CVE-2026-29008 in U-Boot
Summary
by MITRE • 07/08/2026
U-Boot through 2026.04-rc3 contains an integer underflow vulnerability in the tcp_rx_state_machine() function (net/tcp.c) that allows a network-adjacent attacker to crash the bootloader by sending a malformed TCP SYN+ACK packet with a manipulated data offset field causing payload_len to become negative. When the TCP_SYN_SENT handler calls tcp_rx_user_data() without invoking tcp_seg_in_wnd() validation, the negative payload_len is implicitly converted to a large unsigned integer (e.g., 0xFFFFFFD8) and passed to memcpy() in store_block(), causing an immediate crash that prevents device boot and may enable memory corruption when CONFIG_LMB is disabled.
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Analysis
by VulDB Data Team • 07/08/2026
The vulnerability under discussion represents a critical integer underflow condition within the U-Boot bootloader ecosystem that fundamentally compromises system availability and potentially introduces memory corruption risks. This flaw exists in the tcp_rx_state_machine() function located within net/tcp.c and specifically targets the handling of TCP SYN+ACK packets during the initial connection establishment phase. The issue manifests when a network-adjacent attacker crafts a malformed TCP packet with a manipulated data offset field, which directly influences the calculation of payload_len within the TCP processing pipeline. The integer underflow occurs because the data offset field manipulation results in a negative value for payload_len, a condition that arises from improper validation of TCP segment parameters before critical operations are executed.
The technical execution of this vulnerability exploits the absence of proper input validation within the TCP state machine implementation where the TCP_SYN_SENT handler processes incoming user data without first invoking tcp_seg_in_wnd() function to validate segment boundaries and packet integrity. This omission creates a path where malformed data offset fields can propagate through the TCP processing stack unchallenged, leading to the implicit conversion of the negative payload_len value into a large unsigned integer such as 0xFFFFFFD8. The subsequent use of this invalid value in memcpy() operations within the store_block() function constitutes an immediate crash condition that terminates the bootloader execution flow and prevents normal device boot procedures from completing successfully.
The operational impact of this vulnerability extends beyond simple denial-of-service conditions to potentially enable more sophisticated attack vectors when specific configuration options are disabled. When CONFIG_LMB (Large Memory Block) support is disabled in the U-Boot build configuration, the memory corruption risks increase significantly as the bootloader's memory management mechanisms become less robust against malformed input data. This configuration dependency creates a scenario where the vulnerability can transition from a simple crash condition to a potential code execution vector, particularly when attackers can manipulate memory layout and stack contents through carefully crafted network packets. The vulnerability affects all U-Boot versions up to and including 2026.04-rc3, representing a substantial attack surface across numerous embedded systems that rely on this bootloader implementation.
The underlying architecture of this vulnerability aligns with CWE-191, which specifically addresses integer underflow conditions where the result of an arithmetic operation becomes less than the minimum value representable by the data type. This classification directly corresponds to the negative payload_len calculation that occurs during TCP packet processing and demonstrates how seemingly benign network packet manipulation can trigger fundamental memory safety issues within embedded bootloader implementations. From an ATT&CK perspective, this vulnerability maps to T1059.007 (Command and Scripting Interpreter: Python) and T1499.004 (Network Denial of Service: Resource Exhaustion) through its potential for remote exploitation and system availability compromise, while also potentially enabling T1068 (Local Execution: Exploitation for Privilege Escalation) when memory corruption occurs in specific configurations. The vulnerability's network-adjacent requirement means that attackers need physical proximity or network access to the target device, but this limitation does not reduce the severity impact given the critical nature of bootloader compromise and its immediate effect on system boot processes.
The mitigation strategies for this vulnerability require immediate patching of affected U-Boot versions through the official release channels, with security updates incorporating proper input validation checks before arithmetic operations on TCP segment fields. System administrators should prioritize updating all affected devices to patched versions while also implementing network segmentation controls to limit access to bootloader interfaces and reduce attack surface exposure. Additionally, enabling CONFIG_LMB support where possible can provide additional memory safety protections against potential corruption scenarios, though this does not eliminate the immediate crash condition that prevents normal boot operations. Organizations maintaining legacy systems with unpatchable U-Boot versions should consider implementing network-level firewalls or access control lists to prevent unauthorized network access to devices running vulnerable bootloader versions, as the vulnerability's remote exploitability makes it particularly dangerous in operational environments where physical security may be insufficient.