CVE-2026-29008: U-Boot 2026.04-rc3 Integer Underflow DoS via tcp_rx_state_machine()

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-07-08 · updated 2026-07-22
Weakness: CWE-191

Description

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.

Affected products

u-boot: before 2026.04; 2026.04-rc1, 2026.04-rc2, 2026.04-rc3 — fixed in 2026.04

How to fix

Upgrade to a fixed version: u-boot 2026.04. Check the vendor advisory for mitigations.

References

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