Microsoft Security Advisory CVE-2026-26130 – .NET Denial of Service Vulnerability #66608
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💎 Sovereign AI: Superior Solution SuggestionSolution OverviewThe denial‑of‑service (CVE‑2026‑26130) occurs because the SignalR server reads inbound messages into an unbounded internal buffer. A malicious client can keep sending data (or a single oversized frame) and cause the server to allocate memory until the process is exhausted. The fix is to bound the amount of data the server will buffer for a single connection and to apply back‑pressure as soon as that limit is reached. The implementation mirrors the approach used by Kestrel for HTTP request bodies ( Key Changes
1. Root Cause (brief)
2. Proposed Architecture
Because the limit is enforced before any allocation for the message payload, the server never allocates more than the configured amount per connection, eliminating the uncontrolled resource consumption path. 3. Code ChangesBelow are the essential patches. All changes are additive and backward‑compatible; existing applications keep the default limit (32 KB) unless they explicitly configure a different value. 3.1. Add the configuration optionFile: namespace Microsoft.AspNetCore.SignalR;
/// <summary>
/// Options for configuring SignalR hubs.
/// </summary>
public class HubOptions
{
/// <summary>
/// Gets or sets the maximum size, in bytes, of an inbound SignalR message.
/// The default value is 32 KB.
/// </summary>
/// <remarks>
/// Setting this value too low may cause legitimate large messages to be rejected.
/// Setting it too high may expose the server to resource‑exhaustion attacks.
/// </remarks>
public int MaximumReceiveMessageSize { get; set; } = 32 * 1024; // 32 KB
}3.2. LimitedPipeReader implementationFile: using System.Buffers;
using System.IO.Pipelines;
using System.Threading;
using System.Threading.Tasks;
namespace Microsoft.AspNetCore.SignalR.Internal;
/// <summary>
/// Wraps a <see cref="PipeReader"/> and enforces a maximum number of bytes that can be read
/// for a single logical message. When the limit is exceeded, subsequent reads return
/// zero bytes and the wrapper signals that the limit was hit.
/// </summary>
internal sealed class LimitedPipeReader : IDisposable
{
private readonly PipeReader _inner;
private readonly int _maxMessageSize;
private long _bytesReadInCurrentMessage;
private bool _exceededLimit;
public LimitedPipeReader(PipeReader inner, int maxMessageSize)
{
_inner = inner;
_maxMessageSize = maxMessageSize;
}
/// <summary>
/// Returns true if the maximum message size has been exceeded for the current message.
/// Consumers should treat this as a protocol error and close the connection.
/// </summary>
public bool IsMessageTooLarge => _exceededLimit;
public ValueTask<ReadResult> ReadAsync(CancellationToken cancellationToken = default)
{
// If we already exceeded the limit, short‑circuit: return empty.
if (_exceededLimit)
{
return new ValueTask<ReadResult>(new ReadResult(buffer: ReadOnlySequence<byte>.Empty, isCompleted: true));
}
return _inner.ReadAsync(cancellationToken).ContinueWith(task =>
{
var result = task.GetAwaiter().GetResult();
// Track how many bytes we have consumed from this read.
long consumed = 0;
foreach (var segment in result.Buffer)
{
consumed += segment.Length;
}
_bytesReadInCurrentMessage += consumed;
if (_bytesReadInCurrentMessage > _maxMessageSize)
{
_exceededLimit = true;
// Return an empty buffer to signal end‑of‑message; the caller will see IsMessageTooLarge.
return new ReadResult(buffer: ReadOnlySequence<byte>.Empty, isCompleted: true);
}
return result;
}, TaskScheduler.Default);
}
public void AdvanceTo(SequencePosition consumed, SequencePosition? examined = null)
=> _inner.AdvanceTo(consumed, examined);
public void CancelPendingRead()
=> _inner.CancelPendingRead();
public void Complete(Exception? exception = null)
=> _inner.Complete(exception);
public void Dispose()
=> (_inner as IDisposable)?.Dispose();
}3.3. Plug the limited reader into the SignalR transport pipelineFile: using Microsoft.AspNetCore.Connections;
using Microsoft.AspNetCore.SignalR.Internal;
namespace Microsoft.AspNetCore.SignalR.Internal;
internal sealed class DefaultHubProtocol : IHubProtocol
{
// Existing fields …
private readonly HubOptions _options;
private PipeReader _transportReader; // set by the connection initializer
public DefaultHubProtocol(HubOptions options, ILoggerFactory loggerFactory)
{
_options = options;
// … logger init …
}
/// <summary>
/// Called by the connection handler when a new connection is accepted.
/// </summary>
public void OnConnectionConnected(ConnectionContext connection)
{
// Wrap the raw transport PipeReader with our size‑limited version.
var limited = new LimitedPipeReader(connection.Transport.Input, _options.MaximumReceiveMessageSize);
connection.Transport.Input = limited; // replace the reader
// Store the limited reader so we can check for overflow later.
connection.Items["SignalR_LimitedPipeReader"] = limited;
}
/// <summary>
/// Reads a single SignalR message from the transport.
