CVE-2020-15436 Overview
CVE-2020-15436 is a use-after-free vulnerability in fs/block_dev.c in the Linux kernel before version 5.8. The flaw stems from improper access to a certain error field within the block device handling code [CWE-416]. Local users with elevated privileges can leverage this defect to escalate privileges further or trigger a denial of service condition. The vulnerability affects the mainline Linux kernel and downstream products from vendors including NetApp and Broadcom that ship affected kernel versions.
Critical Impact
Local attackers with high privileges can exploit this use-after-free to gain additional privileges or crash the kernel, compromising confidentiality, integrity, and availability of the host system.
Affected Products
- Linux Kernel versions prior to 5.8
- NetApp products including SolidFire & HCI Management Node, H-Series (H410c, H610c, H610s, H615c), AFF and FAS storage arrays (A250, A400, A700s, 500f, 8300, 8700), and Cloud Backup
- Broadcom Brocade Fabric Operating System Firmware
Discovery Timeline
- 2020-11-23 - CVE-2020-15436 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2020-15436
Vulnerability Analysis
The vulnerability resides in fs/block_dev.c, which implements block device file operations in the Linux kernel. A use-after-free condition occurs when code paths improperly access an error field after the underlying memory has been freed. An attacker with local access and elevated privileges can manipulate block device operations to trigger the dangling reference. Successful exploitation leads to memory corruption inside the kernel, which can be steered toward arbitrary kernel code execution or full privilege escalation. Failed exploitation attempts typically result in a kernel oops or panic, producing a denial of service.
Root Cause
The root cause is improper lifetime management of an error field referenced in block device code. The kernel accesses the field through a pointer that has already been released, satisfying the classic use-after-free pattern tracked under [CWE-416]. Race windows between freeing the underlying structure and subsequent error-path access make the issue reachable from userspace through block device interfaces.
Attack Vector
Exploitation requires local access and high privileges on the target system, as reflected in the CVSS attack vector. No user interaction is needed. An attacker invokes block device operations that exercise the vulnerable error path, races the deallocation to keep a stale reference live, and then steers the freed slab object to controlled data. The technical details of the fix are documented in the Linux Kernel Mailing List Post.
No verified public proof-of-concept exploit is available for this CVE. The vulnerability is described in prose only; see the upstream patch on LKML for the source-level mechanics.
Detection Methods for CVE-2020-15436
Indicators of Compromise
- Unexpected kernel oops or panic messages in dmesg or /var/log/kern.log referencing block_dev.c, __blkdev_*, or general protection faults in block I/O code paths.
- KASAN reports flagging use-after-free conditions in block device functions on instrumented kernels.
- Unexplained privilege escalation events where a non-root or service account suddenly executes commands as root.
Detection Strategies
- Inventory kernel versions across the fleet and flag any host running a Linux kernel earlier than 5.8.
- Enable kernel auditing (auditd) for unusual mount, ioctl, and block device open operations originating from non-administrative users.
- Correlate kernel crash telemetry with process execution logs to identify repeated crashes preceding suspicious activity.
Monitoring Recommendations
- Forward kernel ring buffer and audit logs to a centralized logging or SIEM platform for long-term correlation.
- Alert on kernel panics, segfaults inside kernel context, and KASAN findings on production hosts.
- Track baseline behavior for privileged processes and alert on deviations such as new child processes spawned by storage management daemons.
How to Mitigate CVE-2020-15436
Immediate Actions Required
- Upgrade the Linux kernel to version 5.8 or later, or apply the distribution backport that addresses the fs/block_dev.c use-after-free.
- For NetApp appliances, apply the firmware and software updates referenced in the NetApp Security Advisory.
- Audit and restrict accounts that hold elevated privileges, since exploitation requires high local privileges.
Patch Information
The upstream fix was merged into the Linux kernel and is included in version 5.8 and later. Patch context is published on the Linux Kernel Mailing List Post. Vendors shipping affected kernels released coordinated updates, including NetApp, which documents fixed releases in the NetApp Security Advisory. Broadcom Brocade Fabric Operating System Firmware users should consult Broadcom support channels for the corresponding firmware update.
Workarounds
- Limit local shell and administrative access to trusted operators only, since the vulnerability requires local high-privilege access.
- Enforce least-privilege policies and remove unnecessary CAP_SYS_ADMIN capabilities from service accounts and containers.
- Enable kernel hardening features such as KASLR, SMEP, SMAP, and KASAN on test systems to detect or hinder exploitation attempts.
# Verify the running kernel version and confirm it is 5.8 or later
uname -r
# On Debian/Ubuntu, install the latest kernel security update
sudo apt-get update && sudo apt-get install --only-upgrade linux-image-generic
# On RHEL/CentOS, apply available kernel updates
sudo yum update kernel
# Reboot to load the patched kernel
sudo reboot
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
