Which versions of U-Boot are affected by CVE-2026-29008?

Das U-Boot bootloader versions through 2026.04-rc3 contain a denial-of-service vulnerability allowing network-adjacent attackers to crash devices during boot before the OS loads, rendering systems…

How to fix or mitigate CVE-2026-29008?

Within 24 hours: Inventory all devices and systems running Das U-Boot through version 2026.04-rc3; determine network accessibility and criticality of affected bootloaders. Within 7 days: Implement network segmentation to restrict network-adjacent access to boot interfaces via firewalls or VLANs; verify CONFIG_LMB is enabled on all systems to prevent memory corruption variants. Within 30 days: Monitor Das U-Boot project releases for patch availability; develop deployment plan for affected devices; establish boot-failure alerting and device recovery procedures for critical infrastructure.

U-Boot CVE-2026-29008

Q: What is the CVSS score of CVE-2026-29008?

CVE-2026-29008 has a CVSS 4.0 base score of 8.7 (High). CVSS vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X. EPSS exploitation probability: 0.4%.

EUVD-2026-42332 HIGH

Integer Underflow (CWE-191)

2026-07-08 VulnCheck

GHSA-3qwj-q697-4ppr

Buffer Overflow Integer Overflow Suse U Boot

8.7

CVSS 4.0 · Vendor: VulnCheck

Severity by source

Vendor (VulnCheck) PRIMARY

8.7 HIGH

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

vuln.today AI

6.5 MEDIUM

Attacker must answer U-Boot's outbound TCP connection on the local segment during netboot, so AV:A; no auth or UI; impact is boot-time DoS only, hence A:H and C:N/I:N.

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

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

SUSE

6.5 MEDIUM

AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Primary rating from Vendor (VulnCheck).

CVSS VectorVendor: VulnCheck

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Lifecycle Timeline

Analysis Generated

Jul 08, 2026 - 17:01 vuln.today

DescriptionCVE.org

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.

AnalysisAI

Denial of service (with conditional memory corruption) in Das U-Boot bootloader versions through 2026.04-rc3 lets a network-adjacent attacker crash the device before the OS loads by returning a malformed TCP SYN+ACK to a connection U-Boot has initiated. Because the CVSS 4.0 vector (VA:H, no confidentiality/integrity impact) reflects an availability-only bug rooted in an integer underflow (CWE-191), the practical outcome is a bricked/failed boot; when CONFIG_LMB is disabled, the same underflow can corrupt memory. No public exploit identified at time of analysis; not listed in CISA KEV.

Technical ContextAI

U-Boot is the de facto open-source boot/pre-boot firmware for embedded and IoT devices, and includes a lightweight TCP/IP stack (net/tcp.c) used for network operations such as HTTP/wget-style downloads during netboot. The flaw is in tcp_rx_state_machine(): while a connection is in the TCP_SYN_SENT state, the handler calls tcp_rx_user_data() to consume payload without first running tcp_seg_in_wnd() sequence/window validation. An attacker-controlled TCP data-offset field lets the computed payload_len go negative (CWE-191 integer underflow); this signed value is implicitly widened to a large unsigned integer (e.g. 0xFFFFFFD8) and handed to memcpy() inside store_block(), producing an out-of-bounds copy. CPE cpe:2.3:a:u-boot:u-boot:*:*:*:*:*:*:*:* indicates all U-Boot builds containing this TCP code path are in scope.

RemediationAI

Upstream fix available via the U-Boot mailing list (https://lists.denx.de/pipermail/u-boot/2026-May/617853.html); released patched version not independently confirmed from the provided data, so rebuild firmware from a U-Boot revision that adds tcp_seg_in_wnd() validation before tcp_rx_user_data() in the TCP_SYN_SENT path, or backport that patch. As compensating controls until a tagged release is integrated: if network boot is not required, disable the TCP stack / CONFIG_NET TCP features (CONFIG_TCP) in the U-Boot build to remove the vulnerable code path entirely (trade-off: no HTTP/TCP netboot); keep CONFIG_LMB enabled so the underflow degrades to a crash rather than memory corruption (trade-off: LMB memory management overhead); and restrict the boot-time network to a trusted, isolated segment so an attacker cannot answer U-Boot's outbound TCP connections (trade-off: requires controlled provisioning VLAN). Track the VulnCheck advisory (https://www.vulncheck.com/advisories/u-boot-rc3-integer-underflow-dos-via-tcp-rx-state-machine) and your OEM's firmware update for the fixed version. Do not assume 'no patch exists' - absence of a confirmed version number here is a data gap, not confirmation.

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