Linux does a lot of work behind the scenes. It handles memory, manages network connections, runs applications, and keeps everything working together. We don’t usually think about any of this because, well, it’s supposed to just work.
But what happens when something goes wrong?
That’s where CVE-2026-98432 – Linux Kernel IPv4 TCP Use-After-Free Vulnerability comes in.
It’s a critical flaw in the Linux kernel that lets attackers trigger a memory issue by sending specially crafted network packets. This could crash the system, corrupt memory, or even allow remote code execution.
What’s worse, attackers don’t need to log in or trick anyone into clicking a link.
So, what’s actually going wrong? How can sending a few network packets cause so much trouble? What steps should organizations take to address this?
Let’s analyze it.
First, What's Actually Going Wrong?
CVE-2026-98432 is a use-after-free vulnerability, or UAF for short.
Here’s what it means.
Think of memory as a space where programs temporarily keep the information they need. When they’re done with that information, they release the space so it can be used for something else.
That’s perfectly normal.
The problem occurs when a program tries to use that same memory after it has already been released. It shouldn’t happen, but sometimes it does. This is known as a use-after-free vulnerability.
Now, let’s bring that back to Linux.
The kernel manages socket options when it’s handling incoming IPv4 TCP connections. With this vulnerability, it can release the memory holding those options while another operation is still trying to use them.
That’s the problem in a nutshell.
CVE-2026-98432 has a CVSS v3.1 score of 9.8, which puts it in the Critical category. An attacker doesn’t need to authenticate or have access to the system to try exploiting it. They can send specially crafted IPv4 TCP packets containing IP options to an affected system.
There’s another important detail here. Researchers demonstrated remote code execution against systems with a Multipath TCP (MPTCP) listener.
AWS also reported that it had no evidence of the vulnerability being exploited in the wild at the time of its advisory.
But how does the kernel end up releasing memory that’s still being used?
- Track Key Vulnerabilities and Exposures (CVEs)
- Visibility to Risk: Prioritizing CVEs
- Terrain-Aware Defense
So, How Does the Vulnerability Work?
Let’s start with something familiar: a device trying to connect to a Linux server.
When that connection request reaches the server, the Linux kernel has to process it and keep track of the information needed to establish the connection. Some of that information is stored in socket options.
Normally, this process should happen without any problems.
However, in this instance, the kernel moves those socket settings to a newly created child socket from a request socket. Instead of creating a separate copy, it transfers ownership of the existing memory.
Another operation might still be using those same socket options. For example, one CPU could be handling a new connection while another is processing a retransmitted SYN request for that connection.
Now imagine the child socket gets freed while that second operation is still running.
The memory is gone, but the other operation hasn’t finished using it. When it tries to access that memory again, the kernel runs into a use-after-free condition.
Here’s the sequence in simpler terms:
- An attacker sends specially crafted IPv4 TCP packets to a system that's listening for connections.
- The kernel processes the connection and transfers the socket options to a child socket.
- At the same time, another CPU may be handling a retransmitted SYN request and trying to access those same options.
- The child socket gets freed before the other operation finishes.
- The second operation tries to access the memory even though it has already been released.
And that’s how the problem occurs.
This is what’s known as a race condition. Two operations are happening at the same time, and the kernel isn’t handling the memory release safely.
There’s another way to run into a similar problem, too.
It’s called request socket migration. Basically, the kernel moves a connection request from one socket to another. During this process, the original and cloned request sockets can share the same socket options.
That creates another opportunity for one operation to release the memory while another is still using it.
So, the real issue here is timing. The kernel needs to wait until other operations are finished using the memory before releasing it. In this case, it doesn’t always do that.
What Can Actually Happen If Someone Exploits It?
Okay, so we know what’s going wrong inside the kernel. But what does this mean for an organization running Linux systems?
Unfortunately, the impact can be quite serious.
Depending on the conditions, an attacker could potentially trigger:
- System crashes: If the kernel tries to access invalid memory, it could crash the system and interrupt the services running on it.
- Memory corruption: The kernel could end up accessing memory that's already been released, potentially affecting other operations.
- Memory disclosure: Under certain conditions, information stored in kernel memory could be exposed.
- Remote code execution: Researchers demonstrated unauthenticated remote code execution against systems with an MPTCP listener.
The remote attack aspect is particularly important.
An attacker doesn’t need an account, special permissions, or someone to interact with a malicious file or link. They can attempt to trigger the vulnerable code remotely by sending specially crafted network packets.
For organizations running Linux servers that accept incoming TCP connections, that’s definitely something to pay attention to.
How Do You Know If Your Linux Systems Are Affected?
If you’re responsible for a Linux environment, this is probably the next thing on your mind.
CVE-2026-98432 affects Linux kernel versions 4.4 and later. But that doesn’t automatically mean every system running one of these versions is vulnerable.
