EVE OS TPM: SHA1 PCR Sealing Exposes Vault Key
CVE-2023-43635 Published on September 20, 2023
Vault Key Sealed With SHA1 PCRs
Vault Key Sealed With SHA1 PCRs
The measured boot solution implemented in EVE OS leans on a PCR locking mechanism.
Different parts of the system update different PCR values in the TPM, resulting in a unique
value for each PCR entry.
These PCRs are then used in order to seal/unseal a key from the TPM which is used to
encrypt/decrypt the vault directory.
This vault directory is the most sensitive point in the system and as such, its content should
be protected.
This mechanism is noted in Zededas documentation as the measured boot mechanism,
designed to protect said vault.
The code thats responsible for generating and fetching the key from the TPM assumes that
SHA256 PCRs are used in order to seal/unseal the key, and as such their presence is being
checked.
The issue here is that the key is not sealed using SHA256 PCRs, but using SHA1 PCRs.
This leads to several issues:
Machines that have their SHA256 PCRs enabled but SHA1 PCRs disabled, as well
as not sealing their keys at all, meaning the vault is not protected from an attacker.
SHA1 is considered insecure and reduces the complexity level required to unseal the
key in machines which have their SHA1 PCRs enabled.
An attacker can very easily retrieve the contents of the vault, which will effectively render
the measured boot mechanism meaningless.
Vulnerability Analysis
CVE-2023-43635 can be exploited with local system access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to be very high.
Weakness Types
Insufficiently Protected Credentials
The product transmits or stores authentication credentials, but it uses an insecure method that is susceptible to unauthorized interception and/or retrieval.
Reversible One-Way Hash
The product uses a hashing algorithm that produces a hash value that can be used to determine the original input, or to find an input that can produce the same hash, more efficiently than brute force techniques. This weakness is especially dangerous when the hash is used in security algorithms that require the one-way property to hold. For example, if an authentication system takes an incoming password and generates a hash, then compares the hash to another hash that it has stored in its authentication database, then the ability to create a collision could allow an attacker to provide an alternate password that produces the same target hash, bypassing authentication.
Products Associated with CVE-2023-43635
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Affected Versions
LF-Edge, Zededa EVE OS:- Before 9.5.0 is affected.
- Before 9.5.0 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.