Mitigating CVE-2026-31431 “Copy Fail” in Docker Engine

🔑 Enhanced Key Takeaways

•CVE-2026-31431, dubbed "Copy Fail," is a high-severity local privilege escalation (LPE) vulnerability stemming from a logic flaw within the Linux kernel's algif_aead module, part of the AF_ALG (userspace crypto API).
•The exploit mechanism involves abusing an interaction between the AF_ALG socket interface and the splice() system call to achieve a controlled 4-byte write into the kernel's page cache of any readable file, allowing attackers to corrupt in-memory privileged binaries (e.g., /usr/bin/su) without altering the on-disk file.
•This vulnerability affects virtually all Linux distributions running kernels released from 2017 until patched versions are applied, including major distributions like Ubuntu, Amazon Linux, Red Hat Enterprise Linux (RHEL), and SUSE, with a public proof-of-concept (PoC) exploit demonstrating high reliability across these systems.
•Beyond local privilege escalation, "Copy Fail" acts as a container escape primitive, enabling an unprivileged process within a container to gain root access on the host system, thereby compromising all other containers running on that node.
•In addition to updating Docker Engine, interim mitigations include disabling the algif_aead kernel module or implementing strict seccomp profiles to block AF_ALG socket creation for untrusted container workloads, which can prevent exploitation even on unpatched kernels.

🛠️ Technical Deep Dive

Vulnerability Type: Logic flaw in the algif_aead module of the AF_ALG (userspace crypto API) within the Linux kernel's cryptographic subsystem.
Root Cause: An in-place optimization introduced in the Linux kernel in 2017 (commit 72548b093ee3) for AEAD decryption, where the kernel incorrectly reuses source memory as the destination during cryptographic operations.
Exploit Mechanism:
- An unprivileged local user initiates an attack by abusing the interaction between the AF_ALG socket interface and the splice() system call.
- When a file is spliced into a pipe and then into an AF_ALG socket, the socket's input scatterlist directly references the kernel's cached pages of that file, avoiding data duplication.
- During an AEAD decryption operation, the algif_aead.c code is configured for an in-place operation, meaning the same scatterlist serves as both input and output.
- This in-place design, coupled with improper error handling during a failed copy operation, allows an attacker to perform a controlled 4-byte write into the kernel's page cache of any readable file.
- The corruption occurs entirely within the kernel's memory, bypassing traditional user-space security protections.
- Crucially, the kernel does not mark the corrupted page as dirty for writeback, ensuring the on-disk file remains unchanged and evading detection by standard file integrity checks.
- Attackers commonly target setuid binaries (e.g., /usr/bin/su) in the page cache to achieve reliable root privilege escalation.
Affected Kernel Versions: Linux kernels released from 2017 (after commit 72548b093ee3) up to patched versions, specifically before 6.18.22, 6.19.12, or 7.0.
Kernel Mitigation: The upstream fix involves reverting the faulty in-place optimization by mainline commit a664bf3d603d, which modifies crypto/algif_aead.c, crypto/af_alg.c, crypto/algif_skcipher.c, and include/crypto/if_alg.
Userspace/Docker Mitigation: Docker Engine v29.4.3 includes mitigations to block the syscall surface. Other userspace mitigations include persistently disabling the algif_aead kernel module (e.g., echo "install algif_aead /bin/false" > /etc/modprobe.d/disable-algif.conf && rmmod algif_aead) or restricting AF_ALG socket creation through seccomp profiles for containerized workloads.

🔮 Future ImplicationsAI analysis grounded in cited sources

Increased focus on kernel hardening and alternative container isolation technologies.

The 'Copy Fail' vulnerability underscores the inherent risks of the shared kernel model in containerization, likely accelerating the adoption of stronger isolation technologies like microVMs (e.g., Firecracker, Cloud Hypervisor) or gVisor for multi-tenant environments.

Enhanced default security profiles in container runtimes.

Container orchestrators like Docker will likely strengthen default seccomp profiles to restrict potentially exploitable syscalls, such as AF_ALG socket creation, as a defense-in-depth measure, even on patched kernels.

Development of more sophisticated detection mechanisms for in-memory page cache corruption.

Since 'Copy Fail' modifies the in-memory page cache without altering the on-disk file, traditional file integrity checks are ineffective, necessitating advanced runtime monitoring and memory forensics to detect such stealthy attacks.

⏳ Timeline

2017

In-place optimization (commit `72548b093ee3`) introduced into the Linux kernel's cryptographic subsystem, later identified as the root cause of CVE-2026-31431.

2026-04-01

Mainline kernel commit `a664bf3d603d`, which reverts the faulty in-place optimization, is committed upstream.

2026-04-22

CVE-2026-31431 is officially resolved in the Linux kernel, and details are published on NVD.

2026-04-29

Public disclosure of CVE-2026-31431, nicknamed 'Copy Fail,' by researchers, accompanied by the release of public proof-of-concept exploits.

2026-05-01

Microsoft Security Blog publishes an analysis of CVE-2026-31431, highlighting its significant impact on cloud environments and Kubernetes clusters.

2026-05-11

Docker Engine v29.4.3, which includes necessary mitigations for CVE-2026-31431, is released as part of Docker Desktop 4.73.0.

Mitigating CVE-2026-31431 “Copy Fail” in Docker Engine

⚡ 30-Second TL;DR

Key Points

🧠 Deep Insight

🔑 Enhanced Key Takeaways

🛠️ Technical Deep Dive

🔮 Future ImplicationsAI analysis grounded in cited sources

⏳ Timeline

📎 Sources (18)

👉Related Updates