In 2018, a serious vulnerability (CVE-2018-9389) was found in the Linux kernel's IPv6 networking stack. Specifically, in the ip6_append_data function within the ip6_output.c source file, there’s a heap buffer overflow bug. This can allow a local user to escalate privileges—achieving code execution as root without any user interaction. This article will explain the vulnerability in simple terms, examine the affected code, provide resources, and walk through how an exploit could work in practice.

What is CVE-2018-9389?

CVE-2018-9389 is a heap buffer overflow in the Linux kernel's IPv6 code. Attackers exploiting this flaw can run arbitrary code in kernel space, potentially gaining root privileges. All they need is the ability to send specially crafted IPv6 packets from a local account. No special privileges or user involvement is required.

Affected components:
ip6_append_data function in ip6_output.c

Vulnerable kernel versions:
Varies, but seen in versions prior to the upstream fix. If your Linux distribution is from 2018 or earlier, do check your kernel version!

Where's the problem?

Inside the Linux kernel networking code, the ip6_append_data function takes data and appends it to outgoing IPv6 packets. Due to messed up buffer size calculations, an attacker can make the kernel write more data than the buffer can hold—causing a heap buffer overflow.

This is dangerous since:

Here's a simplified version of the problem area (excerpted and cleaned up for clarity)

int ip6_append_data(...) {
    ...
    // Bad size calculation might allow overflow
    int len = ...; // Length attacker controls
    skb = sock_alloc_send_skb(sk, alloclen, ...);
    ...
    memcpy(skb_put(skb, len), from, len); // len can be too big!
}

In the buggy version, if the attacker controls len, it can exceed the size of the allocated buffer, so the memcpy writes past the end—classic heap buffer overflow.

Gain code execution:

- The overflow corrupts kernel data. Sophisticated attackers can place fake objects or function pointers, making the kernel run attacker code.

Exploit Example (Proof of Concept)

Here’s a conceptual PoC (for educational purposes only!) using Python’s socket library. It won’t fully exploit the bug (since that's complex and dangerous), but shows how a crafted IPv6 packet could trigger the path:

import socket

# Create IPv6 raw socket
s = socket.socket(socket.AF_INET6, socket.SOCK_RAW, socket.IPPROTO_RAW)

# Overly large fake payload
payload = b'A' * (65535)  # Example large data

# Target address (change ::1 for real network)
dest = ('::1', )

try:
    s.sendto(payload, dest)
    print("Packet sent; if kernel is vulnerable, could overflow heap.")
except Exception as e:
    print("Error: ", e)

In practice, you'll need a custom kernel exploit to turn the overflow into code execution, which is beyond this scope (and potentially illegal without authorization).

Original References & Further Reading

- Android Security Bulletin (April 2018)
- NVD Detail: CVE-2018-9389
- Patch Commit & Kernel.org Discussion
- Public Exploit Discussion on seclists.org

Conclusion

CVE-2018-9389 is a real-world example of how subtle memory bugs in low-level code can lead to disastrous privilege escalation vulnerabilities. Even if you don’t run a multi-user server, keeping your system patched is crucial—kernel bugs often get chained for bigger attacks.

If you want to learn more or test for this vulnerability, stick to your own systems and always work in safe, authorized environments.

Timeline

Published on: 01/18/2025 00:15:24 UTC
Last modified on: 01/23/2025 19:56:32 UTC