diff --git a/docs/core/compatibility/11.md b/docs/core/compatibility/11.md
index 7d83696490c96..554f327f2b3d5 100644
--- a/docs/core/compatibility/11.md
+++ b/docs/core/compatibility/11.md
@@ -39,6 +39,7 @@ See [Breaking changes in ASP.NET Core 11](/aspnet/core/breaking-changes/11/overv
 
 | Title                                                          | Type of change    |
 |----------------------------------------------------------------|-------------------|
+| [API obsoletions](cryptography/11/obsolete-apis.md)            | Source incompatible |
 | [DSA removed from macOS](cryptography/11/dsa-removed-macos.md) | Behavioral change |
 
 <!--
diff --git a/docs/core/compatibility/core-libraries/11/obsolete-apis.md b/docs/core/compatibility/core-libraries/11/obsolete-apis.md
index 7e07c35c74f30..1b4fac7881b49 100644
--- a/docs/core/compatibility/core-libraries/11/obsolete-apis.md
+++ b/docs/core/compatibility/core-libraries/11/obsolete-apis.md
@@ -2,11 +2,12 @@
 title: "Breaking change: .NET 11 obsoletions with custom IDs"
 titleSuffix: ""
 description: Learn about the APIs that have been marked as obsolete in .NET 11 with a custom diagnostic ID.
-ms.date: 04/03/2026
+ms.date: 07/06/2026
+ai-usage: ai-assisted
 ---
 # API obsoletions with non-default diagnostic IDs (.NET 11)
 
-Some APIs have been marked as obsolete, starting in .NET 11.
+Some APIs are marked as obsolete, starting in .NET 11.
 
 This breaking change is specific to APIs that have been marked as obsolete *with a custom diagnostic ID*. Suppressing the default obsoletion diagnostic ID, which is [CS0618](../../../../csharp/language-reference/compiler-messages/cs0618.md) for the C# compiler, does not suppress the warnings that the compiler generates when these APIs are used.
 
@@ -43,6 +44,7 @@ These obsoletions can affect [source compatibility](../../categories.md#source-c
 
