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CVE-2026-56170

CVE-2026-56170: ASP.NET Core Denial of Service Vulnerability

Allocation of resources without limits or throttling in ASP.NET Core allows an unauthorized attacker to deny service over a network.

(GitHub Advisory)

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CVE-2026-56170

CVE-2026-56170:

7.5

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3.1

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation NoneAutomatable YesTech Impact Partial

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
Microsoft.AspNetCore.App.Runtime.linux-arm	nuget	>= 10.0.0, <= 10.0.5	10.0.6
Microsoft.AspNetCore.App.Runtime.linux-arm64	nuget	>= 10.0.0, <= 10.0.5	10.0.6

Technical Details

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Root Cause Analysis

The vulnerability is a denial-of-service in ASP.NET Core SignalR's stateful reconnect feature. The root cause is in the MessageBuffer.WriteAsyncCore method, which could block indefinitely when backpressure was applied by a client, leading to resource exhaustion on the server. The analysis of the commits between the vulnerable and patched versions revealed the exact code changes that introduced a timeout to mitigate this issue. The primary vulnerable function is Microsoft.AspNetCore.SignalR.Shared.MessageBuffer.WriteAsyncCore, where the unbounded wait occurred. The function Microsoft.AspNetCore.SignalR.HubConnectionContext.WriteCore is also identified as a key part of the vulnerable code path, as it's the server-side function that invokes the vulnerable message buffering logic when stateful reconnect is in use.

Vulnerable functions

Microsoft.AspNetCore.SignalR.Shared.MessageBuffer.WriteAsyncCore

src/SignalR/common/Shared/MessageBuffer.cs

The vulnerability lies in the `WriteAsyncCore` method within the `MessageBuffer` class. When the message buffer is full, the method enters a loop, waiting for an acknowledgment (_waitForAck) from the client before proceeding. Prior to the patch, this wait was not time-bound. A malicious client could intentionally withhold the acknowledgment, causing the server to wait indefinitely. This would tie up server resources, leading to a denial-of-service condition as other clients would be blocked from sending messages.

Microsoft.AspNetCore.SignalR.HubConnectionContext.WriteCore

src/SignalR/server/Core/src/HubConnectionContext.cs

This function is the entry point on the server for writing messages to a client connection. When stateful reconnect is enabled, it calls the vulnerable `MessageBuffer.WriteAsync` method. The patch wraps this call in a helper function that provides a timeout, especially for `CloseMessage`, to prevent the connection from being blocked indefinitely by a malicious client.

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7.5

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CVE-2026-56170: ASP.NET Core Denial of Service Vulnerability

CVE-2026-56170:

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7.5

7.5

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Vulnerable functions

Technical Details

CVE-2026-56170: ASP.NET Core Denial of Service Vulnerability

7.5

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Basic Information

Basic Information

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CVE-2026-56170: ASP.NET Core Denial of Service Vulnerability

CVE-2026-56170:

7.5

-

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Technical Details

Technical Details

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Vulnerable functions

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7.5

7.5

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Vulnerable functions

Technical Details

CVE-2026-56170: ASP.NET Core Denial of Service Vulnerability

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