WARDEN-WP-0026 T02: safe access transports (no secret values on stdout)

- proxy.py: proxy_fetch_to_file (mode-0600 file), build_wrapped_fetch +
  proxy_fetch_wrapped (single-use OpenBao response-wrapping token), _capture_value
  helper, is_bao_kv_fetch.
- warden access: --out FILE, --wrap [--wrap-ttl], --unsafe-stdout. Raw --fetch to a
  non-TTY stdout is refused (exit 6) — captured/piped output is the disclosure risk;
  sanctioned transports are --out / --exec / --wrap.
- canon: anti-pattern (secret value onto captured stdout) + transport table in
  .claude/rules/credential-routing.md; OperatorAccessAssist.md examples + G2 updated.
- tests: file/wrap/build + stdout-guard in tests/test_proxy.py. 293 pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/warden/cli.py

@@ -956,6 +956,10 @@ def _access_proxy(
     do_exec: bool,
     child_argv: list,
     no_policy: bool,
+    out_path: Optional[str] = None,
+    wrap: bool = False,
+    wrap_ttl: str = "5m",
+    unsafe_stdout: bool = False,
 ) -> None:
     """Proxy a non-SSH credential fetch as the caller (WP-0014 T3).
 
@@ -965,9 +969,12 @@ def _access_proxy(
     """
     from warden.proxy import (
         ProxyError,
+        build_wrapped_fetch,
         caller_auth_present,
         proxy_exec,
         proxy_fetch,
+        proxy_fetch_to_file,
+        proxy_fetch_wrapped,
         resolve_fetch_command,
         write_audit,
     )
@@ -1035,11 +1042,38 @@ def _access_proxy(
         else:
             err.print("[yellow]Proxying ungated[/yellow] (--no-policy; gate not enforced).")
 
-    try:
-        resolved = resolve_fetch_command(entry, domain=domain, field=field, path=path)
-    except ProxyError as e:
-        err.print(f"[red]{e}[/red]")
-        raise typer.Exit(2)
+    # Wrapping (WP-0026 T02) uses its own command shape; the value-bearing transports
+    # share the resolved fetch command.
+    if wrap and not is_login:
+        try:
+            resolved = build_wrapped_fetch(entry, path=path, ttl=wrap_ttl)
+        except ProxyError as e:
+            err.print(f"[red]{e}[/red]")
+            raise typer.Exit(2)
+    else:
+        try:
+            resolved = resolve_fetch_command(entry, domain=domain, field=field, path=path)
+        except ProxyError as e:
+            err.print(f"[red]{e}[/red]")
+            raise typer.Exit(2)
+
+    # T02 — the sanctioned fetch transports (file / env / wrapping token) never put a
+    # secret value on stdout. Streaming a value to stdout is the documented anti-pattern:
+    # allowed only to an interactive terminal, and only with an explicit acknowledgment
+    # when stdout is captured/piped (the logged-context disclosure risk).
+    if not is_login and not do_exec and not wrap and not out_path:
+        import sys as _sys
+
+        if not _sys.stdout.isatty() and not unsafe_stdout:
+            err.print(
+                "[red]Refusing to stream a secret value to a non-terminal stdout[/red] "
+                "(captured/piped output is a disclosure risk). Use a sanctioned transport:\n"
+                "  --out FILE     write the value to a mode-0600 file\n"
+                "  --exec -- CMD  inject it into a child process env\n"
+                "  --wrap         return a single-use OpenBao wrapping token to unwrap yourself\n"
+                "Override only for an interactive human session: --unsafe-stdout."
+            )
+            raise typer.Exit(6)
 
     action = "login" if is_login else ("exec" if do_exec else "fetch")
     err.print(
@@ -1052,6 +1086,18 @@ def _access_proxy(
                 err.print("[red]--exec needs a command after `--`[/red], e.g. `-- npm publish`.")
                 raise typer.Exit(2)
             rc = proxy_exec(resolved, env_var=field or "", child_argv=child_argv)
+        elif wrap:
+            token = proxy_fetch_wrapped(resolved)
+            # The wrapping token is not the secret value — safe to hand back on stdout.
+            print(token)
+            err.print(
+                f"[dim]wrapping token (single-use, ttl {wrap_ttl}) — unwrap in your own "
+                f"context: [bold]bao unwrap {'<token>'}[/bold][/dim]"
+            )
+            rc = 0
+        elif out_path:
+            rc = proxy_fetch_to_file(resolved, Path(out_path))
+            err.print(f"[dim]value written to {out_path} (mode 0600); not shown[/dim]")
         else:
             rc = proxy_fetch(resolved)
     except ProxyError as e:
@@ -1087,7 +1133,7 @@ def access(
     output_json: Annotated[bool, typer.Option("--json", help="Output JSON (stable, secret-free)")] = False,
     all_entries: Annotated[bool, typer.Option("--all", help="Include draft entries")] = False,
     do_fetch: Annotated[
-        bool, typer.Option("--fetch", help="Proxy the fetch as the caller; value streams to stdout")
+        bool, typer.Option("--fetch", help="Proxy the fetch as the caller (pair with --out/--wrap; raw stdout is guarded)")
     ] = False,
     do_exec: Annotated[
         bool,
@@ -1099,6 +1145,21 @@ def access(
     path: Annotated[
         Optional[str], typer.Option("--path", help="Override the owner-side path template")
     ] = None,
+    out_path: Annotated[
+        Optional[str],
+        typer.Option("--out", help="Sanctioned transport: write the value to this mode-0600 file, not stdout"),
+    ] = None,
+    wrap: Annotated[
+        bool,
+        typer.Option("--wrap", help="Sanctioned transport: return a single-use OpenBao wrapping token (bao unwrap)"),
+    ] = False,
+    wrap_ttl: Annotated[
+        str, typer.Option("--wrap-ttl", help="TTL for the --wrap response-wrapping token")
+    ] = "5m",
+    unsafe_stdout: Annotated[
+        bool,
+        typer.Option("--unsafe-stdout", help="Acknowledge streaming a value to a captured/piped stdout (anti-pattern)"),
+    ] = False,
     no_policy: Annotated[
         bool,
         typer.Option("--no-policy", help="Acknowledge proxying when the flex-auth gate is not enforced"),
@@ -1134,7 +1195,7 @@ def access(
 
     entry = matches[0]
 
