Add explicit algorithm allowlist to JWT validation

[michaellwest]

M4: No Explicit Algorithm Allowlist in JWT Validation

Severity: MEDIUM
Category: Cryptographic Weakness / Algorithm Confusion
File: src/Spe/Core/Settings/Authorization/SharedSecretAuthenticationProvider.cs
Lines: 235–249 (ComputeHash method)

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Risk Explanation

The ComputeHash method selects the HMAC algorithm based on the alg claim from the untrusted JWT header:

switch (algorithm)
{
    case "HS256": using (var hmac = new HMACSHA256(key)) { return hmac.ComputeHash(data); }
    case "HS384": using (var hmac = new HMACSHA384(key)) { return hmac.ComputeHash(data); }
    case "HS512": using (var hmac = new HMACSHA512(key)) { return hmac.ComputeHash(data); }
    default: return null;
}

The alg: "none" attack is currently blocked by accident — ComputeHash returns null, and the subsequent Convert.ToBase64String(hash) throws a NullReferenceException, caught by the generic catch (Exception) block.

Why this is fragile:

• If anyone refactors to handle null gracefully, the alg: "none" bypass becomes exploitable.
• If new algorithms are added (e.g., RS256), an attacker could force algorithm confusion (classic CVE-2015-9235).
• The defense is not visible or documented — a future developer will not know the NullReferenceException is load-bearing.

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Implementation Plan

Add an explicit algorithm allowlist check before the switch statement, and a fail-closed null check after:

private static readonly HashSet<string> AllowedAlgorithms = new HashSet<string>(StringComparer.Ordinal)
{
    "HS256", "HS384", "HS512"
};

private byte[] ComputeHash(string algorithm, byte[] key, byte[] data)
{
    if (!AllowedAlgorithms.Contains(algorithm))
    {
        PowerShellLog.Warn($"[Auth] action=algorithmRejected algorithm={algorithm}");
        return null;
    }
    // ... existing cases ...
}

And after ComputeHash returns:

var hash = ComputeHash(tokenHeader.Alg, keyBytes, inputBytes);
if (hash == null)
{
    PowerShellLog.Warn("[Auth] action=authFailed reason=unsupportedAlgorithm");
    return false;
}
var computedSignature = Convert.ToBase64String(hash);

Files to modify

File	Change
src/Spe/Core/Settings/Authorization/SharedSecretAuthenticationProvider.cs	Add algorithm allowlist, null check after ComputeHash

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Test Plan

1. Unit test — alg: "none" token: Rejected.
2. Unit test — unknown algorithms: RS256, ES256, PS256, empty, NONE, hs256 (case-sensitive) — all rejected.
3. Unit test — allowed algorithms: HS256, HS384, HS512 — all accepted.
4. Warning is logged when unsupported algorithm is attempted.
5. Existing JWT authentication tests pass.