CVE-2025-40019: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-40019 is a missing input size validation vulnerability in the Linux kernel's crypto: essiv subsystem that can lead to a local denial of service. The flaw exists because the ssize check in essiv_aead_crypt was only applied to encryption paths, leaving decryption and in-place encryption operations unchecked. It was published on October 24, 2025, and affects Linux kernel versions from commit be1eb7f78aa8 up to the respective fixes across multiple stable branches. It carries a CVSS v3.1 base score of 5.5 (Medium) (Red Hat CVE, Feedly).

The root cause is improper validation of input size (related to CWE-20: Improper Input Validation) in the essiv_aead_crypt function within the kernel's ESSIV (Encrypted Salt-Sector Initialization Vector) AEAD cryptographic template. The ssize check was originally placed only on the encryption code path, meaning that when the function was invoked for decryption or in-place encryption, no bounds check was performed on the scatter-gather list size. The fix moves the ssize check to the beginning of essiv_aead_crypt so it applies uniformly to all code paths. A local attacker with low privileges could craft a malicious cryptographic operation request to trigger the missing check, potentially causing a kernel crash (Red Hat CVE, MSRC).

Successful exploitation results in a denial of service by crashing or hanging the kernel during cryptographic operations, affecting system availability. There is no impact on confidentiality or integrity. The scope is limited to the local system, with no lateral movement potential directly from this vulnerability (Red Hat CVE, Feedly).

At least two public proof-of-concept repositories exist: 0xAtharv/CVE-2025-40019-POC (added March 2026) and guard-wait/CVE-2025-40019_POC, as well as xooxo/CVE-2025-40019-Essiv. There is no evidence of in-the-wild exploitation or threat actor attribution at this time. The vulnerability is not listed in the CISA KEV catalog. The EPSS score is approximately 0.014% (0.000140), indicating a very low probability of exploitation in the near term (Feedly).

1. Gain local access: Obtain a low-privileged local user account on a system running a vulnerable Linux kernel version (prior to the fixed versions in the affected stable branches).
2. Identify the vulnerable subsystem: Confirm the kernel uses the essiv AEAD crypto template, which is commonly used with dm-crypt/LUKS disk encryption configurations.
3. Craft a malicious crypto operation: Using the kernel's AF_ALG socket interface or a custom kernel module, invoke the essiv_aead_crypt function with a decryption or in-place encryption request that provides an undersized scatter-gather buffer (i.e., ssize smaller than expected).
4. Trigger the missing check: Because the ssize validation is absent for decryption/in-place paths in vulnerable kernels, the kernel proceeds without bounds checking, leading to an out-of-bounds access or NULL dereference.
5. Achieve denial of service: The kernel crashes or hangs, causing a system outage (Feedly, MSRC).

• Logs: Kernel oops or panic messages in /var/log/kern.log or dmesg referencing essiv_aead_crypt, scatter-gather list errors, or NULL pointer dereferences in the crypto subsystem.
• Process: Unusual processes invoking AF_ALG sockets or cryptographic operations as a low-privileged user, particularly targeting AEAD cipher operations.
• System Behavior: Unexpected kernel crashes or system hangs on systems using dm-crypt/LUKS with ESSIV mode, especially following cryptographic decryption operations.

Apply the upstream kernel patches that move the ssize check to the start of essiv_aead_crypt. Fixed versions include: Linux kernel 6.6.113 or later, 6.12.54 or later, 6.1.157 or later, 5.15.195 or later, 6.17.4 or later, and 6.18-rc1 or later. Distribution-specific updates are available from Red Hat, Debian (DSA-6053-1, DLA-4379-1, DLA-4404-1), Ubuntu (USN-7990-x, USN-8013-x, USN-8015-x, USN-8224-1), SUSE, openSUSE, Oracle Linux, and Amazon Linux. Microsoft Azure Linux 3 users should update to kernel versions beyond azl3_kernel_6.6.112.1-2 and azl3_kernel_6.6.104.2-4. As a workaround, restrict local unprivileged user access on systems where patching is not immediately possible (Red Hat CVE, MSRC, Debian LTS).