CodeQL documentation

Potentially overrunning write

ID: cpp/overrunning-write
Kind: problem
Severity: error
Precision: medium
   - reliability
   - security
   - external/cwe/cwe-120
   - external/cwe/cwe-787
   - external/cwe/cwe-805
Query suites:
   - 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, and 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.


void sayHello()
	char buffer[10];

	// BAD: this message overflows the buffer
	strcpy(buffer, "Hello, world!");

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

In this example, the call to strcpy copies a message of 14 characters (including the terminating null) into a buffer with space for just 10 characters. As such, the last four characters overflow the buffer resulting in undefined behavior.

To fix this issue three changes should be made:

  • Control the size of the buffer using a preprocessor define.
  • Replace the call to strcpy with strncpy, specifying the define as the maximum length to copy. This will prevent the buffer overflow.
  • Consider increasing the buffer size, say to 20 characters, so that the message is displayed correctly.