CVE-2022-30767: Denx U-Boot Buffer Overflow Vulnerability

CVE-2022-30767 Overview

CVE-2022-30767 is a critical buffer overflow vulnerability in the nfs_lookup_reply function within net/nfs.c of Das U-Boot bootloader through version 2022.04 (and through 2022.07-rc2). The vulnerability exists due to an unbounded memcpy operation with a failed length check, which can lead to memory corruption. This issue is particularly concerning as it stems from an incomplete fix for a previous vulnerability, CVE-2019-14196.

Critical Impact

This network-accessible buffer overflow vulnerability in the Das U-Boot bootloader allows remote attackers to potentially achieve code execution during the boot process, compromising the entire system before the operating system loads.

Affected Products

• DENX U-Boot through 2022.04
• DENX U-Boot 2022.07-rc1 and 2022.07-rc2
• Fedora Project Fedora 36

Discovery Timeline

• 2022-05-16 - CVE-2022-30767 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2022-30767

Vulnerability Analysis

The vulnerability resides in the Network File System (NFS) implementation within Das U-Boot, specifically in the nfs_lookup_reply function. Das U-Boot is a widely-used open-source bootloader that provides network boot capabilities for embedded systems, making this vulnerability particularly impactful for IoT devices, embedded systems, and network-booted infrastructure.

The flaw manifests when processing NFS lookup reply packets. The original fix for CVE-2019-14196 attempted to address buffer overflow issues in this code path but failed to properly validate buffer boundaries for all memcpy operations. An attacker with network access can craft malicious NFS responses that trigger the unbounded memory copy, potentially overwriting adjacent memory regions with attacker-controlled data.

Root Cause

The root cause is classified as CWE-120 (Buffer Copy without Checking Size of Input). The nfs_lookup_reply function performs memcpy operations without adequately validating that the source data length fits within the destination buffer bounds. The previous fix for CVE-2019-14196 was incomplete, leaving vulnerable code paths that could still be exploited through specially crafted NFS reply packets.

Attack Vector

This vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker positioned on the same network segment as a device attempting to boot via NFS can intercept or spoof NFS server responses. By sending a malicious NFS lookup reply packet with crafted length values, the attacker can trigger the buffer overflow during the boot process, potentially achieving arbitrary code execution before the operating system is loaded.

// Security patch from net/nfs.c
// Source: https://github.com/u-boot/u-boot/commit/5d14ee4e53a81055d34ba280cb8fd90330f22a96

 	}
 
 	if (supported_nfs_versions & NFSV2_FLAG) {
+		if (((uchar *)&(rpc_pkt.u.reply.data[0]) - (uchar *)(&rpc_pkt) + NFS_FHSIZE) > len)
+			return -NFS_RPC_DROP;
 		memcpy(filefh, rpc_pkt.u.reply.data + 1, NFS_FHSIZE);
 	} else {  /* NFSV3_FLAG */
 		filefh3_length = ntohl(rpc_pkt.u.reply.data[1]);
 		if (filefh3_length > NFS3_FHSIZE)
 			filefh3_length  = NFS3_FHSIZE;
+		if (((uchar *)&(rpc_pkt.u.reply.data[0]) - (uchar *)(&rpc_pkt) + filefh3_length) > len)
+			return -NFS_RPC_DROP;
 		memcpy(filefh, rpc_pkt.u.reply.data + 2, filefh3_length);
 	}

The patch adds proper bounds checking before each memcpy operation, validating that the packet length (len) is sufficient to contain the data being copied. If the check fails, the function returns -NFS_RPC_DROP to safely discard the malformed packet.

Detection Methods for CVE-2022-30767

Indicators of Compromise

• Unusual NFS traffic patterns targeting devices during boot sequences
• Malformed NFS lookup reply packets with inconsistent length fields
• Device boot failures or unexpected behavior following network boot attempts
• Memory corruption symptoms during U-Boot execution phase

Detection Strategies

• Monitor network traffic for anomalous NFS protocol activity during device boot windows
• Implement intrusion detection rules to identify NFS packets with abnormal file handle sizes exceeding standard limits
• Deploy network sensors to detect potential man-in-the-middle attacks targeting PXE/NFS boot environments
• Audit firmware versions across embedded devices to identify systems running vulnerable U-Boot releases

Monitoring Recommendations

• Enable detailed logging on NFS servers to capture all boot-related file system requests
• Implement network segmentation to isolate boot infrastructure from untrusted network segments
• Deploy endpoint detection on systems capable of monitoring pre-OS boot activity
• Establish baseline network behavior for boot processes to identify deviations indicating exploitation attempts

How to Mitigate CVE-2022-30767

Immediate Actions Required

• Update Das U-Boot to a patched version that includes the security fix
• Restrict network access to boot infrastructure using VLANs or firewall rules
• Disable NFS boot functionality if not required for operations
• Implement 802.1X network authentication to prevent unauthorized devices from joining boot networks

Patch Information

The vulnerability has been addressed through commits to the U-Boot repository. The primary fix is available at the U-Boot GitHub commit and the DENX source repository commit. Fedora users should apply updates through the Fedora package announcement. Additional technical details are available in the GitHub Security Lab research and the DENX mailing list post.

Workarounds

• Configure network infrastructure to prevent ARP spoofing and DHCP attacks that could redirect boot traffic
• Use alternative boot methods such as local storage or TFTP with authenticated connections where possible
• Implement network access control lists to restrict which servers can respond to NFS boot requests
• Consider using secure boot implementations that validate bootloader integrity before execution

# Configuration example - Disable NFS boot in U-Boot environment
# Access U-Boot command line during boot

# View current boot configuration
printenv bootcmd

# Set boot to use local storage instead of NFS
setenv bootcmd 'mmc dev 0; fatload mmc 0:1 ${loadaddr} zImage; bootz ${loadaddr}'

# Save environment changes
saveenv

# Alternatively, restrict network boot sources via DHCP server configuration
# In dhcpd.conf - limit NFS server to trusted hosts only