CVE-2021-47656 is a high-severity use-after-free (UAF) vulnerability in the Linux kernel's JFFS2 (Journaling Flash File System version 2) implementation. The flaw arises during the mounting process of a JFFS2 filesystem image. Specifically, the vulnerability occurs when the system assumes that the initial blocks of the JFFS2 image are normal and contain at least one extended attribute (xattr)-related inode, but encounters an abnormal block subsequently. This abnormal block triggers an error in the jffs2_scan_eraseblock() function. Following this, the function jffs2_clear_xattr_subsystem() is called twice erroneously during the filesystem build and mount process (once in jffs2_build_filesystem() and again in jffs2_do_fill_super()). This double invocation leads to a use-after-free condition where memory previously freed is accessed again, causing a kernel memory corruption. The kernel's Kernel Address Sanitizer (KASAN) detects this as a BUG report indicating a read of freed memory. The root cause is improper error handling and cleanup sequencing in the JFFS2 mounting code, where jffs2_clear_xattr_subsystem() is called redundantly after an error return, leading to double free and use-after-free. The fix involves moving the call to jffs2_clear_xattr_subsystem() from one error handling path ('out_inohash') to another ('out_root') to ensure it is only called once, preventing the UAF. This vulnerability is tracked under CWE-416 (Use After Free) and has a CVSS v3.1 base score of 7.8, indicating high severity. Exploitation requires local privileges (low attack vector), low complexity, and no user interaction, but partial privileges (PR:L) are needed. Successful exploitation could lead to full confidentiality, integrity, and availability compromise of the affected system due to kernel memory corruption. No known exploits are currently reported in the wild. The affected versions correspond to specific Linux kernel commits prior to the patch. This vulnerability is relevant to systems using JFFS2, which is commonly used in embedded devices and systems with flash memory storage. The vulnerability could be triggered by mounting a crafted JFFS2 image, potentially leading to privilege escalation or denial of service via kernel crash.

For European organizations, the impact of CVE-2021-47656 depends largely on the deployment of Linux-based embedded systems or devices using JFFS2 filesystems. Many industrial control systems, IoT devices, routers, and embedded appliances in sectors such as manufacturing, telecommunications, and critical infrastructure may rely on Linux kernels with JFFS2 support. Exploitation could allow an attacker with local access to execute arbitrary code in kernel space, escalate privileges, or cause system crashes leading to denial of service. This poses significant risks to operational continuity, data confidentiality, and system integrity. In critical sectors like energy, transportation, and healthcare, such disruptions could have cascading effects on safety and service availability. Additionally, the vulnerability could be leveraged as a stepping stone for lateral movement within networks if attackers gain initial footholds on vulnerable devices. Although exploitation requires local privileges, compromised devices in distributed environments could be targeted by insiders or malware that gains local access. The lack of known exploits in the wild suggests limited immediate threat, but the high severity and kernel-level impact necessitate proactive mitigation to prevent future attacks. Organizations relying on embedded Linux devices or custom Linux distributions should assess their exposure and patch accordingly to maintain security posture.

1. Apply official Linux kernel patches that address CVE-2021-47656 as soon as they become available from trusted sources or Linux distribution vendors. 2. For embedded devices or custom Linux builds, rebuild kernels with the patched JFFS2 code to eliminate the vulnerability. 3. Audit and inventory all devices and systems using JFFS2 filesystems to identify potentially vulnerable endpoints. 4. Restrict local access to systems running vulnerable kernels to trusted personnel only, minimizing risk of local exploitation. 5. Implement strict access controls and monitoring on devices with JFFS2 to detect suspicious mounting operations or kernel errors indicative of exploitation attempts. 6. Where possible, consider migrating from JFFS2 to more modern and actively maintained filesystems that do not exhibit this vulnerability. 7. Employ kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR), SELinux/AppArmor policies, and seccomp filters to reduce exploitation impact. 8. Regularly update and patch embedded device firmware and Linux kernels to incorporate security fixes. 9. For critical infrastructure, implement network segmentation to isolate vulnerable embedded devices and limit attack surface. 10. Conduct penetration testing and vulnerability assessments focusing on embedded Linux devices to validate mitigations.