U-Boot up to 2026.04-rc3 TCP_SYN_SENT handler net/tcp.c tcp_rx_state_machine offset integer underflow

Summaryinfo

A vulnerability classified as very critical has been found in U-Boot up to 2026.04-rc3. The affected element is the function tcp_rx_state_machine of the file net/tcp.c of the component TCP_SYN_SENT handler. This manipulation of the argument offset causes integer underflow. This vulnerability is handled as CVE-2026-29008. The attack can be initiated remotely. There is not any exploit available.

Detailsinfo

A vulnerability classified as very critical was found in U-Boot up to 2026.04-rc3. Affected by this vulnerability is the function tcp_rx_state_machine of the file net/tcp.c of the component TCP_SYN_SENT handler. The manipulation of the argument offset with an unknown input leads to a integer underflow vulnerability. The CWE definition for the vulnerability is CWE-191. The product subtracts one value from another, such that the result is less than the minimum allowable integer value, which produces a value that is not equal to the correct result. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at lists.denx.de. This vulnerability is known as CVE-2026-29008 since 03/03/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 07/08/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Name

• U-Boot

Version

• 2026.04-rc1
• 2026.04-rc2
• 2026.04-rc3

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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