CVE-2026-23333: Linux Kernel Netfilter Overlap Flaw

CVE-2026-23333 Overview

CVE-2026-23333 is a vulnerability in the Linux kernel's netfilter subsystem, specifically affecting the nft_set_rbtree component. The vulnerability arises from improper validation of open interval overlaps within netfilter set operations. Open intervals, which lack an end element, present a validation challenge—particularly when positioned at the end of a set—as the interval validation logic relies on the presence of an end element to perform necessary checks.

This flaw can lead to incorrect set membership validation, potentially allowing attackers to bypass firewall rules or cause unexpected behavior in packet filtering operations.

Critical Impact

Systems running affected Linux kernel versions with netfilter-based firewalls may experience incorrect interval validation in nftables sets, potentially leading to firewall rule bypass or denial of service conditions.

Affected Products

• Linux Kernel (netfilter subsystem)
• Systems utilizing nftables with interval-based sets
• Linux distributions running affected kernel versions

Discovery Timeline

• 2026-03-25 - CVE CVE-2026-23333 published to NVD
• 2026-03-25 - Last updated in NVD database

Technical Details for CVE-2026-23333

Vulnerability Analysis

The vulnerability exists within the nft_set_rbtree implementation in the Linux kernel's netfilter framework. The core issue stems from the handling of open intervals—intervals that do not have a defined end element. When such open intervals are positioned at the end of a set, the validation mechanism fails to properly detect overlapping conditions.

The problem manifests during set element insertion and deletion operations. When adding new interval elements to an existing set, the validation logic must verify that the new interval does not overlap with existing intervals. However, because open intervals lack an end element, the standard overlap detection fails to identify partial overlaps correctly.

Consider a scenario where an existing set contains the range 255.255.255.0-255.255.255.254. When attempting to add a new interval 255.255.255.0-255.255.255.255, the start element overlap should be detected, but the absence of proper open interval handling causes the validation to miss this overlap condition.

Root Cause

The root cause is the absence of an NFT_SET_ELEM_INTERVAL_OPEN flag that would facilitate detection of open interval overlaps. The existing interval validation logic assumes all intervals have both start and end elements, which creates a blind spot for open intervals. The validation relies on finding an existing end element to compare against the new interval's start element, but open intervals at the end of the set lack this end element entirely.

Additionally, the deletion path introduced complications—when deleting open intervals, the start cookie was being updated, which could trigger spurious EEXIST errors when subsequently adding new elements.

Attack Vector

An attacker with the ability to manipulate nftables rules (typically requiring local privileges or access through a management interface) could exploit this vulnerability to:

1. Create overlapping intervals that bypass the intended validation, potentially crafting firewall rules that don't behave as expected
2. Cause denial of service by triggering error conditions during set element operations
3. Bypass firewall restrictions by exploiting the improper interval handling to create rules that match unintended traffic

The attack complexity is elevated as it requires understanding of nftables internals and the ability to craft specific interval patterns that exploit the validation gap.

Detection Methods for CVE-2026-23333

Indicators of Compromise

• Unexpected behavior in nftables interval-based set matching
• Unusual firewall rule bypasses that shouldn't occur based on configured rules
• Error messages in kernel logs related to nft_set_rbtree operations
• Failed set element insertions with unexpected EEXIST errors

Detection Strategies

• Monitor kernel logs for netfilter-related errors using dmesg | grep -i nft
• Audit nftables configurations for interval-based sets that may be affected
• Implement kernel vulnerability scanning to identify affected kernel versions
• Deploy runtime security monitoring to detect anomalous netfilter behavior

Monitoring Recommendations

• Enable kernel auditing for netfilter subsystem operations
• Configure alerting for unexpected nftables rule changes
• Monitor network traffic patterns for signs of firewall rule bypass
• Regularly review nftables rule effectiveness through periodic security testing

How to Mitigate CVE-2026-23333

Immediate Actions Required

• Update the Linux kernel to a patched version containing commit 648946966a08e4cb1a71619e3d1b12bd7642de7b
• Review existing nftables configurations for interval-based sets
• Consider temporarily using alternative set types if interval validation is critical
• Monitor systems for signs of exploitation until patching is complete

Patch Information

The fix is available in the Linux kernel through commit 6db2be971e3d70c9e3f85d39eff7103c2ee2f579. This patch introduces a new flag field to struct nft_set_elem to track whether an element is the last in an add/delete command. This flag, combined with the start element cookie, enables proper detection of partial overlaps involving open intervals.

The patch specifically addresses:

• Adding a last flag to indicate the final element in a command sequence
• Improved overlap detection for start elements against existing end elements
• Proper handling of corner cases involving multiple intervals in a single command
• Fixed deletion path to avoid updating start cookies for open intervals, preventing spurious errors

Workarounds

• Avoid using open intervals at the end of nftables sets until the kernel is patched
• Use explicit closed intervals with defined end elements where possible
• Implement additional validation layers outside of nftables for critical rule sets
• Consider using iptables as a temporary alternative if nftables interval sets are required

# Check current kernel version for vulnerability assessment
uname -r

# View current nftables ruleset for interval-based sets
nft list ruleset | grep -A5 "type.*interval"

# Update kernel on Debian/Ubuntu systems
sudo apt update && sudo apt upgrade linux-image-generic

# Update kernel on RHEL/CentOS systems
sudo yum update kernel