CodeQL documentation

Unbounded write

ID: cpp/unbounded-write
Kind: path-problem
Security severity: 9.3
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. An unexpectedly long input that reaches this code will cause the buffer to overflow. 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 congratulateUser(const char *userName)
	char buffer[80];

	// BAD: this could overflow the buffer if the UserName is long
	sprintf(buffer, "Congratulations, %s!", userName);

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

In this example, the call to sprintf may overflow buffer. This occurs if the argument userName is very long, such that the resulting string is more than the 80 characters allowed.

To fix the problem the call to sprintf should be replaced with snprintf, specifying a maximum length of 80 characters.


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