Bouncy Castle BC-FJA EUVD-2026-28534

Exploitation requires: (1) local attacker access to the system running the vulnerable BC-FJA library (AV:L from CVSS vector), (2) active invocation of AES-GCM cryptographic operations (gcm128w or gcm512w code paths) by the targeted application, (3) Linux x86_64 operating system, and (4) CPU with AVX or AVX-512f SIMD extensions. No authentication, user interaction, or elevated privileges are required (PR:N/UI:N). Systems running BC-FJA on non-x86_64 architectures (ARM, PowerPC, etc.), non-Linux platforms (Windows, macOS, BSD), or with AVX/AVX-512f disabled in BIOS/hypervisor are not vulnerable. Applications that do not use GCM mode encryption are not affected, even if BC-FJA is installed.

The CVSS 4.0 vector (AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L) indicates local access is required (AV:L), with low attack complexity and no prerequisites, but impact is limited to availability (VA:L) with no confidentiality or integrity loss. The CVSS base score of 5.1 reflects this as a low-to-moderate severity issue. Notably, the RE:M remediation effectiveness tag suggests vendor patches or workarounds are available. The Amber U (unmitigated) status indicates the vulnerability remains unaddressed in current versions at report time, but this requires verification against the do-not-publish advisory URL. The availability-only impact significantly reduces real-world risk compared to a remote code execution vulnerability, even though CVSS:4.0 scoring may initially appear concerning; an attacker must have local system access and cannot compromise cryptographic material or plaintext. The exploitation conditions are highly specific: the attack requires the target system to be running affected BC-FJA versions on Linux x86_64 with AVX/AVX-512f capable processors and to actively invoke GCM operations. This limits the practical threat surface compared to server-side cryptographic libraries exposed via network services.

An attacker with local system access runs an application that invokes Bouncy Castle BC-FJA GCM encryption operations on a Linux x86_64 system with AVX or AVX-512f capability. By crafting specific input or timing attacks against the gcm128w or gcm512w assembly modules, the attacker triggers the vulnerability, causing the GCM operation to fail or hang, resulting in denial of service. No remote network access is required, and the attack does not compromise encrypted data or cryptographic keys.