repoward

A GitHub MCP server that stops AI agents acting on what an attacker's issue told them to do.

Status: alpha. It has not had an independent security review. Read the threat model, especially the part about where it runs.

The problem

An agent with GitHub access can read a malicious public issue, read a private repository, and post the contents in a public pull request. Each call looks allowed on its own, so per-tool permissions do not stop the sequence. This is the attack Invariant Labs published against agents using GitHub's MCP server, and repoward's test suite replays it against real repositories.

Flow guard

Before every write, repoward checks what this agent has already read, using facts from GitHub:

This does not depend on the model noticing an attack or on scanning text, so rewording or encoding the data does not get around it. The state survives restarts, and no tool can clear it. A person can:

uvx repoward flow status
uvx repoward flow reset

What it does not do:

• It only sees this server's tools. A shell, a browser or another MCP server is still a way out, so it protects agents whose GitHub access and outbound routes are limited to this server.
• Data can stay in an agent's context after the window (60 minutes by default) ends. Restart a long-running agent to clear its context.
• Reading one private repository and writing to another private repository is allowed, unless you set single_repo: true.
• Repository files and code search results do not count as untrusted input, because they were merged by someone with write access.
• author_association is a coarse signal, and a compromised trusted account counts as trusted.
• Maintainers of public repositories will see more approval requests. That is the cost of the default, and trusted_associations is configurable.

What works today

• Policy: default-deny YAML with roles, repository allowlists and path denylists. See policy.example.yaml.
• Single guarded path: every tool attempt, including unknown tools and invalid arguments, passes through one pipeline and is logged.
• Audit log: a hash-chained record of every decision, with optional HMAC and checkpoints.
• Rate limits: per principal and per tool.
• Read tools: github_list_issues, github_get_issue, github_search_code, github_get_file, ci_get_status and ci_get_job_logs.
• Write tools: github_create_branch, github_commit_files, github_create_pull_request and github_comment. Writes only reach branches with the configured prefix (agent/ by default), pull requests are draft only by default, commit sizes are limited, nothing is forced or retried, and text holding a recognizable credential is refused.
• Dangerous tools: github_merge_pull_request and github_delete_branch need human approval. An approval covers one exact call at one exact commit, works once and expires. Merging is off until a policy turns it on.
• Untrusted content handling: text from GitHub users comes back wrapped in a tag with a random id per response, after invisible characters, hidden HTML comments and images are removed and secrets are redacted. This is a mitigation, not a guarantee: a model can still be fooled, which is why the rules above are enforced outside the model.

Planned

• Least-privilege tokens: short-lived GitHub App tokens scoped to each call.

Who it is for

Agents whose only route to GitHub is this server, such as CI bots and hosted agents. An agent with shell access can run git or read local credentials directly, so this server is not a security boundary for it.

Install

Requires uv. Create a starter policy, which also prints a client config to paste:

uvx repoward init --repo your-org/your-repo

The starter policy gives the server read-only tools. Add --role contributor for write tools. Files live in ~/.repoward, or wherever REPOWARD_HOME points. An existing policy is never overwritten.

Then add it to your MCP client, and use a token set up as described in GitHub token. Avoid committing a config that holds a real token.

claude mcp add --env GITHUB_TOKEN=<your-token> --transport stdio repoward -- uvx repoward serve

{
  "mcpServers": {
    "repoward": {
      "command": "uvx",
      "args": ["repoward", "serve"],
      "env": { "GITHUB_TOKEN": "<your-token>" }
    }
  }
}

{
  "mcpServers": {
    "repoward": {
      "command": "uvx",
      "args": ["repoward", "serve"],
      "env": { "GITHUB_TOKEN": "${env:GITHUB_TOKEN}" }
    }
  }
}

{
  "inputs": [
    { "type": "promptString", "id": "github-token", "description": "GitHub token", "password": true }
  ],
  "servers": {
    "repoward": {
      "type": "stdio",
      "command": "uvx",
      "args": ["repoward", "serve"],
      "env": { "GITHUB_TOKEN": "${input:github-token}" }
    }
  }
}

[mcp_servers.repoward]
command = "uvx"
args = ["repoward", "serve"]
env_vars = ["GITHUB_TOKEN"]

{
  "mcpServers": {
    "repoward": {
      "command": "uvx",
      "args": ["repoward", "serve"],
      "env": { "GITHUB_TOKEN": "$GITHUB_TOKEN" }
    }
  }
}

Any other client that can launch a stdio MCP server runs the same command: uvx repoward serve.

Useful commands:

uvx repoward validate-policy
uvx repoward audit verify

Approving risky actions

A call that needs approval does not run. It returns an approval_id, and a person decides from a terminal:

uvx repoward approvals list
uvx repoward approvals show <id>
uvx repoward approvals approve <id>

The agent then repeats the same call. Pushing new commits changes the commit the call names, so the old approval no longer applies, and GitHub refuses a merge whose head has moved.

If the agent runs as the same OS user as this server and has a shell, it could run these commands or edit the database itself. Run the server and its files under a separate user or in a container for approvals to mean anything.

GitHub token

Use a fine-grained personal access token limited to the repositories you allow, with short expiry. Choose Only select repositories so that repository permissions appear. The read tools need these, all read-only:

To use the write tools, also give it read and write access to Contents, Issues and Pull requests. Leave the Workflows permission off: GitHub then refuses workflow file changes even if your policy allows the path.

Contents: write lets a token push to any branch and merge, and GitHub cannot limit it to agent/ branches. That limit lives in this server, so also protect your default branch with a ruleset that requires pull requests and has no bypass for the token's owner. If the token leaks or the server is misconfigured, that ruleset still holds.

Limits to know about:

• ci_get_status does not show third-party checks reported only through the Checks API, which GitHub documents as unavailable to fine-grained tokens.
• github_search_code covers the default branch and files under 384 KB. New repositories can return nothing until GitHub indexes them, and its handling of real results has so far only been tested against simulated responses.
• github_get_file refuses symlinks, submodules, binary files and files over 1 MB. It walks the Git tree, so each read makes one request per folder in the path.
• ci_get_job_logs fails closed if GitHub moves log storage to a host it does not recognize.
• github_merge_pull_request does not support merge queues or stacked pull requests.
• github_delete_branch checks the commit and then deletes, because GitHub has no conditional delete, so a small window remains between the two.

What the audit log proves

The log is tamper-evident, not tamper-proof.

Set REPOWARD_AUDIT_HMAC_KEY to use HMAC, and store the output of repoward audit checkpoint somewhere the server cannot write. Comment bodies, titles, commit messages and file contents are logged as a length and hash rather than as text. A crash during a write leaves an incomplete last line, and new writes are then refused until an operator repairs the log. Secret redaction is best effort.

Contributing and security

See CONTRIBUTING.md. Please report vulnerabilities as described in SECURITY.md.

License

MIT. This is an independent project, not affiliated with or endorsed by GitHub.