prepare_subprocess_env: remove all passphrase-related env vars and BORGSTORE_REST_PASSWORD

docs/changes.rst

@@ -171,6 +171,12 @@ New features:
 - create/import-tar --json: report the deduplicated size of the new archive, #10335.
   It is also included in the archive_progress JSON output.
 
+Fixes:
+
+- subprocess environment: also remove BORG_NEW_PASSPHRASE, BORG_OTHER_PASSPHRASE, BORG_PASSCOMMAND,
+  BORG_OTHER_PASSCOMMAND, BORG_PASSPHRASE_FD, BORG_OTHER_PASSPHRASE_FD and BORGSTORE_REST_PASSWORD
+  from the environment given to subprocesses (previously only BORG_PASSPHRASE was removed), #6480.
+
 Version 2.0.0b24 (2026-09-02)
 -----------------------------
 

src/borg/archiver/help_cmd.py

@@ -614,6 +614,11 @@ class HelpMixIn:
                 "More than one passphrase environment variable is set". The same applies to the
                 ``BORG_OTHER_*`` variants (which are a separate, independent group).
                 See also BORG_NEW_PASSPHRASE.
+                borg removes the passphrase-related variables (BORG_PASSPHRASE, BORG_NEW_PASSPHRASE,
+                BORG_PASSCOMMAND, BORG_PASSPHRASE_FD and their ``BORG_OTHER_*`` variants) as well as
+                BORGSTORE_REST_PASSWORD from the environment of the subprocesses it starts (like the
+                command given in BORG_PASSCOMMAND, ``--paths-from-command`` / ``--content-from-command``
+                commands, tar filter commands and ``borg with-lock`` commands).
             BORG_PASSCOMMAND (and BORG_OTHER_PASSCOMMAND)
                 When set, use the standard output of the command (trailing newlines are stripped) to answer the
                 passphrase question for encrypted repositories.

src/borg/helpers/process.py

@@ -325,6 +325,23 @@ def is_terminal(fd=sys.stdout):
     return hasattr(fd, "isatty") and fd.isatty() and (not is_win32 or "ANSICON" in os.environ)
 
 
+# Environment variables that prepare_subprocess_env() removes from the environment of subprocesses.
+# The passphrases and the borgstore REST password are secrets, the commands given in BORG_PASSCOMMAND /
+# BORG_OTHER_PASSCOMMAND often embed secrets or paths to them. The *_FD variables refer to file
+# descriptors that the subprocess does not inherit (Popen closes them), so they would be dangling and
+# misleading there.
+SUBPROCESS_ENV_REMOVE = (
+    "BORG_PASSPHRASE",
+    "BORG_NEW_PASSPHRASE",
+    "BORG_OTHER_PASSPHRASE",
+    "BORG_PASSCOMMAND",
+    "BORG_OTHER_PASSCOMMAND",
+    "BORG_PASSPHRASE_FD",
+    "BORG_OTHER_PASSPHRASE_FD",
+    "BORGSTORE_REST_PASSWORD",
+)
+
+
 def prepare_subprocess_env(system, env=None):
     """
     Prepare the environment for a subprocess we are going to create.
@@ -356,8 +373,9 @@ def prepare_subprocess_env(system, env=None):
             lp = env.get(lp_key)
             if lp is not None and getattr(sys, "frozen", False) and hasattr(sys, "_MEIPASS"):
                 env.pop(lp_key)
-    # security: do not give secrets to subprocess
-    env.pop("BORG_PASSPHRASE", None)
+    # security: do not give secrets (and things only meaningful in this process) to the subprocess
+    for name in SUBPROCESS_ENV_REMOVE:
+        env.pop(name, None)
     # for information, give borg version to the subprocess
     env["BORG_VERSION"] = __version__
     return env

src/borg/testsuite/helpers/process_test.py

@@ -1,7 +1,8 @@
 import shutil
 import pytest
 
-from ...helpers.process import popen_with_error_handling
+from ... import __version__
+from ...helpers.process import popen_with_error_handling, prepare_subprocess_env, SUBPROCESS_ENV_REMOVE
 
 
 class TestPopenWithErrorHandling:
@@ -25,3 +26,33 @@ def test_bad_syntax(self, cmd):
     def test_shell(self):
         with pytest.raises(AssertionError):
             popen_with_error_handling("", shell=True)
+
+
+class TestPrepareSubprocessEnv:
+    @pytest.mark.parametrize("name", SUBPROCESS_ENV_REMOVE)
+    @pytest.mark.parametrize("system", (True, False))
+    def test_removes_secrets_from_os_environ(self, monkeypatch, name, system):
+        monkeypatch.setenv(name, "secret")
+        assert name not in prepare_subprocess_env(system=system)
+
+    def test_removes_secrets_from_given_env(self):
+        env = {name: "secret" for name in SUBPROCESS_ENV_REMOVE}
+        env["BORG_REPO"] = "/path/to/repo"
+        result = prepare_subprocess_env(system=True, env=env)
+        assert not set(result) & set(SUBPROCESS_ENV_REMOVE)
+        assert result["BORG_REPO"] == "/path/to/repo"
+        # the given env must not be modified
+        assert env["BORG_PASSPHRASE"] == "secret"
+        assert "BORG_VERSION" not in env
+
+    def test_keeps_other_variables_and_sets_version(self, monkeypatch):
+        monkeypatch.setenv("BORG_REPO", "/path/to/repo")
+        monkeypatch.setenv("BORG_KEY_FILE", "/path/to/keyfile")
+        monkeypatch.setenv("BORGSTORE_REST_USERNAME", "username")
+        monkeypatch.setenv("HOME", "/home/user")
+        env = prepare_subprocess_env(system=True)
+        assert env["BORG_REPO"] == "/path/to/repo"
+        assert env["BORG_KEY_FILE"] == "/path/to/keyfile"
+        assert env["BORGSTORE_REST_USERNAME"] == "username"
+        assert env["HOME"] == "/home/user"
+        assert env["BORG_VERSION"] == __version__