CVE-2026-29008

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.

Score

CVSS 4.0 8.7 High

Status

Analyzed - disclosure@vulncheck.com

Published

2026-07-08

Last modified

2026-07-14

Weaknesses

CWE-191

Affected products

• U-boot (Denx)

References

• https://lists.denx.de/pipermail/u-boot/2026-May/617853.html
• https://u-boot.org/
• https://www.vulncheck.com/advisories/u-boot-rc3-integer-underflow-dos-via-tcp-rx-state-machine
• https://y637f9qq2x.com/posts/u-boot-tcp-nfs-vulns/