CodeQL documentation

Potentially overrunning write with float to string conversion

ID: cpp/overrunning-write-with-float
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 that includes one or more float to string conversions (i.e. the %f format specifier), which may overflow the destination buffer if extreme inputs are given. 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 displayValue(double value)
	char buffer[256];

	// BAD: extreme values may overflow the buffer
	sprintf(buffer, "%f", value);

	MessageBox(hWnd, buffer, "A Number", MB_OK);

In this example, the call to sprintf contains a %f format specifier. Though a 256 character buffer has been allowed, it is not sufficient for the most extreme floating point inputs. For example the representation of double value 1e304 (that is 1 with 304 zeroes after it) will overflow a buffer of this length.

To fix this issue three changes should be made:

  • Control the size of the buffer using a preprocessor define.
  • Replace the call to sprintf with snprintf, specifying the define as the maximum length to copy. This will prevent the buffer overflow.
  • Consider using the %g format specifier instead of %f.