CodeQL documentation

Likely overrunning write

ID: cpp/very-likely-overrunning-write
Kind: problem
Security severity: 9.3
Severity: error
Precision: high
   - reliability
   - security
   - external/cwe/cwe-120
   - external/cwe/cwe-787
   - external/cwe/cwe-805
Query suites:
   - cpp-code-scanning.qls
   - cpp-security-extended.qls
   - cpp-security-and-quality.qls

Click to see the query in the CodeQL repository

The program performs a buffer copy or write operation with no upper limit on the size of the copy. By analyzing the bounds of the expressions involved, it appears that certain inputs will cause a buffer overflow to occur in this case. In addition to causing program instability, techniques exist which may allow an attacker to use this vulnerability to execute arbitrary code.


Always control the length of buffer copy and buffer write operations. strncpy should be used over strcpy, snprintf over sprintf, and in other cases ‘n-variant’ functions should be preferred.


int sayHello(uint32_t userId)
	char buffer[17];

	if (userId > 9999) return USER_ID_OUT_OF_BOUNDS;

	// BAD: this message overflows the buffer if userId >= 1000,
	// as no space for the null terminator was accounted for
	sprintf(buffer, "Hello, user %d!", userId);

	MessageBox(hWnd, buffer, "New Message", MB_OK);
	return SUCCESS;

In this example, the call to sprintf writes a message of 14 characters (including the terminating null) plus the length of the string conversion of `userId` into a buffer with space for just 17 characters. While `userId` is checked to occupy no more than 4 characters when converted, there is no space in the buffer for the terminating null character if `userId >= 1000`. In this case, the null character overflows the buffer resulting in undefined behavior.

To fix this issue these changes should be made:

  • Control the size of the buffer by declaring it with a compile time constant.

  • Preferably, replace the call to sprintf with snprintf, using the defined constant size of the buffer or `sizeof(buffer)` as maximum length to write. This will prevent the buffer overflow.

  • Increasing the buffer size to account for the full range of `userId` and the terminating null character.


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