CVE-2021-3715 in Linux
Summary
by MITRE • 03/03/2022
A flaw was found in the "Routing decision" classifier in the Linux kernel's Traffic Control networking subsystem in the way it handled changing of classification filters, leading to a use-after-free condition. This flaw allows unprivileged local users to escalate their privileges on the system. The highest threat from this vulnerability is to confidentiality, integrity, as well as system availability.
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Analysis
by VulDB Data Team • 06/30/2026
The vulnerability under discussion represents a critical use-after-free condition within the Linux kernel's Traffic Control subsystem, specifically affecting the Routing decision classifier component. This flaw exists in how the kernel handles dynamic modification of classification filters, creating a scenario where freed memory locations can be accessed and potentially manipulated by malicious processes. The issue manifests as a privilege escalation vulnerability that can be exploited by unprivileged local users, fundamentally undermining the security model of Linux systems. From a cybersecurity perspective, this represents a significant weakness in the kernel's memory management and object lifecycle handling mechanisms.
The technical implementation of this vulnerability stems from improper handling of reference counting and memory deallocation within the traffic control filtering system. When classification filters are modified or removed, the kernel fails to properly synchronize access to shared data structures, leading to situations where objects are freed from memory while still being referenced by other components. This classic use-after-free condition allows an attacker to manipulate freed memory locations, potentially corrupting kernel data structures or executing arbitrary code with kernel privileges. The vulnerability is particularly dangerous because it operates within the core networking subsystem where many security controls and access restrictions are enforced.
The operational impact of this privilege escalation flaw extends beyond simple local exploitation, as it can compromise the fundamental integrity of the entire system. Once an unprivileged user gains elevated privileges through this vulnerability, they can access sensitive system resources, modify critical files, establish persistent backdoors, or disable security mechanisms entirely. The threat to confidentiality is particularly severe as attackers can access encrypted data, authentication credentials, and system configuration files that should be protected from unauthorized access. Integrity is compromised through the ability to modify system binaries, kernel modules, and configuration settings, while availability suffers from potential system crashes or denial of service conditions that may occur during exploitation attempts.
The vulnerability aligns with CWE-416, which specifically addresses use-after-free conditions in software implementations, and demonstrates characteristics consistent with ATT&CK technique T1068, which involves exploit for privilege escalation. From a defensive standpoint, system administrators should prioritize immediate patching of affected kernel versions, as this vulnerability provides a direct path to kernel-level compromise without requiring network access or specialized attack infrastructure. Additional mitigations include implementing strict kernel module loading policies, monitoring for suspicious memory access patterns, and employing kernel hardening techniques such as stack canaries and address space layout randomization. Organizations should also conduct thorough security assessments of systems running vulnerable kernel versions to identify potential exploitation attempts and establish proper incident response procedures for handling privilege escalation events.