diff --git a/reviewer/noema_reviewer/github_io.py b/reviewer/noema_reviewer/github_io.py
index e695193f7..d3910833b 100644
--- a/reviewer/noema_reviewer/github_io.py
+++ b/reviewer/noema_reviewer/github_io.py
@@ -59,11 +59,36 @@
 )
 
 
+def _github_cli_environment() -> dict[str, str]:
+    """Build the least-authority environment for reviewer GitHub CLI calls."""
+    safe_env = {
+        "GH_HOST": "github.com",
+        "NO_COLOR": "1",
+    }
+    path = os.environ.get("PATH")
+    if path:
+        safe_env["PATH"] = path
+    token = os.environ.get("GH_TOKEN")
+    if token:
+        safe_env["GH_TOKEN"] = token
+    return safe_env
+
+
+def _redact_delegated_github_token(text: str, child_env: dict[str, str]) -> str:
+    """Remove the exact delegated GitHub token before an error can be retained."""
+    token = child_env.get("GH_TOKEN", "")
+    if not token:
+        return text
+    return text.replace(token, "[REDACTED]")
+
+
 def default_runner(args: Sequence[str], stdin: str | None = None) -> str:
-    """Run a ``gh`` command without a shell and return stdout."""
+    """Run a ``gh`` command with explicit least-authority environment."""
+    child_env = _github_cli_environment()
     completed = subprocess.run(
         list(args),
         input=stdin,
+        env=child_env,
         text=True,
         stdout=subprocess.PIPE,
         stderr=subprocess.PIPE,
@@ -71,7 +96,8 @@ def default_runner(args: Sequence[str], stdin: str | None = None) -> str:
         shell=False,
     )
     if completed.returncode != 0:
-        raise RuntimeError(f"Command failed ({completed.returncode}): {args[0]}\n{completed.stderr.strip()}")
+        detail = _redact_delegated_github_token(completed.stderr.strip(), child_env)
+        raise RuntimeError(f"Command failed ({completed.returncode}): {args[0]}\n{detail}")
     return completed.stdout
 
 
diff --git a/reviewer/tests/test_github_io_environment_boundary.py b/reviewer/tests/test_github_io_environment_boundary.py
new file mode 100644
index 000000000..4eaaa83f5
--- /dev/null
+++ b/reviewer/tests/test_github_io_environment_boundary.py
@@ -0,0 +1,86 @@
+"""Regression coverage for reviewer GitHub CLI subprocess authority."""
+
+from __future__ import annotations
+
+from types import SimpleNamespace
+
+import pytest
+
+from noema_reviewer.github_io import default_runner
+
+
+def test_default_runner_passes_only_reviewed_github_cli_environment(monkeypatch) -> None:
+    """A hostile parent must not become ambient authority for the ``gh`` child."""
+    observed: dict[str, object] = {}
+
+    def fake_run(args, **kwargs):
+        """Capture the subprocess contract without executing GitHub CLI."""
+        observed.update(kwargs)
+        return SimpleNamespace(returncode=0, stdout="ok", stderr="")
+
+    monkeypatch.setenv("PATH", "/reviewed/bin")
+    monkeypatch.setenv("GH_TOKEN", "delegated-github-token")
+    monkeypatch.setenv("GITHUB_TOKEN", "ambient-github-token")
+    monkeypatch.setenv("NVIDIA_NIM_API_KEY", "model-secret")
+    monkeypatch.setenv("NOEMA_LLM_API_KEY", "reviewer-secret")
+    monkeypatch.setenv("NOEMA_REVIEWER_APP_PRIVATE_KEY", "app-private-key")
+    monkeypatch.setenv("HTTPS_PROXY", "http://proxy.invalid")
+    monkeypatch.setenv("HOME", "/hostile/home")
+    monkeypatch.setenv("NODE_OPTIONS", "--require=/hostile/preload.cjs")
+    monkeypatch.setattr("noema_reviewer.github_io.subprocess.run", fake_run)
+
+    assert default_runner(["gh", "api", "user"], None) == "ok"
+    assert observed["env"] == {
+        "PATH": "/reviewed/bin",
+        "GH_TOKEN": "delegated-github-token",
+        "GH_HOST": "github.com",
+        "NO_COLOR": "1",
+    }
+    assert observed["shell"] is False
+
+
+def test_default_runner_keeps_only_pinned_defaults_without_path_or_token(monkeypatch) -> None:
+    """Missing optional launch/token authority must not widen the child environment."""
+    observed: dict[str, object] = {}
+
+    def fake_run(args, **kwargs):
+        """Capture the subprocess contract without executing GitHub CLI."""
+        observed.update(kwargs)
+        return SimpleNamespace(returncode=0, stdout="ok", stderr="")
+
+    monkeypatch.delenv("PATH", raising=False)
+    monkeypatch.delenv("GH_TOKEN", raising=False)
+    monkeypatch.setenv("GITHUB_TOKEN", "ambient-github-token")
+    monkeypatch.setenv("HOME", "/hostile/home")
+    monkeypatch.setattr("noema_reviewer.github_io.subprocess.run", fake_run)
+
+    assert default_runner(["gh", "api", "user"], None) == "ok"
+    assert observed["env"] == {
+        "GH_HOST": "github.com",
+        "NO_COLOR": "1",
+    }
+    assert observed["shell"] is False
+
+
+def test_default_runner_redacts_delegated_token_from_failure_diagnostics(monkeypatch) -> None:
+    """A hostile ``gh`` failure cannot copy the delegated token into retained errors."""
+    token = "delegated-github-token"
+
+    def fake_run(args, **kwargs):
+        """Echo the delegated credential as a hostile child executable might."""
+        return SimpleNamespace(
+            returncode=1,
+            stdout="",
+            stderr=f"authentication failed for {token}; retry token={token}",
+        )
+
+    monkeypatch.setenv("GH_TOKEN", token)
+    monkeypatch.setattr("noema_reviewer.github_io.subprocess.run", fake_run)
+
+    with pytest.raises(RuntimeError) as raised:
+        default_runner(["gh", "api", "user"], None)
+
+    detail = str(raised.value)
+    assert token not in detail
+    assert "authentication failed" in detail
+    assert "retry token=[REDACTED]" in detail