Fixes the symlink handling issue in the authorized_key module (#760)

- Addresses CVE-2026-11837
- Fixes #759

Signed-off-by: Hideki Saito <saito@fgrep.org>

changelogs/fragments/759_cve_2026_11837.yml

@@ -0,0 +1,2 @@
+security_fixes:
+  - authorized_key - fix local privilege escalation via symlink-following when running as root (https://github.com/ansible-collections/ansible.posix/issues/759).

plugins/modules/authorized_key.py

@@ -307,6 +307,17 @@ class keydict(dict):
         return [item[1] for item in self.items()]
 
 
+def _safe_open_write(module, path, follow):
+    flags = os.O_WRONLY | os.O_CREAT | os.O_TRUNC
+    if not follow and hasattr(os, 'O_NOFOLLOW'):
+        flags |= os.O_NOFOLLOW
+    try:
+        fd = os.open(path, flags, int('0600', 8))
+    except OSError as e:
+        module.fail_json(msg="File open failed %s : %s" % (path, to_native(e)))
+    return fd
+
+
 def keyfile(module, user, write=False, path=None, manage_dir=True, follow=False):
     """
     Calculate name of authorized keys file, optionally creating the
@@ -359,7 +370,7 @@ def keyfile(module, user, write=False, path=None, manage_dir=True, follow=False)
                 module.fail_json(msg="Failed to create directory %s : %s" % (sshdir, to_native(e)))
             if module.selinux_enabled():
                 module.set_default_selinux_context(sshdir, False)
-        os.chown(sshdir, uid, gid)
+        os.chown(sshdir, uid, gid, follow_symlinks=follow)
         os.chmod(sshdir, int('0700', 8))
 
     if not os.path.exists(keysfile):
@@ -367,16 +378,13 @@ def keyfile(module, user, write=False, path=None, manage_dir=True, follow=False)
         if not os.path.exists(basedir):
             os.makedirs(basedir)
 
-        f = None
+        fd = _safe_open_write(module, keysfile, follow)
-        try:
+        os.close(fd)
-            f = open(keysfile, "w")  # touches file so we can set ownership and perms
-        finally:
-            f.close()
         if module.selinux_enabled():
             module.set_default_selinux_context(keysfile, False)
 
     try:
-        os.chown(keysfile, uid, gid)
+        os.chown(keysfile, uid, gid, follow_symlinks=follow)
         os.chmod(keysfile, int('0600', 8))
     except OSError:
         pass
@@ -607,7 +615,7 @@ def enforce_state(module, params):
 
     # check current state -- just get the filename, don't create file
     do_write = False
-    params["keyfile"] = keyfile(module, user, do_write, path, manage_dir)
+    params["keyfile"] = keyfile(module, user, do_write, path, manage_dir, follow)
     existing_content = readfile(module, params["keyfile"])
     existing_keys = parsekeys(module, existing_content)
 
@@ -696,6 +704,11 @@ def enforce_state(module, params):
 
         if not module.check_mode:
             writefile(module, filename, new_content)
+            user_entry = pwd.getpwnam(user)
+            uid = user_entry.pw_uid
+            gid = user_entry.pw_gid
+            os.chown(filename, uid, gid, follow_symlinks=follow)
+            os.chmod(filename, int('0600', 8))
         params['changed'] = True
 
     return params

tests/integration/targets/authorized_key/tasks/check_symlink.yml

@@ -0,0 +1,22 @@
+---
+#
+# Check: keysfile is symlink and follow=false
+#
+- name: Try to add key with keysfile as symlink
+  ansible.posix.authorized_key:
+    user: testuser
+    key: "{{ rsa_key_basic }}"
+    state: present
+    manage_dir: false
+    follow: false
+
+- name: Assert target file ownership unchanged
+  ansible.builtin.stat:
+    path: /tmp/symlink_test_target/target_file
+  register: target_file_stat
+
+- name: Verify target file is still owned by root
+  ansible.builtin.assert:
+    that:
+      - target_file_stat.stat.uid == 0
+...

tests/integration/targets/authorized_key/tasks/check_symlink_cleanup.yml

@@ -0,0 +1,12 @@
+---
+- name: Remove testuser
+  ansible.builtin.user:
+    name: testuser
+    state: absent
+    remove: true
+
+- name: Remove symlink target directory
+  ansible.builtin.file:
+    path: /tmp/symlink_test_target
+    state: absent
+...

tests/integration/targets/authorized_key/tasks/check_symlink_setup.yml

@@ -0,0 +1,38 @@
+---
+- name: Create testuser for symlink tests
+  ansible.builtin.user:
+    name: testuser
+    create_home: true
+  register: testuser_result
+
+- name: Create symlink target directory
+  ansible.builtin.file:
+    path: /tmp/symlink_test_target
+    state: directory
+    owner: root
+    mode: '0700'
+
+- name: Create a target file owned by root
+  ansible.builtin.copy:
+    dest: /tmp/symlink_test_target/target_file
+    content: "sensitive data"
+    owner: root
+    mode: '0600'
+
+- name: Remove .ssh directory if exists
+  ansible.builtin.file:
+    path: "{{ testuser_result.home }}/.ssh"
+    state: absent
+
+- name: Create .ssh directory
+  ansible.builtin.file:
+    path: "{{ testuser_result.home }}/.ssh"
+    state: directory
+    mode: '0700'
+
+- name: Create symlink from authorized_keys to target file
+  ansible.builtin.file:
+    src: /tmp/symlink_test_target/target_file
+    dest: "{{ testuser_result.home }}/.ssh/authorized_keys"
+    state: link
+...

tests/integration/targets/authorized_key/tasks/main.yml

@@ -17,7 +17,7 @@
 # You should have received a copy of the GNU General Public License
 # along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
 
-- name: Setup testing environment
+- name: Setup for testing environment
   ansible.builtin.import_tasks: setup_steps.yml
 
 - name: Test for multiple keys handling
@@ -37,3 +37,12 @@
 
 - name: Test for permission denied files
   ansible.builtin.import_tasks: check_permissions.yml
+
+- name: CVE-2026-11837 Setup for symlink tests
+  ansible.builtin.import_tasks: check_symlink_setup.yml
+
+- name: CVE-2026-11837 Test for symlink tests
+  ansible.builtin.import_tasks: check_symlink.yml
+
+- name: CVE-2026-11837 Cleanup symlink test
+  ansible.builtin.import_tasks: check_symlink_cleanup.yml