# CVE-2026-29008: U-Boot integer underflow in tcp_rx_state_machine

Source: https://junglewise.ai/threats/cve-2026-29008-u-boot-integer-underflow-in-tcp-rx-state-machine  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-08. Updated: 2026-07-08  
License: CC BY 4.0 with a link back

CVE-2026-29008 · Severity: high · CVSS 7.5 · Published 2026-07-08

U-Boot is a widely used open-source bootloader responsible for initializing hardware and starting the operating system on embedded devices. A vulnerability in its networking component allows a nearby attacker to crash the device during the boot process by sending a specially crafted network packet. This can prevent the device from starting up successfully, leading to a denial of service that may require physical intervention to recover.

## Quick answers

**Is CVE-2026-29008 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-29008 being exploited in the wild.

**How severe is CVE-2026-29008?** High severity, with a CVSS score of 7.5, published 8 July 2026.

**Which products and versions are affected?** Das U-Boot Project U-Boot through 2026.04-rc3.

**Is there a fix for CVE-2026-29008?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-29008 attacked?** It is attacked from the adjacent network, in device firmware.

**Am I affected by CVE-2026-29008?** Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

An integer underflow exists in the tcp_rx_state_machine() function within net/tcp.c. When the bootloader is in the TCP_SYN_SENT state, it may process a malformed TCP SYN+ACK packet where a manipulated data offset field causes the payload_len calculation to result in a negative value. Because the TCP_SYN_SENT handler fails to invoke tcp_seg_in_wnd() validation, this negative value is implicitly converted to a large unsigned integer when passed to memcpy() in the store_block() function. This results in an out-of-bounds memory access and immediate crash. If CONFIG_LMB is disabled, this could potentially lead to further memory corruption. The vulnerability is exploitable by an unauthenticated attacker on the same local network segment.

## Affected products

- Das U-Boot Project U-Boot through 2026.04-rc3

## Timeline

- 2026-05-08: disclosed: Initial outreach to U-Boot mailing list by VulnCheck
- 2026-07-08: advisory: Public advisory released by VulnCheck and NVD record published

## References

- [https://lists.denx.de/pipermail/u-boot/2026-May/617853.html](https://lists.denx.de/pipermail/u-boot/2026-May/617853.html)
- [https://u-boot.org/](https://u-boot.org/)
- [https://www.vulncheck.com/advisories/u-boot-rc3-integer-underflow-dos-via-tcp-rx-state-machine](https://www.vulncheck.com/advisories/u-boot-rc3-integer-underflow-dos-via-tcp-rx-state-machine)