In recent years, the security of computer networks has become an ever-growing concern as data breaches and cyber-attacks have become increasingly common. One potential area of vulnerability lies in the handling of network packets by the Linux kernel, specifically in its implementation of the IPv4 and IPv6 protocols. While this vulnerability, designated as CVE-2011-3188, may not directly lead to the theft of sensitive information, it can be exploited to cause denial of service (DoS) attacks and potentially hijack network sessions.

In this long-read post, we'll take a closer look at the CVE-2011-3188 vulnerability, including the specifics of how it affects the Linux kernel, the Modified MD4 hashing algorithm it uses, and how potential attackers might exploit it. We'll also provide code snippets and links to crucial original references pertaining to this vulnerability.

The Vulnerability

The Linux kernel, versions prior to 3.1, contains a vulnerability in both its IPv4 and IPv6 implementations. This vulnerability arises from the kernel's use of a modified MD4 hashing algorithm to generate sequence numbers and Fragment Identification values.

Sequence numbers and Fragment Identification values are essential components of network packets. They help in keeping track of the order of packets transmitted and ensure that packets are reconstructed in the correct order on the receiving end.

The problem with the modified MD4 algorithm lies in its predictability: attackers can potentially guess these sequence numbers and Fragment IDs and exploit this vulnerability by sending specially crafted packets, ultimately causing a denial of service (disrupted networking) or even hijacking network sessions.

Original References

For more detailed information on CVE-2011-3188 and its effects on the Linux kernel, refer to the following links:
1. The official CVE entry
2. NVD - National Vulnerability Database
3. Ubuntu Linux Security Notice

Here's a sample code snippet demonstrating the predictability of the modified MD4 hashing algorithm

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include "md4.h"

uint32_t predict_next_sequence_number(uint32_t previous_number) {
    uint32_t next_number;
    MD4_CTX ctx;
    uint8_t digest[16];

    MD4_Init(&ctx);
    MD4_Update(&ctx, &previous_number, sizeof(previous_number));
    MD4_Final(digest, &ctx);

    next_number = ((uint32_t *)digest)[];
    return next_number;
}

int main(int argc, char *argv[]) {
    uint32_t seq_num = xDEADBEEF;
    printf("Initial sequence number: %u\n", seq_num);

    for (int i = ; i < 5; i++) {
        seq_num = predict_next_sequence_number(seq_num);
        printf("Predicted sequence number: %u\n", seq_num);
    }

    return ;
}

In this example, an attacker could predict the next few sequence numbers based on a single known sequence number (xDEADBEEF).

To exploit the vulnerability, an attacker would typically follow these steps

1. Determine the target system's IP address and the target network session. This can be done using various network scanning tools and techniques.

Observe the session's communication and capture a few network packets.

3. Using the predictability of the modified MD4 algorithm, calculate the likely sequence numbers or Fragment IDs in upcoming network packets.
4. Craft malicious packets with the predicted sequence numbers and Fragment IDs and inject them into the network. This could disrupt the session or allow the attacker to hijack the target's network session.

Conclusion

CVE-2011-3188 demonstrates the importance of robust and secure implementations of network protocols, as even seemingly minor vulnerabilities can have potentially serious consequences. By understanding the specifics of this vulnerability and its potential exploitation methods, cybersecurity professionals can take steps to protect their networks and systems from similar attacks in the future. To mitigate the risk posed by this vulnerability, users and administrators should update their Linux kernel to version 3.1 or later, which implements a more secure method for generating sequence numbers and Fragment Identification values.

Timeline

Published on: 05/24/2012 23:55:00 UTC
Last modified on: 02/13/2023 04:32:00 UTC