Recently, a critical vulnerability (CVE-2023-4039) has been discovered affecting the -fstack-protector feature in GCC-based toolchains targeting the AArch64 architecture. The vulnerability allows attackers to exploit an existing buffer overflow in dynamically-sized local variables in an application without being detected. This could result in uncontrolled loss of availability, affecting confidentiality or integrity, and poses significant risks to affected systems. This stack-protector failure only applies to C99-style dynamically-sized local variables or those created using alloca(), and not for statically-sized local variables.

Exploit Details

The vulnerability in question is related to the default behavior of the stack-protector, which, upon detection of an overflow, would terminate the application. However, an attacker who can exploit a buffer overflow without triggering the stack-protector might be able to change the program's flow control to cause an uncontrolled loss of availability or even further impact the confidentiality and integrity of the system.

Here's a sample code snippet to demonstrate the vulnerability

#include <stdio.h>
#include <stdlib.h>

void vulnerable_function(size_t length) {
  char vulnerable_buffer[length]; 
  /* alloca()-based vulnerable_buffer allocations would also be affected */
  memset(vulnerable_buffer, , length);
  fgets(vulnerable_buffer, length, stdin);
  printf("Input: %s\n", vulnerable_buffer);

int main() {
  size_t buffer_size;
  printf("Enter size: ");
  scanf("%lu", &buffer_size);
  return ;

In the example above, since vulnerable_buffer is a dynamically-sized local variable, the stack-protector fails to protect against buffer overflow. An attacker can potentially exploit this vulnerability to manipulate the program's control flow and affect the confidentiality, integrity, or availability of the system.

Original References

1. A detailed explanation of the vulnerability and the official patch can be found in the following mailing list message:
2. The official report on the CVE can be found here:

Mitigation and Recommendation

To mitigate the risk posed by this vulnerability, it is recommended to apply the latest security patches and updates provided by the toolchain vendors. A possible workaround is to avoid using C99-style and alloca()-based dynamically-sized local variables and opt for statically-sized local variables or heap-allocated buffers, ensuring proper bounds checking.


CVE-2023-4039 is a critical vulnerability affecting the -fstack-protector feature in GCC-based toolchains targeting AArch64, impacting C99-style and alloca()-based dynamically-sized local variables. Prompt action by developers to update their toolchain or employ recommended mitigations is vital to maintain system security and protect the confidentiality, integrity, and availability of affected systems.


Published on: 09/13/2023 09:15:00 UTC
Last modified on: 09/14/2023 20:01:00 UTC