Caliptra Core Runtime Firmware CVE-2026-5818

Exploitation requires (1) adjacent-network access to the platform management/RoT mailbox interface rather than the public internet (AV:A), (2) high pre-existing privileges sufficient to invoke ActivateFirmwareCmd against the Caliptra RoT (PR:H), (3) the target system actively using the hitless firmware update path rather than cold-boot updates, and (4) a specific attack prerequisite condition (AT:P) tied to the timing/state of the activate_fw flow. Limiting factors: no user interaction is needed (UI:N) and complexity is low once prerequisites are met (AC:L), but the PR:H and adjacent-vector requirements mean this is not exploitable by an unauthenticated remote attacker - it is a privilege-escalation-into-trust-chain bypass usable only by actors who already control firmware management.

The CVSS 4.0 vector (AV:A/AC:L/AT:P/PR:H/UI:N/VC:N/VI:H/VA:H plus subsequent-system SC:H/SI:H/SA:H) scores 7.2 (High) and reflects an adjacent-vector, high-privilege attack with no confidentiality impact but severe integrity/availability consequences that cascade into downstream systems trusting the RoT. PR:H and AT:P meaningfully constrain practical risk - the attacker must already control the firmware update pipeline or hold elevated platform privileges and meet specific attack prerequisites - so this is not a remote-internet-facing bug. However, because the broken check defeats the RoT's core guarantee of MCU firmware authenticity, successful exploitation breaks the trust chain that any attestation, secure-boot, or confidential-computing claim depends on, which is why the subsequent-system impacts are rated H across the board. EPSS and KEV data were not provided in the input; no public exploit identified at time of analysis, and given the disclosure originated from Caliptra itself, exploitation is currently theoretical rather than observed.

An attacker who has already obtained high-privilege access to the platform management interface (for example, a compromised BMC or malicious insider with firmware-update rights on an adjacent management network) submits a crafted, unsigned or maliciously modified MCU firmware image through the hitless update flow. Because activate_fw mishandles the verification return value, Caliptra accepts the image without validating its signature and activates it, giving the attacker persistent code execution on the MCU with the RoT continuing to attest the system as trusted. No public exploit identified at time of analysis.