CVE-2026-89608: Linux Kernel eCryptfs Buffer Overflow

CVE-2026-89608 Overview

CVE-2026-89608 is a Linux kernel vulnerability in the eCryptfs file system. The flaw resides in ecryptfs_parse_packet_set(), which receives a pointer into the file header but calculates the remaining packet buffer size from PAGE_SIZE - 8. For version 1 headers, the packet set starts later in the header, so this calculation can overstate the available buffer. The bug enables out-of-bounds reads while parsing tag 3 and tag 11 packets from encrypted file headers. Local, authenticated attackers can leverage this condition to disclose kernel memory or trigger denial of service on systems mounting eCryptfs volumes.

Critical Impact
A local low-privileged user can trigger out-of-bounds memory access during eCryptfs packet parsing, resulting in kernel information disclosure or system crash.

Affected Products

Linux kernel with eCryptfs file system support enabled
Distributions shipping vulnerable eCryptfs keystore.c parser code
Systems mounting user or home directory encryption backed by eCryptfs

Discovery Timeline

2026-09-11 - CVE-2026-89608 published to NVD
2026-09-14 - Last updated in NVD database

Technical Details for CVE-2026-89608

Vulnerability Analysis

The vulnerability lives in ecryptfs_parse_packet_set() inside the eCryptfs keystore parser. The function accepts a pointer into an encrypted file header and must determine how much space is available for packet parsing. Instead of using the actual buffer length supplied by the caller, it computes the remaining size as PAGE_SIZE - 8.

For version 1 headers, the packet set does not begin at a fixed offset near the start of the page. The packet set starts later in the header, so PAGE_SIZE - 8 overstates the bytes truly available to the parser. Subsequent tag 3 and tag 11 packet handling then reads past the intended boundary while decoding fields such as encrypted key material and passphrase identifiers.

The fix passes the actual packet set buffer length from the caller and calculates per-packet limits from the remaining bytes in that buffer. The patch also recomputes the remaining length after consuming a tag 3 packet before parsing the following tag 11 packet, preventing cumulative overreads.

Root Cause

The root cause is an incorrect boundary calculation. ecryptfs_parse_packet_set() derived the parsing limit from a page-relative constant rather than the caller-supplied buffer length. This is a classic Out-of-Bounds Read caused by trusting a hard-coded size that does not reflect the true position of the packet set within the header.

Attack Vector

Exploitation requires local access with low privileges. An attacker crafts a malicious eCryptfs-formatted file with a version 1 header positioned so the packet set sits near the tail of the page. When the kernel parses the file through eCryptfs mount operations, the parser reads memory beyond the legitimate buffer. Successful exploitation can leak adjacent kernel memory to userspace or corrupt parser state, causing a kernel oops. Remote exploitation is not possible without prior local access.

See the upstream fixes for implementation details: Linux Kernel Commit 749fa08d and Linux Kernel Commit 2602b79.

Detection Methods for CVE-2026-89608

Indicators of Compromise

Kernel oops or panic messages referencing ecryptfs_parse_packet_set, parse_tag_3_packet, or parse_tag_11_packet in dmesg output.
Unexpected EINVAL or EIO errors when opening files on eCryptfs-mounted directories.
Repeated user-triggered mounts of eCryptfs volumes containing malformed or attacker-controlled header data.

Detection Strategies

Monitor kernel logs for stack traces originating in fs/ecryptfs/keystore.c and correlate with the user account that initiated the mount or file open.
Audit systems for eCryptfs usage using mount | grep ecryptfs and inventory hosts still relying on the deprecated file system.
Track process execution of mount.ecryptfs_private and ecryptfs-mount-private from non-administrative users.

Monitoring Recommendations

Enable kernel auditd rules on openat, mount, and read syscalls targeting paths under user eCryptfs mount points.
Forward kernel ring buffer messages to a centralized log platform to identify repeated eCryptfs parser faults across the fleet.
Alert on any new eCryptfs-encrypted file being introduced into home directories from removable media or network shares.

How to Mitigate CVE-2026-89608

Immediate Actions Required

Apply the upstream Linux kernel patches referenced in the NVD entry and reboot affected hosts.
Restrict use of eCryptfs to trusted users and disable it where alternative encryption such as fscrypt or LUKS is available.
Inventory endpoints and servers running kernels with eCryptfs enabled and prioritize patching for multi-user systems.

Patch Information

The fix is available across multiple stable branches. Refer to the following commits: Linux Kernel Commit 2602b79, Linux Kernel Commit 329de8b9, Linux Kernel Commit 3bd9bb3e, Linux Kernel Commit 66a87559, Linux Kernel Commit 747fd45b, Linux Kernel Commit 749fa08d, Linux Kernel Commit e5d254e6, and Linux Kernel Commit e7573b0e. Consume the kernel package updates from your distribution once available.

Workarounds

Unload the ecryptfs kernel module on systems that do not require it using modprobe -r ecryptfs and blacklist it in /etc/modprobe.d/.
Migrate encrypted home directories to fscrypt or full-disk encryption such as LUKS to remove reliance on eCryptfs parsing paths.
Limit which users can mount eCryptfs volumes by tightening permissions on mount.ecryptfs_private and related helpers.

bash

# Configuration example: disable eCryptfs where not required
echo 'blacklist ecryptfs' | sudo tee /etc/modprobe.d/disable-ecryptfs.conf
sudo modprobe -r ecryptfs
sudo update-initramfs -u