Additionally, it depends on the Linux distribution, the kernel branch, and whether or not the necessary security patches have been installed.
Several vendors, including Amazon Linux, Red Hat, SUSE, and Debian, have published advisories and fixes.
Here’s something else to watch out for: Linux distributions often backport security fixes to older kernel versions.
In other words, a system might already have the fix even if its kernel version looks older. So, don’t rely on the version number alone when checking for vulnerable systems.
If you’re reviewing your environment, start with these questions:
- Are any of your Linux servers running potentially affected kernel builds?
- Have you installed the security updates recommended by your Linux distribution?
- Are these systems accepting incoming IPv4 TCP connections?
- Are you using Multipath TCP or request socket migration?
- Have you rebooted the affected systems after installing the kernel update?
And don’t stop with your physical servers. Check your cloud-based Linux instances, too. They can easily get overlooked when organizations are reviewing their infrastructure.
How Can It Be Fixed?
Now for the important part: fixing the issue.
The good news is that vendors have released security updates. So, if you have an affected system, getting the fixed kernel installed should be your first step.
The upstream fix changes how the kernel releases socket options. It uses kfree_rcu() to make sure memory isn’t released until active readers have finished accessing it.
So, what should you do next?
1. Install the available kernel update
First, check the security advisory for your Linux distribution.
Find the fixed kernel package recommended for your version and install it. The package you need may differ depending on the distribution and release you’re running, so follow the vendor’s instructions.
2. Reboot the affected systems
Don’t skip this step.
Installing a kernel update doesn’t necessarily mean your system is already running the updated kernel. You need to make sure the new kernel has actually been loaded.
AWS specifically notes that affected self-managed EC2 instances must be rebooted after installing the update.
3. Can't patch everything immediately? Use temporary mitigations
We know that updating every system in a large environment isn’t always something you can do overnight.
If you need more time to roll out the patches, AWS recommends a few temporary measures to reduce exposure.
For example, you can disable Multipath TCP using net.mptcp.enabled=0. Applications built with Go 1.24 or later can also use GODEBUG=multipathtcp=0 to disable MPTCP support.
Another option is to block incoming IPv4 packets containing IP options using firewall rules. AWS provides an iptables rule for this, along with guidance for configuring AWS Network Firewall.
Just remember that these measures are workarounds, not permanent fixes. They can help reduce exposure while you’re working on the updates, but they don’t address the underlying vulnerability.
4. Double-check that everything is updated
Once the updates are installed and the systems have been rebooted, check that they’re actually running the fixed kernel.
It’s also worth keeping a simple record of which systems have been updated, which kernel packages they’re running, and when they were last rebooted.
This helps you avoid missing systems during the patching process.
Is Patching Enough? What About Network Monitoring?
Getting the security updates installed should obviously come first. But there’s another thing organizations need to think about.
What about the time between discovering the vulnerability and getting every affected system patched?
You still need to know what’s happening across your network during that period.
Since CVE-2026-98432 involves incoming IPv4 TCP traffic, monitoring network activity can help security teams spot unusual patterns and investigate anything that looks suspicious.
For example, repeated SYN retransmissions, sudden bursts of TCP connection requests, or unexpected IPv4 packets containing IP options might be worth investigating.
To be clear, these aren’t confirmed signs of an attempt to exploit CVE-2026-98432. But they can give security teams additional context when looking into unusual network activity.
This is where network detection and response (NDR) comes in.
Fidelis Network® uses Deep Session Inspection® (DSI) to inspect traffic across ports and protocols. This helps security teams get a closer look at network communications, examine traffic details, and investigate suspicious activity.
It also offers capabilities such as Network DLP, sandboxing, and threat intelligence. These can help teams examine suspicious files and communications, investigate potential data movement, and understand what’s happening across their network.
Of course, network monitoring won’t fix the kernel vulnerability. It doesn’t replace patching, either.
But it gives security teams another way to keep an eye on their environment, investigate unusual activity, and get a better understanding of what’s happening while potentially vulnerable systems are still being updated.
- Content Inspection
- Content Identification
- Full Session Reassembly
So, What Should You Take Away from This?
CVE-2026-98432 is a good example of how something as basic as memory management can turn into a serious security problem.
The Linux kernel can end up accessing memory that’s already been released because two operations are trying to use the same socket options at the same time. Additionally, this may result in system crashes, memory corruption, or even remote code execution, depending on the situation.
Don’t wait to find out if you are impacted if your company uses Linux.
Review your systems, install the recommended kernel updates, and reboot where required. If you need more time to patch everything, look at the temporary mitigations recommended by your Linux vendor.
And while you’re doing all that, keep an eye on your network, too. Knowing what’s happening across your environment can help security teams spot unusual activity and investigate potential threats before they turn into bigger problems.
