CVE-2026-74220 in U-Boot
Summary
by MITRE • 09/30/2026
U-Boot before 2026.10-rc5 contains a buffer overflow in nfs_read_reply() function in net/nfs-common.c that allows attackers to corrupt memory by supplying crafted NFS READ reply lengths. A malicious NFS server can exploit signed integer handling to bypass length validation and write far past the destination buffer, crashing the bootloader or corrupting memory.
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Analysis
by VulDB Data Team • 09/30/2026
The vulnerability identified in U-Boot versions prior to 2026.10-rc5 represents a critical security flaw within the network file system client implementation, specifically located in the nfs_read_reply function found in net/nfs-common.c. This buffer overflow arises from improper handling of signed integer values when processing reply lengths received from an NFS server. In embedded systems and bootloader environments like U-Boot, memory safety is paramount because a compromise at this level can lead to complete system takeover or denial of service before the operating system even loads. The flaw stems from a failure to properly validate input data types during the parsing of network packets, allowing malicious actors to manipulate how length values are interpreted by the software logic.
From a technical perspective, the core issue lies in signed integer handling that bypasses standard length validation checks. When an NFS client receives a READ reply, it must verify that the payload size does not exceed the allocated buffer capacity to prevent overwriting adjacent memory regions. However, due to this vulnerability, crafted packets containing specific negative or excessively large values can trick the application into treating them as valid positive lengths or bypassing boundary checks entirely. This allows an attacker to write data far beyond the destination buffer's allocated space, resulting in heap or stack corruption depending on where the nfs_read_reply function operates within the memory layout of U-Boot. Such out-of-bounds writes corrupt critical control structures, potentially overwriting return addresses, pointers, or other sensitive variables stored nearby in memory.
The operational impact of this vulnerability is severe for any device relying on U-Boot as its primary bootloader. A successful exploitation can lead to immediate system crashes by triggering segmentation faults or general protection faults during the boot process. More dangerously, if an attacker carefully crafts the overflow payload, they may achieve arbitrary code execution within the privileged context of the bootloader. This could allow them to modify boot parameters, inject malicious kernel images, disable security features such as Secure Boot verification, or establish persistence mechanisms that survive OS reinstalls since U-Boot often resides in persistent flash memory separate from the main operating system storage.
This vulnerability aligns with CWE-120, which describes buffer copy without checking size limits, and more specifically reflects issues related to signed integer conversion errors that lead to out-of-bounds writes. In terms of attack vectors, it falls under MITRE ATT&CK techniques involving network-based exploitation where an adversary interacts directly with the target system over a network protocol. Since NFS is commonly used in embedded Linux environments for diskless booting or configuration management, this vulnerability poses a significant risk in IoT devices, industrial control systems, and networking equipment that utilize U-Boot and rely on remote file access during initialization phases.
Mitigation strategies primarily involve upgrading to U-Boot version 2026.10-rc5 or later where the signed integer handling has been corrected and proper bounds checking is enforced for all incoming NFS reply lengths. Organizations should also implement network segmentation to restrict access to NFS servers from untrusted networks, ensuring that only authorized devices can communicate with boot servers during the initialization phase. Additionally, enabling Secure Boot features in U-Boot provides an additional layer of defense by verifying the integrity of loaded images and preventing unauthorized modifications even if lower-level memory corruption occurs. Regular auditing of bootloader configurations and disabling unnecessary network services like NFS client functionality when not required further reduces the attack surface available to potential adversaries targeting embedded infrastructure.