Anonymous GitHub Vulnerable to Cross-Site Scripting via Malicious GitHub Repository Content

### Summary

Anonymous GitHub fetches repository content (e.g., markdown files) from GitHub's API and renders it without sanitization. On the client side, markdown is parsed with `marked` (with `sanitize: false`) and injected into the DOM via `$sce.trustAsHtml()` + `ng-bind-html`, bypassing AngularJS's built-in XSS protection. An attacker can craft a malicious GitHub repository whose README executes arbitrary JavaScript in the Anonymous GitHub origin.

### Details

#### README fetched from GitHub API

The server fetches the README via GitHub's REST API and stores the raw markdown in MongoDB:

```typescript
// https://github.com/tdurieux/anonymous_github/blob/b2d77faa6c6f35ad9ae6ed46e3a3fa4681ac84c2/src/core/source/GitHubRepository.ts#L162-L174
const ghRes = await oct.repos.getReadme({
owner: this.owner,
repo: this.repo,
ref: selected?.commit,
});
const readme = Buffer.from(
ghRes.data.content,
ghRes.data.encoding as BufferEncoding
).toString("utf-8");
selected.readme = readme;
await model.save();
```

It is then served to the client with no sanitization:

```typescript
// https://github.com/tdurieux/anonymous_github/blob/b2d77faa6c6f35ad9ae6ed46e3a3fa4681ac84c2/src/server/routes/repository-private.ts#L254-L260
return res.send(
await repo.readme({
accessToken: token,
force: req.query.force == "1",
branch: req.query.branch as string,
})
);
```

#### Client-side rendering via `$sce.trustAsHtml()` + `ng-bind-html`

The client fetches the raw README, parses it with `renderMD()` (which uses `marked` with `sanitize: false`), then bypasses AngularJS sanitization:

```javascript
// https://github.com/tdurieux/anonymous_github/blob/b2d77faa6c6f35ad9ae6ed46e3a3fa4681ac84c2/public/script/app.js#L1219-L1226
const res = await $http.get(`/api/repo/${o.owner}/${o.repo}/readme`, {
params: { force: force === true ? "1" : "0", branch: $scope.source.branch },
});
$scope.readme = res.data;
```

```javascript
// https://github.com/tdurieux/anonymous_github/blob/b2d77faa6c6f35ad9ae6ed46e3a3fa4681ac84c2/public/script/app.js#L1339-L1343
const html = renderMD(
$scope.anonymize_readme,
`https://github.com/${o.owner}/${o.repo}/raw/${$scope.source.branch}/`
);
$scope.html_readme = $sce.trustAsHtml(html); // sink: bypasses Angular XSS protection
```

The `renderMD()` function explicitly disables sanitization:

```javascript
// https://github.com/tdurieux/anonymous_github/blob/b2d77faa6c6f35ad9ae6ed46e3a3fa4681ac84c2/public/script/utils.js#L165-L176
marked.setOptions({
sanitize: false, // HTML in markdown is preserved as-is
// ...
});
return marked.parse(md, { renderer });
```

The resulting HTML is bound to the DOM via `ng-bind-html`, which trusts the string marked by `$sce.trustAsHtml()` and inserts it as innerHTML.

### Impact

1. **Stored XSS**: Any malicious GitHub repository can execute JavaScript in the Anonymous GitHub origin when a user anonymizes it or views its content
2. **Account Takeover**: Steal authentication tokens and session cookies
3. **Data Exfiltration**: Access other users' anonymization configurations and private repository data via `/api/user` and `/api/repo/list`

### Proof of Concept


1. Create a GitHub repository with a malicious `README.md`:

```markdown
# Innocent README
<img src=x onerror="alert(document.domain)">
```

2. On Anonymous GitHub, enter the malicious repository URL to anonymize it
3. The XSS executes immediately when the README preview is rendered on the anonymize page

### Remediation

1. Sanitize markdown output with DOMPurify before rendering (the dependency already exists but is unused)
2. Serve HTML files with `Content-Disposition: attachment` or in a sandboxed iframe on a separate origin
3. Replace `$sce.trustAsHtml()` with proper `ngSanitize` usage
4. HTML-escape filenames and paths in directory listing templates
5. Add Content Security Policy headers

### Credits

Zhengyu Liu, Jingcheng Yang