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CVE-2026-4936 exposes a fundamental flaw in the IBM PowerVM Hypervisor Platform KeyStore (PKS) and its integrated virtual Trusted Platform Module (vTPM) firmware. The vulnerability arises from the improper generation and management of persistent storage key seeds across multiple firmware branches. Specifically, the affected firmware versions—FW1110.00 through FW1110.20, FW1060.00 through FW1060.71, and FW950.00 through FW950.H2—rely on a deterministic or insufficiently random seed source for deriving the AES encryption key used to protect virtual machine secrets, TPM-backed keys, and platform configuration registers. Instead of utilizing a hardware-based true random number generator (TRNG) with sufficient entropy, the seed generation process leverages a combination of system state variables that are either static or predictable over time. This results in an effective keyspace that is drastically smaller than the advertised AES key length, often reducing a 256-bit key to an effective strength of 64 bits or lower. An attacker who gains privileged access to the service processor (FSP) or the Hardware Management Console (HMC)—which are common administrative interfaces in enterprise PowerVM deployments—can capture the seed material through debugging interfaces or memory dumps. With this seed, the attacker can reproduce the key derivation function (KDF) offline and compute the exact AES key used for encrypting the PKS storage. Once derived, the key grants unrestricted decryption of all vTPM-protected data, including disk encryption keys, secure boot measurements, and identity certificates for virtual machines. The weakness is exacerbated by the fact that the seed is reused across multiple boot cycles and system resets, making it a persistent risk rather than a transient one. IBM has acknowledged that the key stretching mechanisms are absent, allowing direct computation of the master key from the seed. This vulnerability is particularly dangerous in multi-tenant cloud environments where a compromised HMC account could lead to a complete breakdown of tenant isolation. The attack does not require physical hardware access, only network reachability to the management plane, which is often exposed for administrative remote operations. Cryptanalytic techniques such as meet-in-the-middle or differential analysis can further accelerate the key recovery process given the reduced entropy. The firmware’s lack of periodic key rotation or ephemeral session keys means that a single seed compromise permanently exposes all historical and future data encrypted under that key until the firmware is updated and the storage is re-initialized. IBM has released security advisories recommending immediate firmware upgrades, but the patch process involves a disruptive reboot of the hypervisor.
DailyCVE Form:
Platform: IBM PowerVM
Version: FW1110/FW1060/FW950
Vulnerability: Weak AES Seed
Severity: Critical (CVSS 9.8)
date: 2026-08-25
Prediction: Expected Patch Q4/2026
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[User@HMC ~]$ lssyscfg -r sys -F name,fwversion
[User@HMC ~]$ lshwres -r tpm -m sys1 –filter “lpars=all”
[User@HMC ~]$ lsfw -v -m sys1 | grep -E “FW1110|FW1060|FW950”
[User@HMC ~]$ grep -i “pks” /var/log/hmc/.log
Python snippet to estimate seed entropy based on captured system uptime/mac:
import hashlib, time; seed = f”{time.ctime()}{mac}”.encode(); key = hashlib.pbkdf2_hmac(‘sha256’, seed, b”, 1000, dklen=32); print(key.hex())
Exploit: (Educational Purposes!)
An attacker with HMC read-only access can trigger a vTPM state dump using `viosvcmd -p vios1 -c “tpm2_getcap handles”` to enumerate active objects. By reading the persistent storage blob via `cat /proc/ppc64/pks/seed` (after escalating to root on the service processor), the seed bytes are extracted. A custom OpenCL cracker iterates over the effective keyspace (2^64) to derive the AES-GCM key, then decrypts the PKS archive using openssl enc -aes-256-gcm -d -K <derived_key> -iv <iv> -in pks.bin -out decrypted.bin. The recovered master key allows forging new TPM certificates and decrypting all guest logical partition (LPAR) LUKS headers.
Protection: from this CVE
Immediately upgrade to FW1110.21, FW1060.72, or FW950.H3 once released. As a temporary mitigation, restrict HMC and FSP access to trusted IP subnets using firewall rules (iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/8 -j ACCEPT). Disable the vTPM feature for non-essential VMs via chsyscfg -r lpar -m sys1 -i "name=vm1,virtual_tpm=0". Enable audit logging on all management sessions and rotate the PKS master secret using the `pksreset` command (if available) after each administrative event.
Impact:
Complete compromise of platform confidentiality: attackers can extract customer encryption keys, decrypt virtual disk images, and bypass secure boot attestation. Integrity of the entire trust chain (from hypervisor to guest OS) is broken, allowing undetected firmware implants. This leads to data theft in shared IaaS clouds, loss of regulatory compliance (GDPR/HIPAA), and permanent reputational damage. The persistent nature of the weak key also enables cross-tenant attacks, as one compromised LPAR’s TPM material can be used to decrypt another tenant’s protected state if the same hypervisor manages both.
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Sources:
Reported By: nvd.nist.gov
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