 ## See also
 
+- [Cryptography obsoletions (.NET 11)](../../cryptography/11/obsolete-apis.md)
 - [API obsoletions with non-default diagnostic IDs (.NET 10)](../10.0/obsolete-apis.md)
 - [API obsoletions with non-default diagnostic IDs (.NET 9)](../9.0/obsolete-apis-with-custom-diagnostics.md)
 - [API obsoletions with non-default diagnostic IDs (.NET 8)](../8.0/obsolete-apis-with-custom-diagnostics.md)
diff --git a/docs/core/compatibility/cryptography/11/obsolete-apis.md b/docs/core/compatibility/cryptography/11/obsolete-apis.md
new file mode 100644
index 0000000000000..9fa8c69a5cbcc
--- /dev/null
+++ b/docs/core/compatibility/cryptography/11/obsolete-apis.md
@@ -0,0 +1,49 @@
+---
+title: "Breaking change: .NET 11 obsoletions in cryptography"
+titleSuffix: ""
+description: Learn about the cryptography APIs that have been marked as obsolete in .NET 11.
+ms.date: 07/06/2026
+ai-usage: ai-assisted
+---
+# Cryptography obsoletions (.NET 11)
+
+Some cryptography APIs are marked as obsolete, starting in .NET 11.
+
+For obsoletions in other core .NET library areas, see [API obsoletions with non-default diagnostic IDs (.NET 11)](../../core-libraries/11/obsolete-apis.md).
+
+## Previous behavior
+
+Previously, you could use these APIs without any compile-time warnings about obsoletion.
+
+## New behavior
+
+In .NET 11 and later versions, using these APIs produces a compile-time diagnostic.
+
+The following table lists the custom diagnostic IDs and their corresponding warning messages.
+
+| Diagnostic ID | Description | Severity |
+|---------------|-------------|----------|
+| [SYSLIB0065](../../../../fundamentals/syslib-diagnostics/syslib0065.md) | The `set` accessor of <xref:System.Security.Cryptography.AsnEncodedData.RawData?displayProperty=nameWithType> is obsolete. Use the constructor of the appropriate type to decode data, or use <xref:System.Security.Cryptography.AsnEncodedData.CopyFrom(System.Security.Cryptography.AsnEncodedData)?displayProperty=nameWithType> for mutable scenarios. | Warning |
+
+## Version introduced
+
+.NET 11 Preview 6
+
+## Type of breaking change
+
+These obsoletions can affect [source compatibility](../../categories.md#source-compatibility).
+
+## Recommended action
+
+If you must keep use the obsolete APIs, suppress the warning by using the custom diagnostic ID.
+
+## Affected APIs
+
+### SYSLIB0065
+
+- `set` accessor of <xref:System.Security.Cryptography.AsnEncodedData.RawData?displayProperty=fullName>
+
+## See also
+
+- [SYSLIB0065: AsnEncodedData.RawData setter is obsolete](../../../../fundamentals/syslib-diagnostics/syslib0065.md)
+- [API obsoletions with non-default diagnostic IDs (.NET 11)](../../core-libraries/11/obsolete-apis.md)
diff --git a/docs/core/compatibility/toc.yml b/docs/core/compatibility/toc.yml
index 1395fa691738d..88869b0c91be1 100644
--- a/docs/core/compatibility/toc.yml
+++ b/docs/core/compatibility/toc.yml
@@ -36,6 +36,8 @@ items:
                 href: core-libraries/11/ziparchive-createasync-eager-load.md
           - name: Cryptography
             items:
+              - name: API obsoletions
+                href: cryptography/11/obsolete-apis.md
               - name: DSA removed from macOS
                 href: cryptography/11/dsa-removed-macos.md
           - name: Extensions
diff --git a/docs/fundamentals/syslib-diagnostics/obsoletions-overview.md b/docs/fundamentals/syslib-diagnostics/obsoletions-overview.md
index b759faaeb706f..7497bacaee2e6 100644
--- a/docs/fundamentals/syslib-diagnostics/obsoletions-overview.md
+++ b/docs/fundamentals/syslib-diagnostics/obsoletions-overview.md
@@ -88,6 +88,7 @@ The following table lists the `SYSLIB0XXX` obsoletions in .NET 5+.
 | [SYSLIB0061](syslib0061.md) | Warning | The `Queryable` <xref:System.Linq.Queryable.MaxBy``2(System.Linq.IQueryable{``0},System.Linq.Expressions.Expression{System.Func{``0,``1}},System.Collections.Generic.IComparer{``0})?displayProperty=nameWithType> and <xref:System.Linq.Queryable.MinBy``2(System.Linq.IQueryable{``0},System.Linq.Expressions.Expression{System.Func{``0,``1}},System.Collections.Generic.IComparer{``0})?displayProperty=nameWithType> taking an `IComparer<TSource>` are obsolete. Use the new ones that take an `IComparer<TKey>`. |
 | [SYSLIB0062](syslib0062.md) | Warning | <xref:System.Xml.Xsl.XsltSettings.EnableScript?displayProperty=nameWithType> is obsolete. |
 | [SYSLIB0064](syslib0064.md) | Warning | <xref:System.Security.Cryptography.RSACryptoServiceProvider.Encrypt(System.Byte[],System.Boolean)?displayProperty=nameWithType> and <xref:System.Security.Cryptography.RSACryptoServiceProvider.Decrypt(System.Byte[],System.Boolean)?displayProperty=nameWithType> are obsolete. Use the overloads that accept an <xref:System.Security.Cryptography.RSAEncryptionPadding> instead. |
+| [SYSLIB0065](syslib0065.md) | Warning | The `set` accessor of <xref:System.Security.Cryptography.AsnEncodedData.RawData?displayProperty=nameWithType> is obsolete. Use the constructor of the appropriate type to decode data, or use <xref:System.Security.Cryptography.AsnEncodedData.CopyFrom(System.Security.Cryptography.AsnEncodedData)?displayProperty=nameWithType> for mutable scenarios. |
 
