Memory Leak in Aspnet with Hmac Signatures

A memory leak in an ASP.NET application that uses HMAC signatures typically arises when signature computation or caching retains objects longer than necessary. HMAC operations in .NET involve cryptographic primitives such as HMACSHA256 or HMACSHA512. If instances of these classes or related buffers are stored in long-lived caches, static fields, or singleton services without cleanup, unmanaged resources and large internal buffers may not be released promptly, leading to a gradual increase in memory usage.

In ASP.NET, common patterns that can combine poorly with HMAC usage include per-request caching of HMAC instances, storing computed signatures in static dictionaries keyed by request-specific data, or failing to dispose of streams that feed into signature algorithms. For example, if a developer caches a HMACSHA256 keyed by user or session and never evicts entries, the associated buffers and internal state persist for the lifetime of the app domain, growing with each distinct key. This is especially risky in high-throughput APIs where many unique keys can accumulate quickly.

Another scenario involves large payloads or file uploads processed for signing. Streaming the entire request body into memory to compute an HMAC, and then retaining buffers or objects tied to that stream, can keep significant memory referenced. If the application also uses dependency injection with a singleton HMAC provider that holds onto buffers or transformation contexts, the leak compounds because the same instance is reused across requests without releasing internal state.

Unlike crashes, memory leaks in this combination degrade performance over time: increased GC pressure, higher CPU for collection, and eventual out-of-memory conditions under sustained load. Detection requires observing memory trends across requests and correlating spikes with signature activity, rather than immediate failures. Because HMAC usage is often invisible in high-level monitoring, leaks can remain unnoticed until service stability is impacted.

To mitigate memory leaks when using HMAC signatures in ASP.NET, focus on limiting object lifetime, avoiding static retention, and ensuring proper disposal. Use short-lived instances of HMAC algorithms within the scope of each operation, and avoid caching them unless absolutely necessary with strict eviction policies.

Example 1: Stateless per-request HMAC computation with disposal

using System.Security.Cryptography;
using System.Text;

public static string ComputeHmac(string message, string base64Key)
{
    byte[] key = Convert.FromBase64String(base64Key);
    // Using using ensures the HMAC instance and its internal buffers are released.
    using var hmac = new HMACSHA256(key);
    byte[] hash = hmac.ComputeHash(Encoding.UTF8.GetBytes(message));
    return Convert.ToBase64String(hash);
}

Example 2: Cached signatures with size-bounded eviction (MemoryCache)

using System.Runtime.Caching;

public class SignatureCache
{
    private readonly ObjectCache _cache = MemoryCache.Default;
    private readonly int _maxItems = 1000;
    private int _currentCount;

    public string GetOrAdd(string key, Func compute, DateTimeOffset absoluteExpiration)
    {
        if (_cache.Get(key) is string cached)
            return cached;

        var computed = compute();
        var policy = new CacheItemPolicy { AbsoluteExpiration = absoluteExpiration };
        // Optional: react to cache pressure to avoid unbounded growth.
        _cache.Set(key, computed, policy);
        return computed;
    }

    // Call this periodically or via a background signal to trim.
    public void TrimIfNeeded()
    {
        if (_currentCount >= _maxItems)
        {
            // Simplified: in production use cache enumerations and evict oldest.
            // This is illustrative to show bounded retention.
        }
    }
}

Example 3: Avoid singleton HMAC holders; prefer transient or scoped services

// In Startup.cs or Program.cs
// Do NOT register HMAC instances as singleton if they hold buffers.
builder.Services.AddScoped(); // or transient, depending on usage pattern

// HmacProvider encapsulates key management without caching computed signatures.
public class HmacProvider
{
    private readonly byte[] _key;
    public HmacProvider(IConfiguration config)
    {
        _key = Convert.FromBase64String(config["Hmac:Key"]);
    }

    public byte[] Sign(byte[] data)
    {
        // Create and dispose within the method scope.
        using var hmac = new HMACSHA256(_key);
        return hmac.ComputeHash(data);
    }
}

Additional practices

• Dispose of HMAC instances and any streams used for input immediately after use.
• Avoid storing computed signatures in long-lived static structures; if caching is required, use bounded caches with eviction and size limits.
• Profile memory under load to confirm that signature-related allocations are collected and not promoted to higher generations unnecessarily.