CVE-2021-38575: Tianocore EDK2 Buffer Overflow Vulnerability

CVE-2021-38575 Overview

CVE-2021-38575 is a critical buffer overflow vulnerability discovered in the NetworkPkg/IScsiDxe component of the TianoCore EDK2 UEFI firmware implementation. This BIOS/UEFI vulnerability affects the iSCSI driver used during network-based boot operations and can be exploited remotely without authentication.

The vulnerability exists in the iSCSI driver's handling of network data during boot operations. Attackers on the same network segment can send specially crafted iSCSI packets to exploit buffer overflows in the affected driver, potentially achieving code execution at the firmware level before the operating system loads.

Critical Impact

Remote attackers can exploit buffer overflows in UEFI firmware's iSCSI driver to execute arbitrary code at the firmware level, potentially establishing persistent malware that survives OS reinstallation.

Affected Products

• TianoCore EDK2 (all affected versions)
• Insyde Kernel 5.0
• Insyde Kernel 5.1
• Insyde Kernel 5.2
• Insyde Kernel 5.3
• Insyde Kernel 5.4
• Insyde Kernel 5.5

Discovery Timeline

• 2021-12-01 - CVE-2021-38575 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2021-38575

Vulnerability Analysis

This vulnerability represents a severe firmware-level security flaw affecting the iSCSI boot functionality in UEFI implementations based on TianoCore EDK2. The buffer overflow vulnerabilities (classified under CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer, and CWE-124: Buffer Underwrite) occur in the IScsiDxe driver component within the NetworkPkg module.

The attack requires network access and targets systems configured for iSCSI boot or that have the iSCSI DXE driver loaded. While the attack complexity is high due to the specific conditions required for exploitation, successful exploitation requires no privileges or user interaction, making it particularly dangerous in enterprise environments that leverage network boot infrastructure.

Root Cause

The root cause of CVE-2021-38575 lies in improper bounds checking within the IScsiDxe driver when processing iSCSI protocol data. The driver fails to adequately validate the length of incoming data before copying it into fixed-size buffers, creating classic buffer overflow conditions. Additionally, the presence of CWE-124 (Buffer Underwrite) indicates that certain operations may write data before the beginning of the intended buffer, corrupting adjacent memory structures.

These vulnerabilities are particularly concerning because they exist at the firmware level, below the operating system. Code executing in this pre-boot environment operates with the highest system privileges and can modify critical system configurations, including Secure Boot settings.

Attack Vector

The attack vector for CVE-2021-38575 is network-based, targeting systems during the UEFI boot phase when the iSCSI driver processes network traffic. An attacker positioned on the same network segment as the target system can send malicious iSCSI responses during the boot process.

The exploitation scenario typically involves:

1. Identifying target systems configured for iSCSI boot or with the iSCSI driver active
2. Intercepting or spoofing iSCSI traffic during the boot sequence
3. Sending crafted packets containing oversized or malformed data to trigger buffer overflow conditions
4. Achieving code execution in the DXE (Driver Execution Environment) phase of UEFI boot

Successful exploitation can result in firmware-level code execution, enabling attackers to install persistent implants that survive operating system reinstallation, bypass Secure Boot protections, or completely compromise the system's boot process.

Detection Methods for CVE-2021-38575

Indicators of Compromise

• Unexpected modifications to UEFI firmware variables or configuration
• Anomalous network traffic to/from systems during boot operations, particularly on iSCSI ports (TCP 3260)
• Secure Boot integrity failures or unexpected boot behavior
• Evidence of firmware updates or modifications not sanctioned by administrators

Detection Strategies

• Monitor network traffic for malformed or anomalous iSCSI protocol packets targeting boot infrastructure
• Implement UEFI firmware integrity verification using hardware-based attestation mechanisms (TPM measurements)
• Deploy network intrusion detection signatures targeting iSCSI protocol anomalies
• Conduct regular firmware version audits to identify systems running vulnerable EDK2-based implementations

Monitoring Recommendations

• Enable logging on iSCSI target servers to detect unusual connection attempts or malformed requests
• Implement network segmentation to isolate boot infrastructure from general network traffic
• Use SentinelOne Singularity Platform to monitor for post-exploitation indicators at the OS level
• Configure alerts for unexpected firmware update events or UEFI variable modifications

How to Mitigate CVE-2021-38575

Immediate Actions Required

• Identify all systems using iSCSI boot or TianoCore EDK2-based firmware and prioritize patching
• Disable iSCSI boot functionality on systems where it is not required
• Implement network segmentation to restrict access to boot infrastructure
• Apply vendor-supplied firmware updates from system manufacturers

Patch Information

TianoCore has addressed this vulnerability and released fixes tracked in TianoCore Bug Report #3356. System manufacturers using EDK2 have released updated firmware incorporating these fixes. Insyde has published Security Advisory SA-2023025 with specific guidance for affected Insyde Kernel versions 5.0 through 5.5.

Organizations should contact their system vendors for firmware updates applicable to their specific hardware. Debian users can reference the Debian LTS Announcement for package updates.

Workarounds

• Disable iSCSI boot functionality in UEFI settings where not required for business operations
• Implement strict network access controls to prevent unauthorized systems from communicating with boot infrastructure
• Use dedicated, isolated network segments (VLANs) for iSCSI boot traffic
• Enable Secure Boot and ensure firmware integrity measurements are configured with TPM

# Example: Verify current firmware version on Linux systems
sudo dmidecode -t bios

# Check for UEFI Secure Boot status
mokutil --sb-state

# Review iSCSI configuration (disable if not needed)
iscsiadm -m session