Microsoft EUVD-2026-17965

Summary

@tinacms/graphql uses string-based path containment checks in FilesystemBridge:

• path.resolve(path.join(baseDir, filepath))
• startsWith(resolvedBase + path.sep)

That blocks plain ../ traversal, but it does not resolve symlink or junction targets. If a symlink/junction already exists under the allowed content root, a path like content/posts/pivot/owned.md is still considered "inside" the base even though the real filesystem target can be outside it.

As a result, FilesystemBridge.get(), put(), delete(), and glob() can operate on files outside the intended root.

Details

The current bridge validation is:

function assertWithinBase(filepath: string, baseDir: string): string {
  const resolvedBase = path.resolve(baseDir);
  const resolved = path.resolve(path.join(baseDir, filepath));
  if (
    resolved !== resolvedBase &&
    !resolved.startsWith(resolvedBase + path.sep)
  ) {
    throw new Error(
      `Path traversal detected: "${filepath}" escapes the base directory`
    );
  }
  return resolved;
}

But the bridge then performs real filesystem I/O on the resulting path:

public async get(filepath: string) {
  const resolved = assertWithinBase(filepath, this.outputPath);
  return (await fs.readFile(resolved)).toString();
}

public async put(filepath: string, data: string, basePathOverride?: string) {
  const basePath = basePathOverride || this.outputPath;
  const resolved = assertWithinBase(filepath, basePath);
  await fs.outputFile(resolved, data);
}

public async delete(filepath: string) {
  const resolved = assertWithinBase(filepath, this.outputPath);
  await fs.remove(resolved);
}

This is a classic realpath gap:

1. validation checks the lexical path string
2. the filesystem follows the link target during I/O
3. the actual target can be outside the intended root

This is reachable from Tina's GraphQL/local database flow. The resolver builds a validated path from user-controlled relativePath, but that validation is also string-based:

const realPath = path.join(collection.path, relativePath);
this.validatePath(realPath, collection, relativePath);

Database write and delete operations then call the bridge:

await this.bridge.put(normalizedPath, stringifiedFile);
...
await this.bridge.delete(normalizedPath);

Local Reproduction

This was verified llocally with a real junction on Windows, which exercises the same failure mode as a symlink on Unix-like systems.

Test layout:

• content root: D:\bugcrowd\tinacms\temp\junction-repro4
• allowed collection path: content/posts
• junction inside collection: content/posts/pivot -> D:\bugcrowd\tinacms\temp\junction-repro4\outside
• file outside content root: outside\secret.txt

Tina's current path-validation logic was applied and used to perform bridge-style read/write operations through the junction.

Observed result:

{
  "graphqlBridge": {
    "collectionPath": "content/posts",
    "requestedRelativePath": "pivot/owned.md",
    "validatedRealPath": "content\\posts\\pivot\\owned.md",
    "bridgeResolvedPath": "D:\\bugcrowd\\tinacms\\temp\\junction-repro4\\content\\posts\\pivot\\owned.md",
    "bridgeRead": "TOP_SECRET_FROM_OUTSIDE\\r\\n",
    "outsideGraphqlWriteExists": true,
    "outsideGraphqlWriteContents": "GRAPHQL_ESCAPE"
  }
}

That is the critical point:

• the path was accepted as inside content/posts
• the bridge read outside\secret.txt
• the bridge wrote outside\owned.md

So the current containment check does not actually constrain filesystem access to the configured content root once a link exists inside that tree.

Impact

• Arbitrary file read/write outside the configured content root
• Potential delete outside the configured content root via the same assertWithinBase() gap in delete()
• Breaks the assumptions of the recent path-traversal fixes because only lexical traversal is blocked
• Practical attack chains where the content tree contains a committed symlink/junction, or an attacker can cause one to exist before issuing GraphQL/content operations

The exact network exploitability depends on how the application exposes Tina's GraphQL/content operations, but the underlying bridge bug is real and independently security-relevant.

Recommended Fix

The containment check needs to compare canonical filesystem paths, not just string-normalized paths.

For example:

1. resolve the base with fs.realpath()
2. resolve the candidate path's parent with fs.realpath()
3. reject any request whose real target path escapes the real base
4. for write operations, carefully canonicalize the nearest existing parent directory before creating the final file

In short: use realpath-aware containment checks for every filesystem sink, not path.resolve(...).startsWith(...) alone.

Resources

• packages/@tinacms/graphql/src/database/bridge/filesystem.ts
• packages/@tinacms/graphql/src/database/index.ts
• packages/@tinacms/graphql/src/resolver/index.ts