Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
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.
Real-world impact
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."
How attackers exploit it
Step-by-step attacker path
- 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.
Vulnerable code example
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;} Secure code example
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.
Prevention checklist
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.
Detection signals
How to detect CWE-761
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
Plexicus auto-detects CWE-761 and opens a fix PR in under 60 seconds.
Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.
Frequently asked questions What is CWE-761?
How serious is CWE-761?
What languages or platforms are affected by CWE-761?
How can I prevent CWE-761?
How does Plexicus detect and fix CWE-761?
Where can I learn more about CWE-761?
Frequently asked questions
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.
How serious is CWE-761?
MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.
What languages or platforms are affected by CWE-761?
MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.
How can I prevent CWE-761?
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.
How does Plexicus detect and fix CWE-761?
Plexicus's SAST engine matches the data-flow signature for CWE-761 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.
Where can I learn more about CWE-761?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/761.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
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…
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