2021-02-12 Update

This issue has been fixed with the .NET Framework February Security and Quality Rollup:

ASP.NET

Addresses an issue where after installing the update released on October 20th, some ASP.Net applications fail during precompilation – likely with a message that contains the words “Error ASPCONFIG.”

2021-01-21 Update

The KB article has been updated with known issues where this bug has been addressed.
There is a workaround available, you can verify it by looking at the workaround branch:

Workaround
Customers who observe new unexpected failures or functional issues can implement an application setting by adding (or merging) the following code to the application configuration file. Setting either “true” or “false” will avoid the issue. However, we recommend that you set this value to “true” for sites that do not rely on cookieless features.
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <appSettings>
        <add key="aspnet:DisableAppPathModifier" value="true" />
    </appSettings>
</configuration>

Reproduce issue introduced with KB4578969

This is a short minimum reproduceable example for an issue introduced with the latest .NET Framework Update KB4578969.

Description of the issue

When publishing an ASP.NET site with pre-compile enabled and KB4578969 applied, there is an error when publishing the site.
This only happens, if there is a timeout attribute with a non-standard value. (e.g. a string placeholder)

Introduced parsing error

error ASPCONFIG: The value of the property 'timeout' cannot be parsed. The error is: Input string was not in a correct format.

AspNetPreCompile:
  C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\aspnet_compiler.exe -v / -p \AspNetWebConfigTransformIssueRepro\WebConfigTransformRepro\obj\Release\AspnetCompileMerge\Source -u -c \AspNetWebConfigTransformIssueRepro\WebConfigTransformRepro\obj\Release\AspnetCompileMerge\TempBuildDir
\AspNetWebConfigTransformIssueRepro\WebConfigTransformRepro\obj\Release\AspnetCompileMerge\Source\web.config(9): error ASPCONFIG: The value of the property 'timeout' cannot be parsed. The err
or is: Input string was not in a correct format. [\AspNetWebConfigTransformIssueRepro\WebConfigTransformRepro\WebConfigTransformRepro.csproj]
Done Building Project "\AspNetWebConfigTransformIssueRepro\WebConfigTransformRepro\WebConfigTransformRepro.csproj" (default targets) -- FAILED.

Done Building Project "\AspNetWebConfigTransformIssueRepro\WebConfigTransformRepro.sln" (default targets) -- FAILED.


Build FAILED.

Repro instructions

Prerequisites

Visual Studio 2019 with the ASP.NET and web development workload

Steps to reproduce it with this repository

Clone this repository
Execute the powershell script build.ps1

Steps to reproduce it yourself

Create a new ASP.NET Framework 4.8 project
Add one of the following web.config configuration elements:
configuration/system.web/sessionState with a timeout attribute
configuration/system.web/authentication/forms with a timeout attribute
Change the value of timeout to any other value than integer, e.g. {TIMEOUT}
Add a publish profile for file system publish with pre-compiling enabled
Try to publish the site

Note
This reproducable example uses a web.config transform instead to show the probably more suited use case for this issue. (Publish with placeholders and replace them in the deployment or update process)

kapsiR / AspNetWebConfigTransformIssueRepro