CVE-2026-74221 in U-Boot
Summary
by MITRE • 09/30/2026
U-Boot before 2026.10-rc5 contains a buffer overflow in nfs_readlink_reply() function in net/nfs-common.c when processing NFS server responses. A malicious NFS server can send crafted READLINK replies with negative or oversized symlink length values to corrupt memory and crash the bootloader.
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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_readlink_reply function found in net/nfs-common.c. This buffer overflow arises from insufficient validation of input data received during the processing of NFS server responses. The core technical issue stems from the bootloader's failure to adequately verify the length values associated with symbolic link replies before attempting to copy or process them into fixed-size buffers. When a malicious NFS server sends crafted READLINK replies containing negative or excessively large symlink length fields, the application logic proceeds without appropriate bounds checking, leading to memory corruption that can destabilize the entire boot environment and potentially allow for arbitrary code execution if the attacker has control over other aspects of the system state.
From an operational perspective, this vulnerability poses a severe risk during the early stages of device initialization where U-Boot serves as the primary bootloader. An attacker with network access to the target machine can exploit this flaw by hosting a rogue NFS server that delivers specially crafted packets designed to trigger the overflow. The immediate impact is typically a denial of service manifested through a system crash or reboot loop, effectively preventing the operating system from loading and rendering the device inoperable. However, given the nature of buffer overflows in C-based embedded systems, there exists a significant potential for more advanced exploitation scenarios where memory corruption could be leveraged to execute arbitrary code with the privileges held by the bootloader process, which often includes direct hardware access and minimal security restrictions compared to standard user-space applications.
This flaw aligns closely with CWE-120, Buffer Copy without Checking Size of Input Classic Buffer Overflow, as it involves writing data beyond the boundaries of a allocated buffer due to unchecked input lengths. Furthermore, in the context of the MITRE ATT&CK framework for IoT and embedded systems, this vulnerability facilitates techniques associated with Initial Access via network services and potentially Execution through bootloader exploitation. The attack vector is classified as Network-based, requiring the adversary to have connectivity to the target's NFS service during the boot phase or when U-Boot attempts to load configurations over the network. This highlights a common weakness in embedded firmware where convenience features like network booting are implemented without rigorous security hardening against untrusted external inputs.
Mitigation strategies primarily involve upgrading to U-Boot version 2026.10-rc5 or later, which includes patches addressing this specific input validation deficiency. In environments where immediate patching is not feasible, administrators should restrict network access to NFS servers used during the boot process using firewall rules or VLAN segmentation to ensure that only trusted and verified sources can provide file system data. Additionally, disabling unnecessary network services in U-Boot configuration if they are not required for the specific deployment scenario reduces the attack surface significantly. Security teams monitoring embedded device fleets should prioritize updating bootloader firmware as part of their routine maintenance cycles, recognizing that vulnerabilities at this level compromise the foundational trust anchor of the device's security posture and can lead to persistent compromises if exploited in conjunction with other weaknesses.