Critical SCTP Flaw CVE-2026-53225 Exposes Uninitialized Kernel Memory in WSL and Azure — Microsoft Issues Patch

Microsoft has released a security patch for the Linux kernel addressing CVE-2026-53225, a serious vulnerability in the Stream Control Transmission Protocol (SCTP) that could enable attackers to read uninitialized kernel memory in Windows Subsystem for Linux (WSL) and Azure Linux virtual machines. Disclosed on June 27, 2026, the flaw resides in the kernel’s __sctp_rcv_asconf_lookup() function and is triggered by a malformed ASCONF (Address Configuration Change) chunk. Successful exploitation may leak sensitive data such as encryption keys, credentials, or other random kernel heap residues.

Administrators of WSL and Azure workloads are urged to apply the update immediately. The vulnerability, rated a 5.6 Medium severity under CVSS v3.1, is technically a local information disclosure, but its presence in shared environments like WSL where a Windows host can interact with the Linux guest amplifies the risk. While no active exploits have been reported in the wild, the low attack complexity and the ease of crafting a proof-of-concept make it a ticking time bomb.

What is SCTP and Why Does It Matter for WSL and Azure?

SCTP is a transport-layer protocol, similar to TCP and UDP, designed for telephony signaling and multimedia communication. It provides reliable, message-oriented delivery with features like multi-homing and multi-streaming. While not as ubiquitous as TCP, SCTP is still used in carrier-grade telecom, WebRTC data channels, and some cloud-native applications.

In the context of WSL 2, the Linux kernel that runs under the Windows hypervisor includes SCTP support by default. This means a malicious process running inside WSL—or even on the Windows host—can send specially crafted SCTP packets to a local listening service inside the Linux environment, triggering the vulnerability. Similarly, Azure Linux VMs that have enabled SCTP (for example, for applications like Diameter or SIP) are exposed if an attacker gains access to the same network segment.

The real danger lies in the fact that the vulnerable code path is reachable without authentication. A remote attacker able to send packets to an SCTP-enabled service could exploit the flaw to exfiltrate fragments of kernel memory. Because the kernel shares a single memory space, any sensitive data that happens to reside in the read range—session keys, tokens, file buffers, or even private user data—could be leaked.

Technical Deep Dive: Uninitialized Memory Read in __sctp_rcv_asconf_lookup()

At the heart of the flaw is the SCTP ASCONF mechanism, which allows peers to dynamically add, delete, or change IP addresses associated with an existing SCTP association. When a host receives an ASCONF chunk, the kernel performs a lookup to see whether the chunk pertains to an existing association. The function __sctp_rcv_asconf_lookup() is responsible for this task.

According to the kernel commit fixing the issue, the bug occurs when a peer sends an ASCONF chunk containing an address parameter that already appears in the association’s address list. In such a case, the function attempts to store a pointer to the address structure. However, due to a logic error in the length validation of the incoming parameter, the pointer may end up pointing to a region of kernel memory that has not been properly initialized. Instead of returning the intended address struct, it returns whatever data happened to be in that memory location from previous allocations.

The result is a classic uninitialized memory read (CWE-125). An attacker can control the length and offset of the read by manipulating the ASCONF parameter fields, effectively reading chunks of kernel memory one fragment at a time. While the kernel typically zeroes allocations (kmalloc, kzalloc), memory reuse in slab caches can leave behind residual data from earlier operations. This could include cryptographic keys, TCP sequence numbers, or even plaintext passwords if they were recently processed by the kernel.

Exploitation requires that the target system has an SCTP association in the ESTABLISHED state. However, certain SCTP implementations may prematurely create associations upon receiving an INIT chunk without fully establishing the connection, widening the attack surface. Security researchers at Microsoft’s Offensive Research & Security Engineering (MORSE) team, who discovered the vulnerability in February 2026, confirmed that a crafted sequence of SCTP packets can force the kernel into a state where the flawed lookup path is reached.

Affected Systems and Versions

CVE-2026-53225 affects all Linux kernel versions from 4.14 up to and including the 6.9 stable tree. The bug was introduced during a refactoring of ASCONF handling in 2017 and went unnoticed for years. The following table summarizes the fix status across common distributions and environments:

For Azure customers, Microsoft has already rolled out patched kernel images for its supported Linux distributions (Ubuntu, Debian, RHEL, SUSE) via Azure Marketplace. VMs that use auto‑update mechanisms will receive the fix automatically. However, environment‑specific kernels like the WSL2 kernel require manual intervention.

The CVSS Score and Why It’s More Than Just a Medium

CVE-2026-53225 was assigned a CVSS v3.1 score of 5.6 (Medium), with the following vector string: AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N. The score reflects the local attack vector and the limited integrity and availability impact. Yet, many security professionals argue that in real‑world deployments, the actual risk is higher.

The “local” attack vector may minimize the score, but in a WSL2 setup, a local user or malware on the Windows host can easily send malicious packets to the Linux guest because the two share the same network namespace via a virtual switch. Moreover, if an Azure VM runs an SCTP‑based service that is reachable from the internet, the attack vector effectively becomes network‑based. The scope change (“S:C”) indicates that a successful compromise of the Linux kernel can affect other components, such as the Windows host in WSL, making this a cross‑boundary information leak.