-    if do_fetch or do_exec:
+    if do_fetch or do_exec or out_path or wrap:
         _access_proxy(
             entry,
             domain=domain,
@@ -1143,6 +1204,10 @@ def access(
             do_exec=do_exec,
             child_argv=list(ctx.args),
             no_policy=no_policy,
+            out_path=out_path,
+            wrap=wrap,
+            wrap_ttl=wrap_ttl,
+            unsafe_stdout=unsafe_stdout,
         )
         return
 

src/warden/proxy.py

@@ -205,6 +205,102 @@ def proxy_fetch(resolved: ResolvedFetch) -> int:
     return completed.returncode
 
 
+def _capture_value(resolved: ResolvedFetch) -> str:
+    """Run the fetch and return its stdout (the value) minus one trailing newline.
+
+    The value transits warden's memory (the accepted proxy tradeoff for the
+    non-stdout transports) but is never written to disk or log by this function.
+    """
+    env = _caller_env()
+    if resolved.argv is not None:
+        fetched = subprocess.run(  # noqa: S603
+            resolved.argv, stdout=subprocess.PIPE, stderr=None, stdin=None,
+            env=env, check=False, text=True,
+        )
+    else:
+        fetched = subprocess.run(  # noqa: S602
+            resolved.shell_cmd, shell=True, stdout=subprocess.PIPE, stderr=None,
+            stdin=None, env=env, check=False, text=True,
+        )
+    if fetched.returncode != 0:
+        raise ProxyError(
+            f"fetch failed (exit {fetched.returncode}) — check caller auth and the path."
+        )
+    value = fetched.stdout
+    if value.endswith("\n"):
+        value = value[:-1]
+    return value
+
+
+def proxy_fetch_to_file(resolved: ResolvedFetch, out_path: Path) -> int:
+    """Fetch the value and write it to ``out_path`` at mode 0600 — never to stdout.
+
+    A sanctioned transport (WP-0026 T02): the value goes to a private file the
+    caller controls, not a terminal or a logged stream. The file is created with
+    O_EXCL semantics widened to truncate-if-owned so a re-fetch overwrites, but the
+    mode is forced to 0600 before any bytes are written.
+    """
+    value = _capture_value(resolved)
+    # Open with restrictive mode from the start; do not echo the value anywhere.
+    fd = os.open(str(out_path), os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
+    try:
+        os.chmod(out_path, 0o600)  # enforce even if the file pre-existed with looser mode
+        with os.fdopen(fd, "w") as fh:
+            fh.write(value)
+    finally:
+        value = ""  # noqa: F841 — best-effort scrub of the local reference
+    return 0
+
+
+def is_bao_kv_fetch(entry: RouteEntry) -> bool:
+    """True when a lane's fetch is a plain ``bao kv get`` (wrappable, WP-0026 T02)."""
+    return bool(entry.fetch_command and entry.fetch_command.strip().startswith("bao kv get"))
+
+
+def build_wrapped_fetch(
+    entry: RouteEntry, *, path: Optional[str] = None, ttl: str = "5m"
+) -> ResolvedFetch:
+    """Build a response-wrapping fetch: ``bao kv get -wrap-ttl=<ttl> -format=json <path>``.
+
+    Response wrapping returns a single-use, short-TTL *wrapping token* instead of the
+    secret value — the sanctioned way to move a value between processes (WP-0026 T02).
+    The caller unwraps it in their own context (`bao unwrap`). Only valid for plain
+    ``bao kv get`` lanes; the whole secret is wrapped (a per-field ``-field`` read
+    cannot be wrapped).
+    """
+    if not is_bao_kv_fetch(entry):
+        raise ProxyError(
+            f"{entry.id!r} is not a plain `bao kv get` lane — response wrapping "
+            "(--wrap) is unavailable. Use --out FILE or --exec instead."
+        )
+    target = path or entry.path_template
+    if not target or _PLACEHOLDER.search(target):
+        raise ProxyError(
+            "--wrap needs a concrete path — supply --path or a resolved path_template."
+        )
+    return ResolvedFetch(argv=["bao", "kv", "get", f"-wrap-ttl={ttl}", "-format=json", target])
+
+
+def proxy_fetch_wrapped(resolved: ResolvedFetch) -> str:
+    """Run a wrapping fetch and return the wrapping *token* (not the secret value).
+
+    The token is single-use and short-lived; it is not itself the credential, so it
+    is safe to hand back on stdout. Parses OpenBao's ``-format=json`` wrap_info.
+    """
+    raw = _capture_value(resolved)
+    try:
+        data = json.loads(raw)
+        token = data["wrap_info"]["token"]
+    except (json.JSONDecodeError, KeyError, TypeError) as e:
+        raise ProxyError(
+            "could not parse a wrapping token from the fetch output "
+            "(is response wrapping supported for this path?)."
+        ) from e
+    if not token:
+        raise ProxyError("empty wrapping token returned.")
+    return str(token)
+
+
 def proxy_exec(resolved: ResolvedFetch, *, env_var: str, child_argv: List[str]) -> int:
     """Fetch the value and inject it into a child command's environment only.