CVE-2023-25000 - HashiCorp Vault’s Shamir Secret Sharing Cache-Timing Vulnerability Explained

---

Introduction

In early 2023, a critical vulnerability was discovered in HashiCorp Vault, affecting its use of Shamir's Secret Sharing. Tracked as CVE-2023-25000, this flaw exposed the vault's “unseal” cryptographic process to potential cache-timing side channel attacks. This post explains the vulnerability using simple language, shows proof-of-concept code, and provides clear references for further reading.

Vault can use a scheme called “Shamir’s Secret Sharing” to split the master key into pieces ("shares") that only reconstruct the key if enough people combine their shares. This process is called “unseal” — Vault won’t unlock unless enough trusted people provide their portions.

Where Was the Problem?

To do its cryptographic math, Vault used a library method that relied on precomputed lookup tables for its calculations. This can be fast—but it’s not always safe! If table lookups differ according to secret data, attackers can sometimes detect patterns in the hardware's memory access *timing*.

Imagine you see how long it takes to open a combination lock. If some numbers are slower, an attacker might use timing to guess the right numbers. Here, that’s what a clever attacker can do with Vault’s cryptographic table lookups—if they can watch the underlying hardware.

Run Vault on shared or multi-tenant systems (like a cloud VM or server with other users),

- Have attackers able to watch very closely (side channel access to the host’s hardware, such as cache-timing),

Support frequent Vault unseal operations (to collect enough data).

If attackers can do all these, they might discover some of the Shamir shares’ bits, making it much easier to brute force the rest and eventually open the vault.

Vulnerable Code Pattern: Table-Based Field Arithmetic

In Vault’s underlying cryptographic code, the computations for Shamir's secret sharing can look something like:

// Example: field multiplication using a lookup table
// (simplified for illustration)
func mul(a, b byte) byte {
    return mulTable[a][b]  // Table-based: a and b select memory locations
}

Each time a different a or b is used, a different memory cell is hit. Modern CPUs can leak which were accessed if someone is spying on timing via the cache. More info in Google's Project Wycheproof guide.

Fix: Use arithmetic that doesn’t have data-dependent timing, like this (secure but slower)

// Field arithmetic without lookup tables (constant-time)
func mul(a, b byte) byte {
    // perform multiplication using only arithmetic, not table lookups
    var product byte = 
    for i := ; i < 8; i++ {
        if (b & (1 << i)) !=  {
            product ^= a << i
        }
    }
    return product
}

This avoids the problem because it always does the same memory operations, regardless of input.

Proof-of-Concept: How Would an Attacker Exploit This?

Suppose an attacker can execute code on the same machine. Each time someone unseals Vault, the attacker observes CPU cache timing. By analyzing the cache delays, the attacker infers which lookup table rows Vault’s code used—connected to the values of the Shamir shares.

Narrow Down possible values for the Vault secret shares.

2. Brute Force through a smaller set, instead of the full randomness the shares were supposed to have.

Even though real-world attacks require a lot of work and opportunity, this shrinks the security gap.

Example attack toolkits

- Cachetiming attack example by OpenSSL
- Research paper on constant-time cryptography

1.11.9

The main fix was to replace table-based field arithmetic with constant-time operations—computations whose memory patterns don’t depend on secret data.

Their fix made it much harder for attackers to gain any benefit by watching memory timings.

Official security advisory

- HashiCorp Security Bulletin (CVE-2023-25000)

Avoid running Vault alongside untrusted code on the same physical hardware.

- Limit access: Only trusted admins should be able to start or restart Vault or perform unseal operations.

References

- HashiCorp Vault CVE-2023-25000 Security Bulletin
- CVE Details Page
- Shamir’s Secret Sharing explained (Wikipedia)
- Cache Timing Attacks explained (Project Wycheproof)
- Constant-time cryptography guide

Conclusion

CVE-2023-25000 is a real-world reminder that cryptography isn’t just math—it’s also how that math is implemented. Old-school lookup tables can leak secrets, especially in multi-tenant environments. Always prefer constant-time implementations and keep your infrastructure up-to-date.

Timeline

Published on: 03/30/2023 01:15:00 UTC
Last modified on: 04/06/2023 14:45:00 UTC