CVE-2021-28950: Medium Vulnerability in Linux Kernel

CVE-2021-28950 is a medium-severity vulnerability affecting the Linux kernel, specifically in the fs/fuse/fuse_i.h file. It has been classified with a CVSS score of 5.5, indicating a moderate level of risk. This vulnerability allows for a "stall on CPU" condition, which occurs when a retry loop continuously encounters the same bad inode, identified as CID-775c5033a0d1. The impact of this vulnerability primarily affects system availability, leading to potential service disruptions.

The vulnerability affects all Linux kernel versions prior to 5.11.8, making it crucial for organizations running older versions to assess their exposure. The nature of the vulnerability means it can be triggered locally by an attacker with low privileges, thus heightening the importance for organizations to implement timely patches to mitigate the risk.

Currently, there are no known exploits or proofs of concept publicly available for this vulnerability, which is fortunate given its potential impact. Nonetheless, organizations should remain vigilant and monitor for any developments related to this CVE, as previously unexploited vulnerabilities can quickly become targets.

Given the medium severity of CVE-2021-28950 and its potential to impact system availability, organizations should prioritize patching immediately. Staying current with kernel updates and employing regular vulnerability assessments will be key strategies in maintaining a secure environment.

Vulnerability Details

The issue arises from a flaw in the Linux kernel before version 5.11.8. The vulnerability is categorized under CWE-834, which pertains to the improper handling of resource management. Due to its LOCAL attack vector, the complexity of exploiting this vulnerability is low, and it requires low privileges with no user interaction necessary, making it particularly concerning.

The CVSS v3.1 vector for this vulnerability is CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H. It highlights that while confidentiality and integrity impacts are nonexistent, the availability impact is rated as high.

Technical Analysis

The root cause of CVE-2021-28950 lies in the Linux kernel's handling of inodes within the FUSE filesystem, where a retry loop can lead to unresponsive CPU states. This condition can be triggered when the kernel continually identifies a bad inode, which prevents the proper function of file operations and leads to performance degradation.

The attack vector is local, meaning that an attacker would need access to the system to exploit the vulnerability. The attack complexity is classified as low, and it requires low privileges, allowing a malicious user to potentially cause significant disruption without sophisticated skills or tools.

With no user interaction required, this vulnerability poses a risk to environments where multiple users have access to the system. If an attacker successfully exploits this vulnerability, it can lead to high availability impacts, resulting in service interruptions and degraded system performance.

Risk & Impact Analysis

The real-world risk associated with CVE-2021-28950 is significant for organizations utilizing affected versions of the Linux kernel. The potential for CPU stalls can disrupt critical services, leading to downtime and loss of productivity. The availability impact is particularly concerning, as it can affect operational continuity and user experience.

Organizations relying on Linux systems should assess their deployment configurations and prioritize updates to mitigate this vulnerability. Given the increasing prevalence of local attacks targeting system availability, addressing this CVE should be a priority in vulnerability management programs.

The urgency of addressing CVE-2021-28950 is underscored by its potential to cause widespread impact in environments where multiple users can trigger the flaw. Organizations should schedule remediation as part of their regular patch management cycle to minimize exposure to this vulnerability.

Affected Versions

The vulnerability affects all versions of the Linux kernel prior to 5.11.8, including specific distributions such as Debian 9.0 and Fedora versions 33 and 34. Organizations using these versions should ensure they apply the necessary patches to secure their systems.

Mitigation & Remediation

To mitigate CVE-2021-28950, organizations should upgrade their Linux kernel to version 5.11.8 or later. If immediate patching is not feasible, consider implementing workarounds such as restricting access to affected systems and monitoring for abnormal CPU usage. Additionally, ensuring proper configuration hardening can reduce the risk of exploitation.

Organizations may find it beneficial to engage in continuous security testing to identify and address vulnerabilities proactively. For those needing assistance, services such as continuous penetration testing can help validate the effectiveness of remedial actions.

Detection Guidance

To detect potential exploitation of CVE-2021-28950, organizations should monitor system logs for indications of unusual CPU usage patterns and retry loops involving inode access. Behavioral anomalies in file operations may also indicate attempts to exploit this vulnerability.

AppSecure Threat Intelligence Insight

CVE-2021-28950 represents a significant risk for organizations using vulnerable versions of the Linux kernel. The incident underscores the importance of timely updates and robust vulnerability management practices. Security teams should be aware of the potential for similar vulnerabilities in the future and maintain vigilance in monitoring their systems.

To enhance security posture, organizations are encouraged to adopt a comprehensive vulnerability management program and consider integrating penetration testing methodologies into their security strategies.

By learning from past vulnerabilities like CVE-2021-28950, organizations can better prepare against future threats and enhance their overall security resilience.