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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.

PUBLISHED Reserved 2026-03-03 | Published 2026-07-08 | Updated 2026-07-14 | Assigner VulnCheck




HIGH: 8.7CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

HIGH: 7.5CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Problem types

Integer Underflow (Wrap or Wraparound)

Product status

Default status
affected

Any version
affected

Credits

Kazuma Matsumoto, a security researcher at GMO Cybersecurity by IERAE, Inc. finder

VulnCheck coordinator

References

y637f9qq2x.com/posts/u-boot-tcp-nfs-vulns/ technical-description exploit

lists.denx.de/pipermail/u-boot/2026-May/617853.html (Mailing List Outreach) issue-tracking

u-boot.org/ (Project Webpage) product

www.vulncheck.com/...-underflow-dos-via-tcp-rx-state-machine third-party-advisory

cve.org (CVE-2026-29008)

nvd.nist.gov (CVE-2026-29008)

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