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How the CVE Works:
Attestable AMIs use a Unified Kernel Image (UKI) measured by the UEFI firmware. The UKI embeds a kernel command line containing a critical dm-verity hash for root filesystem integrity. When UEFI Secure Boot is disabled, the systemd-boot loader appends any externally provided command line to the embedded one. An attacker with ‘UefiData’ modification privileges (e.g., an account operator) can install a boot variable that adds parameters (like ‘rd.verity=disable’) to bypass dm-verity. This manipulation is measured separately in Platform Configuration Register 12 (PCR12) by systemd-boot. Existing Key Management Service (KMS) attestation policies often only validate PCR4 (UKI measurement) and ignore PCR12. By exploiting this gap, an attacker can achieve a persistent boot compromise without altering the PCR4 value, breaking the chain of trust from firmware to the root filesystem.
dailycve form:
Platform: AWS NitroTPM
Version: <1.1.0
Vulnerability: Boot Integrity Bypass
Severity: Critical
date: 2024
Prediction: Patch November 2024
What Undercode Say:
` Manually add PCR12 to KMS policy`
`kms:RecipientAttestation:NitroTPMPCR12:000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000`
` Example nitro-tpm-pcr-compute JSON output`
`{
“Measurements”: {
“HashAlgorithm”: “SHA384”,
“PCR4”: ““,
“PCR7”: ““,
“PCR12”: “000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000”
}
}`
How Exploit:
- Attacker gains IAM privileges allowing `ModifyInstanceAttribute` for
UefiData. - Secure Boot is disabled on the target Attestable AMI instance.
- Attacker sets a boot variable with a malicious kernel command-line fragment (e.g.,
rd.verity=disable).
4. On reboot, systemd-boot appends this fragment.
- Kernel parses the combined command line, disabling integrity checks.
- Attacker achieves persistent root access despite PCR4-based attestation.
Protection from this CVE:
- Upgrade `nitro-tpm-pcr-compute` to version 1.1.0+.
- Manually add PCR12 validation to KMS policies.
- Enable and enforce UEFI Secure Boot (validated via PCR7).
- Follow least-privilege for `UefiData` modification.
Impact:
Complete root filesystem integrity bypass. Persistent compromise of attested instances. Elevation for account operators.
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Sources:
Reported By: github.com
Extra Source Hub:
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