CodeQL documentation

Potential use after free

ID: cpp/use-after-free
Kind: path-problem
Security severity: 9.3
Severity: warning
Precision: high
   - reliability
   - security
   - external/cwe/cwe-416
Query suites:
   - cpp-code-scanning.qls
   - cpp-security-extended.qls
   - cpp-security-and-quality.qls

Click to see the query in the CodeQL repository

This rule finds accesses through a pointer of a memory location that has already been freed (i.e. through a dangling pointer). Such memory blocks have already been released to the dynamic memory manager, and modifying them can lead to anything from a segfault to memory corruption that would cause subsequent calls to the dynamic memory manger to behave erratically, to a possible security vulnerability.

WARNING: This check is an approximation, so some results may not be actual defects in the program. It is not possible in general to compute the values of pointers without running the program with all input data.


Ensure that all execution paths that access memory through a pointer never access that pointer after it is freed.


void f() {
	char* buf = new char[SIZE];
	if (error) {
		delete buf; //error handling has freed the buffer
	log_contents(buf); //but it is still used here for logging


  • I. Gerg. An Overview and Example of the Buffer-Overflow Exploit. IANewsletter vol 7 no 4. 2005.

  • M. Donaldson. Inside the Buffer Overflow Attack: Mechanism, Method & Prevention. SANS Institute InfoSec Reading Room. 2002.

  • Common Weakness Enumeration: CWE-416.

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