The vulnerability in Caliptra Core Runtime Firmware's aes_256_gcm_update module arises from a missing cryptographic step that leads to an incorrect GCM authentication tag. Specifically, when the streaming AES-256-GCM API is used with empty AAD, the hardware GHASH accumulator state is not preserved after the first update call. Consequently, the final authentication tag excludes the first batch of processed ciphertext, enabling undetected modifications to that ciphertext portion. This affects firmware versions 2.0.0 through 2.0.1 and 2.1.0.

An attacker with low privileges and network access could modify ciphertext processed by the vulnerable AES-256-GCM implementation without detection, due to the incorrect authentication tag. This undermines the integrity guarantees of the cryptographic operation, potentially allowing tampering with encrypted data. However, the vulnerability does not allow privilege escalation or denial of service directly and requires specific conditions (empty AAD and use of streaming API).

No official fix or patch is currently available for this vulnerability. Users should monitor the vendor's advisory for updates regarding remediation. Until a fix is provided, avoid using the streaming AES-256-GCM API with empty AAD in the affected firmware versions if possible.