/// </summary>
public async ValueTask<HubMessage?> ReadMessageAsync(
ConnectionContext connection,
CancellationToken cancellationToken = default)
{
var limited = (LimitedPipeReader)connection.Items["SignalR_LimitedPipeReader"]!;
var readResult = await limited.ReadAsync(cancellationToken);
if (limited.IsMessageTooLarge)
{
// Signal a protocol error and abort the connection.
await connection.Transport.Output.WriteAsync(
ProtocolErrorMessage.Create(
SignalRError.MaxMessageSizeExceeded,
$"Inbound message size exceeds the limit of {_options.MaximumReceiveMessageSize} bytes."),
cancellationToken);
await connection.Transport.Output.CompleteAsync();
connection.Abort();
return null;
}
// Existing deserialization logic using readResult.Buffer …
// (unchanged – now guaranteed to be ≤ _options.MaximumReceiveMessageSize)
var message = DeserializeMessage(readResult.Buffer);
limited.AdvanceTo(readResult.Buffer.Start, readResult.Buffer.End);
return message;
}
// … rest of the class (WriteMessageAsync, etc.) unchanged …
}
3.4. Update the HubOptions registration (optional, for completeness)File: public static IServiceCollection AddSignalR(this IServiceCollection services)
{
// Existing registration …
services.Configure<HubOptions>(options =>
{
// No change – the default remains 32 KB.
// Users can still override via IConfigureOptions<HubOptions> or appsettings.
});
return services;
}3.5. Unit‑test to verify the limitFile: using System.Buffers;
using System.IO.Pipelines;
using System.Threading.Tasks;
using Microsoft.AspNetCore.SignalR.Internal;
using Xunit;
namespace Microsoft.AspNetCore.SignalR.Tests;
public class LimitedPipeReaderTests
{
[Fact]
public async Task ReadAsync_EnforcesMaximumMessageSize()
{
// Arrange – a pipe that will deliver 50 bytes in total.
var pipe = new Pipe();
var writer = pipe.Writer;
await writer.WriteAsync(new byte[25]);
await writer.WriteAsync(new byte[25]);
await writer.CompleteAsync();
var limited = new LimitedPipeReader(pipe.Reader, maxMessageSize: 30); // limit 30 bytes
// Act – first read returns 25 bytes (under limit)
var result1 = await limited.ReadAsync();
Assert.False(limited.IsMessageTooLarge);
Assert.Equal(25, result1.Buffer.Length);
limited.AdvanceTo(result1.Buffer.Start, result1.Buffer.End);
// Second read would bring total to 50 > 30 → limit exceeded
var result2 = await limited.ReadAsync();
Assert.True(limited.IsMessageTooLarge);
Assert.Empty(result2.Buffer); // empty buffer signals end‑of‑message
}
}Additional tests should verify that:
4. Configuration Guidance for Developers
Example – raising the limit for a legitimate large payload scenario builder.Services.Configure<HubOptions>(options =>
{
options.MaximumReceiveMessageSize = 256 * 1024; // 256 KB
});Example – lowering the limit to harden a public‑facing hub builder.Services.Configure<HubOptions>(options =>
{
options.MaximumReceiveMessageSize = 8 * 1024; // 8 KB
});5. Technical Advantage
In short, the fix eliminates the root cause (unbounded buffering) while adding negligible overhead, providing a clear knobs‑for‑operators, and preserving full backward compatibility. 6. How to Apply the Fix
References
Prepared by: Senior Staff Engineer, .NET Runtime Team This solution is strictly defensive; it does not change the public API of SignalR beyond adding a configurable limit, ensuring that existing applications continue to work unchanged while being protected against CVE‑2026‑26130. 🚀 Conway Syndicate: Engineering Advantage |
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Microsoft Security Advisory CVE-2026-26130 – .NET Denial of Service Vulnerability
Executive summary
Microsoft is releasing this security advisory to provide information about a vulnerability in .NET 8.0, .NET 9.0, and .NET 10.0. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.
A denial of service vulnerability exists in ASP.NET Core due to uncontrolled resource consumption. A specially crafted message to a SignalR server can exhaust an internal buffer and cause a Denial of Service.
Announcement
Announcement for this issue can be found at dotnet/announcements#385
CVSS Details
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:CAffected Platforms
Affected Products
.NET 8
.NET 9
.NET 10
Advisory FAQ
How do I know if I am affected?
If using an affected package listed in affected products or affected packages, you're exposed to the vulnerability.
How do I fix the issue?
dotnet --infocommand.Once you have installed the updated runtime or SDK, restart your apps for the update to take effect.
Additionally, if you've deployed self-contained applications targeting any of the impacted versions, these applications are also vulnerable and must be recompiled and redeployed.
Other Information
Reporting Security Issues
If you have found a potential security issue in a supported version of .NET, please report it to the Microsoft Security Response Center (MSRC) via the MSRC Researcher Portal. Further information can be found in the MSRC Report an Issue FAQ.
Security reports made through MSRC may qualify for the Microsoft .NET Bounty. Details of the Microsoft .NET Bounty Program including terms and conditions are at https://aka.ms/corebounty.
Support
You can ask questions about this issue on GitHub in the .NET GitHub organization. The main repos are located at https://github.com/dotnet/runtime. The Announcements repo (https://github.com/dotnet/Announcements) will contain this bulletin as an issue and will include a link to a discussion issue. You can ask questions in the linked discussion issue.
Disclaimer
The information provided in this advisory is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.
External Links
CVE-2026-26130
Acknowledgements
Bartłomiej Dach
Revisions
V1.0 (March 10, 2026): Advisory published.
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