CVE-2023-39192 Overview
CVE-2023-39192 is an out-of-bounds read vulnerability [CWE-125] in the Netfilter subsystem of the Linux kernel. The flaw resides in the xt_u32 module, which fails to validate fields within the xt_u32 structure before use. A local attacker with elevated privileges can manipulate size fields to read memory beyond the intended array boundaries. Successful exploitation results in kernel memory disclosure or a kernel crash leading to denial of service. The vulnerability affects multiple Linux distributions including Red Hat Enterprise Linux 8 and Fedora 38. Red Hat and Debian have published security advisories and corresponding patches addressing the issue.
Critical Impact
A local privileged attacker can trigger a kernel out-of-bounds read in the Netfilter xt_u32 module, causing information disclosure or a system crash.
Affected Products
- Linux Kernel (upstream, versions prior to the Netfilter xt_u32 fix)
- Red Hat Enterprise Linux 8
- Fedora 38
Discovery Timeline
- 2023-10-09 - CVE-2023-39192 published to the National Vulnerability Database (NVD)
- 2024-01 - Debian LTS announces patched kernel packages
- 2024-05 - Red Hat releases security advisories RHSA-2024:2950 and RHSA-2024:3138
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2023-39192
Vulnerability Analysis
The Netfilter framework provides packet filtering capabilities used by iptables and related utilities in the Linux kernel. The xt_u32 extension matches arbitrary 32-bit values within packet payloads against user-supplied rules. To configure this match, userspace passes an xt_u32 structure into the kernel containing fields that describe the byte ranges to extract and compare.
The vulnerability stems from missing validation of the size fields within the xt_u32 structure when the rule is loaded. The kernel trusts the supplied size values and uses them as indices into fixed-size arrays. When an attacker supplies a size value exceeding the array bounds, the matching logic reads kernel memory outside the structure. This yields either disclosure of adjacent kernel memory contents or a fault that crashes the kernel.
Exploitation requires local access and the CAP_NET_ADMIN capability, since modifying Netfilter rules is a privileged operation. The issue is classified under [CWE-125] Out-of-Bounds Read.
Root Cause
The xt_u32 module accepts user-controlled size fields from the xt_u32 structure without bounds-checking them against the declared array dimensions. The kernel uses these untrusted values as loop or index limits when traversing the match data, allowing reads to extend past the allocated buffer.
Attack Vector
An attacker with local access and CAP_NET_ADMIN (for example, inside a user namespace where this capability is delegated, or via a compromised privileged process) submits a crafted Netfilter rule containing an xt_u32 match with out-of-range size fields. When the kernel evaluates the rule against traffic, the unchecked size triggers an out-of-bounds read. The result is either kernel memory disclosure usable to defeat KASLR and stage further attacks, or a kernel oops causing denial of service.
No public proof-of-concept exploit is listed in Exploit-DB, and the vulnerability is not present on the CISA Known Exploited Vulnerabilities catalog. Technical details are documented in the Red Hat CVE Analysis CVE-2023-39192 and the Zero Day Initiative Advisory ZDI-CAN-18408.
Detection Methods for CVE-2023-39192
Indicators of Compromise
- Unexpected kernel oops or panic entries in dmesg or /var/log/kern.log referencing xt_u32 or Netfilter match evaluation.
- Loading of the xt_u32 kernel module on systems that do not normally use it, observable through lsmod and /proc/modules.
- Creation of unusual iptables or nftables rules invoking the u32 match by non-administrative workloads.
Detection Strategies
- Audit kernel module load events using auditd rules covering init_module and finit_module syscalls, focusing on xt_u32.
- Monitor invocations of iptables -m u32 and equivalent nftables operations via process execution telemetry.
- Track creation of user namespaces by unprivileged processes, since these are a common path to obtaining CAP_NET_ADMIN for local kernel attacks.
Monitoring Recommendations
- Forward kernel ring buffer messages and auditd events to a centralized logging or SIEM platform for correlation with process lineage.
- Alert on repeated kernel faults originating from the Netfilter code path on a single host within a short interval.
- Baseline expected use of xt_u32 per host role and flag deviations for review.
How to Mitigate CVE-2023-39192
Immediate Actions Required
- Apply the kernel updates referenced in Red Hat Security Advisory RHSA-2024:2950 and Red Hat Security Advisory RHSA-2024:3138 on affected RHEL 8 systems.
- Update Debian-based hosts using the packages described in the Debian LTS Announce Message.
- Reboot systems after kernel package installation to load the patched image.
Patch Information
Red Hat addressed the flaw through kernel errata RHSA-2024:2950 and RHSA-2024:3138. Debian shipped fixes in updated kernel packages announced via the Debian LTS channel in January 2024. Fedora 38 users should install the latest kernel through dnf update. Tracking details are available in Red Hat Bugzilla Report #2226784.
Workarounds
- Blacklist the xt_u32 module on systems that do not require it by adding install xt_u32 /bin/true to a file under /etc/modprobe.d/.
- Restrict CAP_NET_ADMIN and disable unprivileged user namespaces where workloads do not need them by setting kernel.unprivileged_userns_clone=0 on supported distributions.
- Apply mandatory access control policies (SELinux or AppArmor) to confine processes that manipulate Netfilter rules.
# Configuration example: prevent loading of the vulnerable xt_u32 module
echo "install xt_u32 /bin/true" | sudo tee /etc/modprobe.d/disable-xt_u32.conf
sudo rmmod xt_u32 2>/dev/null || true
# Disable unprivileged user namespaces to reduce local privilege escalation surface
echo "kernel.unprivileged_userns_clone=0" | sudo tee /etc/sysctl.d/99-userns.conf
sudo sysctl --system
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
