A critical vulnerability, identified as CVE-2023-39192, has been discovered in the Netfilter subsystem of the Linux kernel. This vulnerability exists in the xt_u32 module, which does not properly validate the fields in the xt_u32 structure. The flaw can be exploited by a local privileged attacker, resulting in an out-of-bounds read by setting the size fields with a value beyond the array boundaries. This could potentially lead to a system crash or unauthorized information disclosure.

In this post, we will dive into the details of the vulnerability, code snippets illustrating the issue, and possible exploits that could arise from this flaw.

Vulnerability Details

The vulnerability stems from the improper validation of fields in the xt_u32 structure within the xt_u32 module. The xt_u32 module is responsible for matching 32-bit words within IP/TCP/UDP packets. As this module doesn't verify the field values, an attacker can set the size fields beyond the array boundaries, causing an out-of-bounds read.

The following code snippet illustrates the problematic part in the Linux kernel, present in "net/netfilter/xt_u32.c":

  static int u32_mt(const struct sk_buff *skb, struct xt_action_param *par)
	const struct xt_u32 *info = par->matchinfo;
	unsigned int nnums = ;
	unsigned int nvals = ;
	const u_int32_t *num32;
	bool results = 1;

	if (skb_linearize(skb) < )
		return false;

	for (; nvals < info->nvals; nvals++) {
		num32 = skb_header_pointer(skb, info->value[nvals].off,
					       sizeof(*num32), &_num32);
		if (num32 == NULL)
			return false;

		num32 = ntohl(*num32);

		if (info->value[nvals].mask == xFFFFFFFFU) {
				if (num32 - info->value[nvals].min <= info->value[nvals].max - info->value[nvals].min)

			/* Multiple ranges */
			for (nnums = ; nnums < info->value[nvals].nnums; nnums++) {
				if (num32 == info->value[nvals].num[nnums])
		if (nnums != info->value[nvals].nnums)

The issue exists because "info->value[nvals].off" and "info->nvals" (number of comparison values) are not properly checked against the maximum allowed values, leading to an out-of-bounds read.


An attacker can exploit this vulnerability by crafting malicious input that sets the size fields with values beyond the array boundaries. This can potentially crash the system or cause information disclosure depending on how the system is set up and what other security mitigations are in place.

To exploit this vulnerability, an attacker would need local access to the system and privileges to set the Netfilter configuration. The following pseudo-code illustrates an example of how to exploit this vulnerability:

  1. Gain local access to the target Linux system
  2. Elevate privileges to the root or an equally privileged user
  3. Configure the malicious Netfilter rules with the crafted input
  4. Monitor the system for crash or information leakage, and use it according to the attacker's objectives

Original References

The vulnerability was initially discovered by [security researcher's name] and has been reported to the Linux kernel developers through the proper channels. The following links provide more information on the vulnerability:

1. Linux Kernel Mailing List
2. CVE-2023-39192
3. National Vulnerability Database


CVE-2023-39192 is a critical vulnerability in the Netfilter subsystem of the Linux kernel, as it exposes systems to the risk of crashes and information disclosure. It is highly recommended for administrators to apply patches and implement the necessary security mitigations to prevent exploitation of this vulnerability.


Published on: 10/09/2023 18:15:10 UTC
Last modified on: 11/07/2023 04:17:27 UTC