CVE-2026-31681
Description
A flaw was found in the Linux kernel's netfilter xt_multiport module. This vulnerability arises from insufficient validation of range encoding within the checkentry function. A local attacker can exploit this by crafting malformed multiport rules, which causes the ports_match_v1() function to read beyond its intended memory boundary. This out-of-bounds read can lead to a denial of service (DoS) or potentially disclose sensitive information.
Statement
An out of bounds read can occur in the xt_multiport v1 match because the range encoding uses pflags and the matcher treats any non zero pflags entry as the start of a port range and then unconditionally consumes the next ports element as the range end. The checkentry path previously validated protocol flags and count but did not validate that every range start has a following element and that two consecutive elements are not both marked as range starts. A local attacker with CAP_NET_ADMIN can install a malformed rule with an invalid pflags layout and later trigger the match during packet processing. This can lead to a kernel crash. For the CVSS the PR is L because installing or modifying iptables or ip6tables rules requires administrative networking privileges.
Mitigation
In order to trigger the issue, it requires the ability to create user/net namespaces.
On non-containerized deployments of Red Hat Enterprise Linux 8, it is recommended to disable user namespaces by setting user.max_user_namespaces to 0:
# echo "user.max_user_namespaces=0" > /etc/sysctl.d/userns.conf
# sysctl -p /etc/sysctl.d/userns.conf
On containerized deployments, such as Red Hat OpenShift Container Platform, do not use this mitigation as the functionality is needed to be enabled.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 6.1 | 5.5 | N/A |
| Attack Vector | Local | Local | N/A |
| Attack Complexity | Low | Low | N/A |
| Privileges Required | Low | Low | N/A |
| User Interaction | None | None | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | Low | None | N/A |
| Integrity Impact | None | None | N/A |
| Availability Impact | High | High | N/A |
Vector
Red Hat: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H
NVD: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Confidentiality
Technical Impact: Read Memory
An attacker could get secret values such as cryptographic keys, PII, memory addresses, or other information that could be used in additional attacks.
Confidentiality
Technical Impact: Bypass Protection Mechanism
Out-of-bounds memory could contain memory addresses or other information that can be used to bypass ASLR and other protection mechanisms in order to improve the reliability of exploiting a separate weakness for code execution.
Availability
Technical Impact: DoS: Crash, Exit, or Restart
An attacker could cause a segmentation fault or crash by causing memory to be read outside of the bounds of the buffer. This is especially likely when the code reads a variable amount of data and assumes that a sentinel exists to stop the read operation, such as a NUL in a string.
Other
Technical Impact: Varies by Context
The read operation could produce other undefined or unexpected results.
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