CVE-2023-6817: Linux Kernel Privilege Escalation Flaw

CVE-2023-6817 Overview

CVE-2023-6817 is a use-after-free vulnerability in the Linux kernel's netfilter: nf_tables subsystem. The flaw resides in the nft_pipapo_walk function, which fails to skip inactive elements during a set walk. This oversight enables double deactivations of PIPAPO (Pile Packet Policies) elements, resulting in a use-after-free condition [CWE-416]. A local unprivileged attacker with the ability to create user or network namespaces can exploit this issue to escalate privileges to root. The vulnerability affects Linux kernel versions through 6.7-rc4 and was addressed in upstream commit 317eb9685095678f2c9f5a8189de698c5354316a.

Critical Impact
Local attackers can leverage this nf_tables use-after-free to gain root privileges on affected Linux systems, with full confidentiality, integrity, and availability impact.

Affected Products

Linux Kernel (multiple branches prior to the fix commit)
Linux Kernel 6.7-rc1 through 6.7-rc4
Downstream distributions including Debian LTS and Siemens industrial products (per advisories SSA-265688 and SSA-398330)

Discovery Timeline

2023-12-18 - CVE-2023-6817 published to NVD
2026-05-12 - Last updated in NVD database

Technical Details for CVE-2023-6817

Vulnerability Analysis

The vulnerability resides in the PIPAPO set backend of the nf_tables netfilter framework. PIPAPO is the data structure that backs nft_set instances containing concatenated ranges, used for high-performance packet classification. During set traversal, the kernel invokes nft_pipapo_walk to iterate over every element registered in the set.

The defect is that nft_pipapo_walk does not check whether an element is marked inactive before passing it to the walker callback. Inactive elements are those that have been removed from the active generation but still reside in the set pending garbage collection. When a transaction that removes such an element is followed by another deactivation request through a walk operation, the kernel deactivates the same element twice. Double deactivation corrupts the element's reference accounting and triggers freeing of memory that other code paths still hold pointers to, producing a classic use-after-free.

An attacker with CAP_NET_ADMIN inside a user namespace can craft nf_tables netlink commands that orchestrate the conflicting deactivations. The resulting dangling pointer can be reclaimed with attacker-controlled data, leading to kernel memory corruption and local privilege escalation.

Root Cause

The root cause is missing state validation in the PIPAPO walk implementation. The function iterates over all elements without filtering out those whose NFT_SET_ELEM_INACTIVE flag is set for the current generation. The fix in commit 317eb9685095678f2c9f5a8189de698c5354316a adds an active-generation check so that walkers skip inactive elements, preventing the double-deactivation primitive.

Attack Vector

Exploitation is local. The attacker must execute code on the target host and possess the ability to issue nf_tables netlink commands. On most modern distributions, unprivileged users can acquire this capability by creating an unprivileged user namespace, then issuing crafted NFT_MSG_NEWSET, NFT_MSG_NEWSETELEM, and NFT_MSG_DELSETELEM requests against a PIPAPO-backed set. The complete exploitation flow described in public OSS-Security disclosures consists of:

Creating a user namespace and entering it with CAP_NET_ADMIN over a new network namespace.
Allocating an nft_set with the PIPAPO backend and populating it with elements.
Triggering a set walk that interacts with already-deactivated elements to cause the double deactivation.
Spraying kernel objects into the freed allocation to hijack a function pointer and pivot to kernel-mode code execution.

The vulnerability mechanism is documented in the upstream commit and in the OpenWall OSS-Security thread; refer to the Linux Kernel Commit Update and the OpenWall OSS Security Discussion for technical specifics.

Detection Methods for CVE-2023-6817

Indicators of Compromise

Unexpected kernel oops or general protection fault entries in dmesg referencing nft_pipapo_walk, nft_pipapo_deactivate, or nf_tables.
Processes spawning shells with UID 0 after invoking unshare(CLONE_NEWUSER|CLONE_NEWNET) followed by heavy nf_tables netlink activity.
KASAN reports identifying use-after-free reads or writes in the nft_pipapo element slab cache.

Detection Strategies

Audit execve and unshare syscalls correlated with socket(AF_NETLINK, ..., NETLINK_NETFILTER) from non-root users.
Hunt for unusual nft or custom binaries that rapidly create and destroy sets containing concatenated ranges.
Track kernel ring buffer messages and crash reports for signatures matching the affected functions.

Monitoring Recommendations

Forward auditd syscall telemetry for unshare, setns, and clone with namespace flags to a central log store for correlation.
Alert on local accounts that suddenly transition to UID 0 outside of approved escalation tooling such as sudo or pkexec.
Monitor for kernel panics and reboots on systems running pre-patch kernels, which can indicate failed exploitation attempts.

How to Mitigate CVE-2023-6817

Immediate Actions Required

Inventory all Linux systems and identify kernels that do not include commit 317eb9685095678f2c9f5a8189de698c5354316a.
Apply distribution kernel updates from your vendor; Debian LTS, Ubuntu Livepatch (LSN-0100-1), and Siemens have shipped fixes.
Reboot or apply a live patch after upgrading to activate the corrected kernel image.
Prioritize multi-tenant hosts, container nodes, and systems where unprivileged users can execute code.

Patch Information

The upstream fix is committed as 317eb9685095678f2c9f5a8189de698c5354316a in the mainline Linux kernel tree. The patch modifies nft_pipapo_walk to skip elements that are inactive for the current generation, eliminating the double-deactivation primitive. Distribution backports are available through the Debian LTS Announcement, the Packet Storm Security Notice, and Siemens advisories SSA-265688 and SSA-398330.

Workarounds

Disable unprivileged user namespaces by setting kernel.unprivileged_userns_clone=0 (Debian/Ubuntu) or user.max_user_namespaces=0 to remove the unprivileged attack path.
Restrict the nf_tables module by blacklisting it where firewall functionality is not required, using /etc/modprobe.d/ configuration.
Apply seccomp profiles in container runtimes that block unshare and setns for workloads that do not require namespace creation.
Enforce least-privilege execution policies so untrusted users cannot run arbitrary binaries on sensitive hosts.

bash

# Disable unprivileged user namespaces (Debian/Ubuntu)
echo 'kernel.unprivileged_userns_clone=0' | sudo tee /etc/sysctl.d/99-cve-2023-6817.conf
sudo sysctl --system

# Verify the running kernel includes the fix commit
uname -r
apt-cache policy linux-image-$(uname -r)

# Optional: blacklist nf_tables where not needed
echo 'blacklist nf_tables' | sudo tee /etc/modprobe.d/blacklist-nftables.conf