 ## EXTOBS obsoletions
 
diff --git a/docs/fundamentals/syslib-diagnostics/syslib0065.md b/docs/fundamentals/syslib-diagnostics/syslib0065.md
new file mode 100644
index 0000000000000..45c35bc0aa787
--- /dev/null
+++ b/docs/fundamentals/syslib-diagnostics/syslib0065.md
@@ -0,0 +1,73 @@
+---
+title: SYSLIB0065 warning - AsnEncodedData.RawData setter is obsolete
+description: Learn about the obsoletion of the AsnEncodedData.RawData property setter. Use of this setter generates compile-time warning SYSLIB0065.
+ms.date: 07/05/2026
+ai-usage: ai-assisted
+f1_keywords:
+  - SYSLIB0065
+---
+
+# SYSLIB0065: AsnEncodedData.RawData setter is obsolete
+
+Starting in .NET 11, the `set` accessor of <xref:System.Security.Cryptography.AsnEncodedData.RawData?displayProperty=nameWithType> is obsolete. Setting this property in code generates warning `SYSLIB0065` at compile time.
+
+## Reason for obsoletion
+
+<xref:System.Security.Cryptography.AsnEncodedData> represents an ASN.1-encoded object. Many types derive from it, such as <xref:System.Security.Cryptography.X509Certificates.X509BasicConstraintsExtension>, and cache their decoded representation so that property access doesn't repeatedly decode the ASN.1.
+
+Because `RawData` is not `virtual`, setting it on a derived instance causes a discrepancy between the cached decoded state and the new raw data. The derived type can't detect that the raw data changed, so it continues to return stale decoded values. For example:
+
+```csharp
+X509BasicConstraintsExtension extension = new(
+    certificateAuthority: true,
+    hasPathLengthConstraint: true,
+    pathLengthConstraint: 3,
+    critical: true);
+
+X509BasicConstraintsExtension decoded = new();
+decoded.RawData = extension.RawData;
+Console.WriteLine(decoded.CertificateAuthority);      // Unexpectedly prints False
+Console.WriteLine(decoded.HasPathLengthConstraint);   // Unexpectedly prints False
+Console.WriteLine(decoded.PathLengthConstraint);      // Unexpectedly prints 0
+```
+
+## Workaround
+
+To maintain coherency between the decoded state and the underlying data, use the constructor of the appropriate type to decode data. Treat instances as read-only—construct a new instance instead of reusing an existing one.
+
+```csharp
+// Instead of setting RawData, use the constructor.
+X509BasicConstraintsExtension decoded = new(extension, extension.Critical);
+```
+
+If you need mutable behavior, use <xref:System.Security.Cryptography.AsnEncodedData.CopyFrom(System.Security.Cryptography.AsnEncodedData)?displayProperty=nameWithType>. The `CopyFrom` method is `virtual`, so derived types can invalidate their decoded state when the raw data changes.
+
+## Suppress a warning
+
+If you must use the obsolete setter, you can suppress the warning in code or in your project file.
+
+To suppress only a single violation, add preprocessor directives to your source file to disable and then re-enable the warning.
+
+```csharp
+// Disable the warning.
+#pragma warning disable SYSLIB0065
+
+// Code that uses obsolete API.
+// ...
+
+// Re-enable the warning.
+#pragma warning restore SYSLIB0065
+```
+
+To suppress all `SYSLIB0065` warnings in your project, add a `<NoWarn>` property to your project file.
+
+```xml
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+   ...
+   <NoWarn>$(NoWarn);SYSLIB0065</NoWarn>
+  </PropertyGroup>
+</Project>
+```
+
+For more information, see [Suppress warnings](obsoletions-overview.md#suppress-warnings).
diff --git a/docs/navigate/tools-diagnostics/toc.yml b/docs/navigate/tools-diagnostics/toc.yml
index 06f6442817523..d5ceb222cd3a3 100644
--- a/docs/navigate/tools-diagnostics/toc.yml
+++ b/docs/navigate/tools-diagnostics/toc.yml
@@ -4093,6 +4093,8 @@ items:
             href: ../../fundamentals/syslib-diagnostics/syslib0062.md
           - name: SYSLIB0064
             href: ../../fundamentals/syslib-diagnostics/syslib0064.md
+          - name: SYSLIB0065
+            href: ../../fundamentals/syslib-diagnostics/syslib0065.md
       - name: Experimental features
         items:
           - name: Overview