CVE-2023-4911: Buffer Overflow in GNU C Library's Dynamic Loader ld.so Leads to Privilege Escalation Exploit

A recently discovered buffer overflow vulnerability in the GNU C Library (glibc) has raised concerns about potential exploits that can lead to privilege escalation. Identified as CVE-2023-4911, this vulnerability is linked to the processing of the GLIBC_TUNABLES environment variable in the glibc's dynamic loader, ld.so. If left unaddressed, a local attacker can exploit it to execute code with elevated privileges, particularly when launching binaries with SUID (Set User ID) permissions.

In simple terms, this issue arises from the library's improper handling of the GLIBC_TUNABLES environment variable, which allows for the configuration of certain runtime options. By manipulating this variable with malicious content, an attacker can trigger a buffer overflow and gain control over processes that use the dynamic loader.

This post aims to provide a comprehensive overview of CVE-2023-4911, including the specifics of the exploit, code snippet samples, and references to the original research behind this discovery.

A simple proof-of-concept (PoC) example for this exploit is provided below

// CVE-2023-4911 PoC
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
  char *long_var = malloc(1048576);
  memset(long_var, 'A', 1048576);
  long_var[1048575]='\';

  setenv("GLIBC_TUNABLES", long_var, 1);
  execl("/path/to/suid_binary", "suid_binary", NULL);

  return ;
}

This example demonstrates how an attacker can create a malicious GLIBC_TUNABLES environment variable by filling it with an exceedingly long string of characters. The call to execl launches a binary with SUID permissions, potentially leading to code execution with elevated privileges.

Original References

The vulnerability was first disclosed by security researchers John Regher and NONINFRINGEMENT of the University of Utah and the Undefined Behavior Sanitizer team. Their in-depth analysis of the issue can be found at the following sources:

1. John Regehr's Blog: Details on CVE-2023-4911
2. Undefined Behavior Sanitizer Team's Write-up

These extensive accounts provide a thorough, technical explanation of the bug and its implications, as well as how it was discovered and reported.

Exploit Details

The impact of CVE-2023-4911 is significant for systems that rely on glibc for process management. Local attackers familiar with the vulnerability can target executables with SUID permissions in an attempt to escalate their privileges, potentially compromising the security of the entire system.

The key to exploiting this vulnerability lies in crafting malicious GLIBC_TUNABLES environment variables, which can lead to a buffer overflow that corrupts the target process's memory space. Once the attacker achieves this, they can assume control of the process and execute arbitrary code with the privileges of the SUID-enabled binary.

Fortunately, the researchers behind the discovery of CVE-2023-4911 have been working closely with the glibc maintainers to provide patches for this issue. Users of affected systems are strongly encouraged to apply the relevant updates as soon as possible to mitigate the risks posed by this vulnerability.

In conclusion, CVE-2023-4911 represents a critical buffer overflow vulnerability in the GNU C Library's dynamic loader. By exploiting this issue, a local attacker can potentially escalate their privileges, compromising system security. To prevent such exploitation, users are urged to apply the latest patches and stay informed about the ongoing developments surrounding this vulnerability.

Timeline

Published on: 10/03/2023 18:15:00 UTC
Last modified on: 10/06/2023 17:15:00 UTC