Fix HTML injection in code block titles

[JamesNK]

Description

Fenced code block language metadata was written directly into the code block title while the rendered Markdown was later trusted as HTML. A crafted language value could therefore escape the title element and inject arbitrary HTML even though raw HTML is disabled in the Markdown pipeline.

This change HTML-encodes the visible code block title at the renderer boundary. The existing attribute and code body rendering behavior is unchanged.

Security considerations

This closes an HTML injection path for untrusted Markdown by treating fenced code block language metadata as untrusted input and encoding it at the HTML sink. A regression test covers a compact SVG event-handler payload. A formal security review has not yet been completed.

Validation:

MarkdownProcessorTests: 26 passed, 0 failed

Fixes # (issue)

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 18866

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 18866"

[Copilot]

Pull request overview

Fixes HTML injection through fenced code-block language metadata by escaping the displayed title at the rendering boundary.

Changes:

• HTML-encodes code-block titles before rendering.
• Adds regression coverage for an SVG event-handler payload.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Aspire.Dashboard/Model/Markdown/HighlightedCodeBlockRenderer.cs	Escapes untrusted title text.
tests/Aspire.Dashboard.Tests/Markdown/MarkdownProcessorTests.cs	Verifies malicious language metadata is encoded.

[github-actions]

Tests selector (audit mode)

The full test matrix and all jobs still run in audit mode. The tests and jobs below are what selective CI would run under enforcement.

7 / 100 test projects · 4 jobs, from 2 changed files.

Selected test projects (7 / 100)

Aspire.Cli.EndToEnd.Tests, Aspire.Dashboard.Components.Tests, Aspire.Dashboard.Tests, Aspire.Hosting.DotnetTool.Tests, Aspire.Hosting.RemoteHost.Tests, Aspire.Managed.Tests, Aspire.Playground.Tests

Selected jobs (4)

cli-starter, deployment-e2e, extension-e2e, polyglot

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How these were chosen — grouped by what changed

🔧 src/Aspire.Dashboard/Model/Markdown/HighlightedCodeBlockRenderer.cs (changed source)
→ 5 via the project graph: Aspire.Dashboard.Components.Tests, Aspire.Hosting.DotnetTool.Tests (2 hops), Aspire.Hosting.RemoteHost.Tests (2 hops), Aspire.Managed.Tests (2 hops), Aspire.Playground.Tests (2 hops)

📦 affected project Aspire.Managed
→ 1 test: Aspire.Cli.EndToEnd.Tests

🧪 tests/Aspire.Dashboard.Tests/Markdown/MarkdownProcessorTests.cs (changed test)
→ 1 directly: Aspire.Dashboard.Tests

Job reasons

Job	Triggered by
cli-starter	• affected project Aspire.Managed • selected test Aspire.Cli.EndToEnd.Tests
deployment-e2e	affected project Aspire.Managed
extension-e2e	• src/Aspire.Dashboard/Model/Markdown/HighlightedCodeBlockRenderer.cs • affected project Aspire.Dashboard
polyglot	affected project Aspire.Managed

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Selection computed for commit 10d0a49.