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CWE-761 Variant Incomplete
Free of Pointer not at Start of Buffer
This vulnerability occurs when a program incorrectly frees a memory pointer that no longer points to the beginning of the allocated heap buffer, often due to pointer arithmetic.
Definition
What is CWE-761?
This vulnerability occurs when a program incorrectly frees a memory pointer that no longer points to the beginning of the allocated heap buffer, often due to pointer arithmetic.
This issue typically happens when you allocate memory using functions like `malloc()`, `calloc()`, or `realloc()`, and then later modify the pointer—for example, by incrementing it to traverse a data structure. When you later pass this offset pointer to `free()`, the memory manager cannot correctly identify the original memory block's metadata, leading to heap corruption.
This corruption can cause immediate crashes, unpredictable behavior, or even create opportunities for attackers to manipulate program data or execution flow. To prevent this, always ensure you free the exact pointer returned by the allocation function, or use a separate tracking variable to preserve the original starting address.
Auswirkungen in der Praxis
Real-world CVEs caused by CWE-761
-
function "internally calls 'calloc' and returns a pointer at an index... inside the allocated buffer. This led to freeing invalid memory."
Wie Angreifer es ausnutzen
Angreiferpfad Schritt für Schritt
- 1
In this example, the programmer dynamically allocates a buffer to hold a string and then searches for a specific character. After completing the search, the programmer attempts to release the allocated memory and return SUCCESS or FAILURE to the caller. Note: for simplification, this example uses a hard-coded "Search Me!" string and a constant string length of 20.
- 2
However, if the character is not at the beginning of the string, or if it is not in the string at all, then the pointer will not be at the start of the buffer when the programmer frees it.
- 3
Instead of freeing the pointer in the middle of the buffer, the programmer can use an indexing pointer to step through the memory or abstract the memory calculations by using array indexing.
- 4
This code attempts to tokenize a string and place it into an array using the strsep function, which inserts a \0 byte in place of whitespace or a tab character. After finishing the loop, each string in the AP array points to a location within the input string.
- 5
Since strsep is not allocating any new memory, freeing an element in the middle of the array is equivalent to free a pointer in the middle of inputstring.
Verwundbares Codebeispiel
Vulnerable C
In this example, the programmer dynamically allocates a buffer to hold a string and then searches for a specific character. After completing the search, the programmer attempts to release the allocated memory and return SUCCESS or FAILURE to the caller. Note: for simplification, this example uses a hard-coded "Search Me!" string and a constant string length of 20.
#define SUCCESS (1)
#define FAILURE (0)
int contains_char(char c){
char *str;
str = (char*)malloc(20*sizeof(char));
strcpy(str, "Search Me!");
while( *str != NULL){
if( *str == c ){
```
/* matched char, free string and return success */*
free(str);
return SUCCESS;}
*/* didn't match yet, increment pointer and try next char */*
str = str + 1;}
*/* we did not match the char in the string, free mem and return failure */*
free(str);
return FAILURE;} Sicheres Codebeispiel
Secure C
Instead of freeing the pointer in the middle of the buffer, the programmer can use an indexing pointer to step through the memory or abstract the memory calculations by using array indexing.
#define SUCCESS (1)
#define FAILURE (0)
int cointains_char(char c){
char *str;
int i = 0;
str = (char*)malloc(20*sizeof(char));
strcpy(str, "Search Me!");
while( i < strlen(str) ){
if( str[i] == c ){
```
/* matched char, free string and return success */*
free(str);
return SUCCESS;}
*/* didn't match yet, increment pointer and try next char */*
i = i + 1;}
*/* we did not match the char in the string, free mem and return failure */*
free(str);
return FAILURE;} 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-761
- Implementation When utilizing pointer arithmetic to traverse a buffer, use a separate variable to track progress through memory and preserve the originally allocated address for later freeing.
- Implementation When programming in C++, consider using smart pointers provided by the boost library to help correctly and consistently manage memory.
- Architecture and Design Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, glibc in Linux provides protection against free of invalid pointers.
- Architecture and Design Use a language that provides abstractions for memory allocation and deallocation.
- Testing Use a tool that dynamically detects memory management problems, such as valgrind.
Erkennungssignale
How to detect CWE-761
Führe dynamische Application-Security-Tests gegen den Live-Endpoint aus.
Beobachte Runtime-Logs auf ungewöhnliche Exception-Traces, fehlerhafte Eingaben oder Versuche, Autorisierung zu umgehen.
Code Review: Markiere jeden neuen Code, der Eingaben von dieser Oberfläche ohne validierte Framework-Helper verarbeitet.
CWE-761
Don't catalog this weakness. Prove it's reachable.
Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.
Häufig gestellte Fragen Was ist CWE-761?
Wie gravierend ist CWE-761?
Welche Sprachen oder Plattformen sind von CWE-761 betroffen?
Wie kann ich CWE-761 verhindern?
Wie erkennt und behebt Plexicus CWE-761?
Wo erfahre ich mehr über CWE-761?
Frequently asked questions
Was ist CWE-761?
This vulnerability occurs when a program incorrectly frees a memory pointer that no longer points to the beginning of the allocated heap buffer, often due to pointer arithmetic.
Wie gravierend ist CWE-761?
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-761 betroffen?
MITRE hat für diese CWE keine betroffenen Plattformen spezifiziert — sie kann in den meisten Anwendungs-Stacks auftreten.
Wie kann ich CWE-761 verhindern?
When utilizing pointer arithmetic to traverse a buffer, use a separate variable to track progress through memory and preserve the originally allocated address for later freeing. When programming in C++, consider using smart pointers provided by the boost library to help correctly and consistently manage memory.
Wie erkennt und behebt Plexicus CWE-761?
Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-761 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-761?
MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/761.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.
Verwandte Schwachstellen
Weaknesses related to CWE-761
CWE-763 Parent
Release of Invalid Pointer or Reference
This vulnerability occurs when a program tries to free a memory resource back to the system but uses an incorrect deallocation method or…
CWE-762 Sibling
Mismatched Memory Management Routines
This vulnerability occurs when a program uses incompatible functions to allocate and free memory. For example, freeing memory with a…
Bereit, das Wesentliche zu validieren?
Bereit zu validieren, was zählt.
Plexicus ist Proof-Driven AppSec: validierte Funde, kontextuelles Verständnis und geprüfte Remediation — in Evidenz verankert, mit Ihnen gescoped.