diagnose the issues.
CWE-786 Base Incomplet
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 buffer.
Définition
What is CWE-786?
This vulnerability occurs when software attempts to read from or write to a memory location positioned before the official start of a buffer.
This flaw, often called a 'buffer under-read' or 'under-write,' happens when a pointer or index is incorrectly positioned. Common triggers include decrementing a pointer beyond the buffer's first element, performing pointer arithmetic that steps back too far, or directly using a negative index value to access array-like structures.
Accessing memory before a buffer's valid range can lead to unpredictable behavior. It may read sensitive data from unrelated parts of memory, corrupt critical program state, or cause an immediate crash, creating opportunities for denial-of-service or information disclosure attacks.
Impact réel
Real-world CVEs caused by CWE-786
-
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)
-
Buffer underflow from an all-whitespace string, which causes a counter to be decremented before the buffer while looking for a non-whitespace character.
-
Buffer underflow resultant from encoded data that triggers an integer overflow.
-
Product sets an incorrect buffer size limit, leading to "off-by-two" buffer underflow.
-
Negative value is used in a memcpy() operation, leading to buffer underflow.
-
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 example asks a user for an offset into an array to select an item.
- 4
The programmer allows the user to specify which element in the list to select, however an attacker can provide an out-of-bounds offset, resulting in a buffer over-read (CWE-126).
- 5
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.
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-786
- 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-786
Plexicus détecte automatiquement CWE-786 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-786 ?
Quelle est la gravité de CWE-786 ?
Quels langages ou plateformes sont affectés par CWE-786 ?
Comment puis-je prévenir CWE-786 ?
Comment Plexicus détecte et corrige CWE-786 ?
Où puis-je en savoir plus sur CWE-786 ?
Frequently asked questions
Qu'est-ce que CWE-786 ?
This vulnerability occurs when software attempts to read from or write to a memory location positioned before the official start of a buffer.
Quelle est la gravité de CWE-786 ?
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-786 ?
MITRE lists the following affected platforms: C, C++.
Comment puis-je prévenir CWE-786 ?
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-786 ?
Le moteur SAST de Plexicus reconnaît la signature de flux de données de CWE-786 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-786 ?
MITRE publie la définition canonique à https://cwe.mitre.org/data/definitions/786.html. Vous pouvez également consulter la documentation OWASP et NIST pour des conseils adjacents.
Faiblesses associées
Weaknesses related to CWE-786
CWE-119 Parent
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.…
CWE-120 Frère
Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
This vulnerability occurs when a program copies data from one memory location to another without first verifying that the source data will…
CWE-123 Frère
Write-what-where Condition
A write-what-where condition occurs when an attacker can control both the data written and the exact memory location where it's written,…
CWE-125 Frère
Out-of-bounds Read
An out-of-bounds read occurs when software accesses memory outside the boundaries of a buffer, array, or similar data structure, reading…
CWE-130 Frère
Improper Handling of Length Parameter Inconsistency
This vulnerability occurs when a program reads a structured data packet or message but fails to properly validate that the declared length…
CWE-466 Frère
Return of Pointer Value Outside of Expected Range
This vulnerability occurs when a function returns a memory pointer that points outside the expected buffer range, potentially exposing…
CWE-787 Frère
Out-of-bounds Write
This vulnerability occurs when software incorrectly writes data outside the boundaries of its allocated memory buffer, either beyond the…
CWE-788 Frère
Access of Memory Location After End of Buffer
This vulnerability occurs when software attempts to read from or write to a memory buffer using an index or pointer that points past the…
CWE-805 Frère
Buffer Access with Incorrect Length Value
This vulnerability occurs when software reads from or writes to a buffer using a loop or sequential operation, but mistakenly calculates…
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