Because the kernel memory read yields arbitrary data, the true impact depends on what secrets happen to be in the slab cache at the time of exploitation. Determined attackers could perform targeted reads by flushing caches strategically, turning a medium‑severity bug into a high‑impact data breach.

How to Patch and Verify

Windows Subsystem for Linux

WSL 2 employs a Microsoft‑provided Linux kernel that is maintained separately from the mainline. To update the kernel to the fixed version, run the following command in PowerShell or Command Prompt:

wsl --update

This will download and install the latest kernel from Microsoft’s servers. After the update, ensure the new kernel version is active by executing inside your WSL distribution:

uname -r

The output should show a version equal to or higher than 5.15.150.1-msft or 6.6.33.1-msft, depending on your WSL configuration.

If you have custom WSL kernel sources, apply the commit 9f2d1f8a (“sctp: Fix uninitialized memory read in __sctp_rcv_asconf_lookup”) from the Microsoft WSL2‑kernel repository.

Azure Linux Virtual Machines

For Azure VMs using platform‑managed kernels, the patches are delivered through your distribution’s standard update mechanism. On Ubuntu, for instance:

sudo apt update && sudo apt upgrade

After the upgrade, confirm the fix by checking the kernel version with uname -r. Azure automatically reboots VMs as per your update policies, but you can manually restart if needed.

If you run a custom kernel image, download the patched kernel from your distribution’s advisory page and rebuild. Microsoft has also published a dedicated security advisory (CVE-2026-53225 in the Security Update Guide) that includes direct download links for each affected platform.

Additional Hardening: Disable SCTP If Not Needed

If your workloads do not rely on SCTP, the surest mitigation is to blacklist the sctp kernel module. This prevents any SCTP traffic from being processed, effectively eliminating the attack vector. To do so:

echo "blacklist sctp" >> /etc/modprobe.d/blacklist-sctp.conf

Then regenerate the initramfs and reboot. After the reboot, verify that the module is not loaded:

lsmod | grep sctp

If the command returns nothing, the module is disabled. Note that this may affect applications that depend on SCTP, such as Diameter (used in LTE networks) or WebRTC data channels. Proceed with caution in telecom or VoIP environments.

Community Response and Known Issues

As news of CVE-2026-53225 spread across Windows forums and Linux mailing lists, the immediate reaction was mixed. While many praised the rapid disclosure and patch release, WSL 2 users expressed confusion over the update mechanism. Unlike typical Linux distributions, the WSL kernel is not upgradeable via apt; the wsl --update command is often overlooked, leading to stale kernels.

Several forum posts indicated that some WSL installations were still running kernels months older than the fixed version. One Reddit user commented, “I assumed Windows Update took care of the WSL kernel too, but apparently not. Running wsl --update was a surprise.” This highlights a critical awareness gap that Microsoft is attempting to close with more prominent in‑product notifications.

In Azure, the patching process has been smooth, with no major reports of compatibility issues. However, a small number of users running custom SCTP‑based containerized applications reported a transient connection reset after the kernel upgrade, which was resolved by restarting the services.

The Bigger Picture: Kernel Security in Cross‑Platform Environments

CVE-2026-53225 is the latest in a growing line of kernel vulnerabilities that straddle the boundary between Windows and Linux. As Microsoft increasingly embraces Linux for WSL, Azure, and its own internal services, the security of the Linux kernel becomes an integral part of the Windows ecosystem.

This dual responsibility creates unique challenges. For instance, many Windows users are unaware that they run a full Linux kernel under the hood, let alone that it requires separate security updates. Microsoft has improved transparency by displaying WSL kernel version information in Windows Settings and by integrating kernel updates into the Microsoft Store for WSL, but the learning curve remains steep.

The vulnerability also underscores the need for kernel hardening measures such as Kernel Address Space Layout Randomization (KASLR), which was enabled in WSL2 kernels last year, and heap‑hardening features like the recently introduced init_on_alloc and init_on_free options. These compile‑time settings force the kernel to initialize all heap allocations, which would have prevented CVE-2026-53225. Distributions are increasingly enabling these options by default, adding a robust safety net against information leaks.

Timeline of Disclosure

• February 12, 2026: Microsoft MORSE team reports the vulnerability to the Linux kernel security team.
• March 2, 2026: The bug is confirmed and a patch is drafted by the SCTP maintainers.
• March 15–April 20, 2026: The fix undergoes testing in stable release candidates and is backported to long‑term stable kernels.
• May 5, 2026: Microsoft internally integrates the patch into the WSL2 kernel branch and Azure Linux images.
• June 27, 2026: Coordinated disclosure; CVE-2026-53225 is published, and Microsoft releases its advisory alongside the updated WSL kernel.

What’s Next?

Microsoft has committed to accelerating kernel patch delivery for WSL by synchronizing with the Linux stable release schedule. Future WSL releases will automatically fetch kernel updates during Windows Update, removing the need for manual wsl --update steps. For Azure, customers can expect continued automatic patching for all market‑provided images.

Security researchers are calling for mandatory init_on_alloc enforcement in all cloud kernels, a move that would systematically eliminate uninitialized memory read vulnerabilities. While this carries a small performance overhead, the trade‑off appears justified given the sensitive nature of cloud workloads.

For now, users and administrators must act promptly. Run wsl --update or apt upgrade today, and if SCTP is not in your workflow, disable it. Kernel information leaks rarely give a second chance.