CVE-2025-21826 in Linux
Summary
by MITRE • 03/06/2025
In the Linux kernel, the following vulnerability has been resolved:
netfilter: nf_tables: reject mismatching sum of field_len with set key length
The field length description provides the length of each separated key field in the concatenation, each field gets rounded up to 32-bits to calculate the pipapo rule width from pipapo_init(). The set key length provides the total size of the key aligned to 32-bits.
Register-based arithmetics still allows for combining mismatching set key length and field length description, eg. set key length 10 and field description [ 5, 4 ] leading to pipapo width of 12.
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Analysis
by VulDB Data Team • 02/16/2026
The vulnerability identified as CVE-2025-21826 resides within the Linux kernel's netfilter subsystem, specifically in the nf_tables component that manages packet filtering and manipulation rules. This flaw represents a critical inconsistency in how the kernel handles set key length validation during rule processing, creating potential pathways for arbitrary code execution or denial of service attacks. The vulnerability manifests when the kernel processes packet filtering rules that involve set-based lookups, where the system fails to properly validate the relationship between individual field lengths and the total set key length, leading to incorrect memory management and potential exploitation.
The technical implementation of this vulnerability stems from the pipapo (pipapo is a data structure used for efficient packet classification) rule width calculation mechanism. When processing set key definitions, the kernel calculates the total rule width by rounding individual field lengths up to 32-bit boundaries during the pipapo_init() initialization process. However, the system allows for register-based arithmetic operations that can combine mismatched set key lengths with field length descriptions, creating a scenario where the calculated pipapo width does not align with the actual key structure. For instance, when a set key length of 10 bytes is combined with a field description of [5, 4], the system computes a pipapo width of 12 bytes, which creates a fundamental mismatch between expected and actual memory layout. This mismatch occurs because the field length description implies a specific key structure while the set key length suggests a different total size, leading to incorrect memory allocation patterns and potential buffer overflows.
The operational impact of this vulnerability extends beyond simple memory corruption, as it can enable sophisticated attack vectors within the kernel's networking stack. An attacker could potentially exploit this inconsistency to manipulate packet filtering rules in ways that bypass security controls or cause system instability through controlled memory corruption. The vulnerability affects systems running Linux kernels that implement netfilter with nf_tables functionality, particularly those utilizing complex packet filtering rules with set-based lookups. The flaw is particularly dangerous because it operates at the kernel level where privilege escalation is not required, and the effects can range from denial of service conditions that disrupt network services to more severe scenarios where attackers could execute arbitrary code with kernel privileges. According to CWE classification, this vulnerability maps to CWE-129: Improper Validation of Array Index, as it involves incorrect validation of memory access patterns based on mismatched key length calculations.
Mitigation strategies for CVE-2025-21826 should prioritize immediate kernel updates from vendors such as Red Hat, Ubuntu, and other Linux distributions that have released patches addressing the core validation issue. System administrators should implement monitoring for unusual network filtering rule behavior and ensure that all packet filtering rules undergo validation checks before deployment in production environments. The fix typically involves strengthening the validation logic within the pipapo initialization process to ensure that field length descriptions align properly with the total set key length, preventing the calculation of mismatched widths that could lead to memory corruption. Additionally, organizations should consider implementing network segmentation and access controls to limit potential attack surfaces, while maintaining regular security audits of their packet filtering configurations to detect any anomalous rule structures that might indicate exploitation attempts. The ATT&CK framework categorizes this vulnerability under T1068: Exploitation for Privilege Escalation, as it represents a kernel-level exploit path that could be leveraged to gain elevated privileges and compromise system integrity.