TASK-016: Validate delete_log_path query parameter with allowlist

- Extract path validation logic into shared helper function in
  backend/app/utils/path_utils.py (validate_log_path)
- Refactor AddLogPathRequest to use the helper function
- Apply the same validation to DELETE /api/config/jails/{name}/logpath
  endpoint by validating the log_path query parameter
- Return HTTP 422 with descriptive error if validation fails
- Add comprehensive unit tests for path validation
- Update Backend-Development.md with usage examples

This prevents path-traversal attacks on the delete_log_path endpoint
by ensuring all log paths are within allowlisted directories.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Backend-Development.md

@@ -219,27 +219,47 @@ For fields that require complex validation (e.g., file paths that must be within
 
 ```python
 from pydantic import field_validator
+from app.utils.path_utils import validate_log_path
 
 class AddLogPathRequest(BaseModel):
     log_path: str = Field(..., description="Absolute path to the log file to monitor.")
 
     @field_validator("log_path", mode="after")
     @classmethod
-    def validate_log_path(cls, value: str) -> str:
+    def validate_log_path_field(cls, value: str) -> str:
         """Validate that the log path is within allowed directories."""
-        settings = get_settings()
-        resolved_path = Path(value).resolve()
-        for allowed_dir in settings.allowed_log_dirs:
-            if resolved_path.is_relative_to(Path(allowed_dir).resolve()):
-                return value
-        raise ValueError(f"Path {value!r} is outside allowed directories")
+        return validate_log_path(value)
+```
+
+**Path Validation Helper:**
+
+For query parameters and other contexts where Pydantic validators cannot be used directly, use the `validate_log_path()` helper from `app.utils.path_utils`:
+
+```python
+from fastapi import HTTPException, status
+from app.utils.path_utils import validate_log_path
+
+@router.delete("/{name}/logpath")
+async def delete_log_path(
+    name: str,
+    log_path: str = Query(...),
+) -> None:
+    try:
+        validate_log_path(log_path)
+    except ValueError as e:
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail=str(e),
+        ) from e
+    # ... rest of handler
 ```
 
 **Key points:**
-- Use `mode="after"` to validate after Pydantic's basic type coercion.
+- Use `mode="after"` in model validators to validate after Pydantic's basic type coercion.
 - Raise `ValueError` if validation fails; Pydantic converts it to an HTTP 400 response.
-- **Never use string prefix matching** for path validation (e.g., `path.startswith("/var/log")`). Use `Path.is_relative_to()` to avoid bypasses like `/var/log_evil/file.log`.
-- Resolve symlinks before validating to prevent symlink-based escapes.
+- For query parameters that cannot use Pydantic validators, use the `validate_log_path()` helper and raise HTTP 422.
+- **Never use string prefix matching** for path validation (e.g., `path.startswith("/var/log")`). The helper uses `Path.relative_to()` to prevent bypasses like `/var/log_evil/file.log`.
+- Symlinks are resolved before validating to prevent symlink-based escapes.
 
 ---
 

Docs/Tasks.md

@@ -1,37 +1,3 @@
-## TASK-015 — `GlobalConfigUpdate.log_target`/`log_level` have no validation
-
-**Severity:** High
-
-### Where found
-`backend/app/models/config.py` — `GlobalConfigUpdate`. `backend/app/services/config_service.py` — `update_global_config()`.
-
-### Why this is needed
-`log_target` is forwarded raw to fail2ban via the Unix socket. fail2ban (running as root) creates or opens the file at that path if it does not exist. `log_level` is forwarded raw without checking it is a valid fail2ban log level. Both fields represent an injection path into fail2ban's internal state from an authenticated but potentially compromised account.
-
-### Goal
-Validate both fields before forwarding to fail2ban.
-
-### What to do
-1. Change `log_target` in `GlobalConfigUpdate` to accept only:
-   - `Literal["STDOUT", "STDERR", "SYSLOG"]`, or
-   - A path validated the same way as `AddLogPathRequest.log_path` (see TASK-014).
-2. Change `log_level` to `Literal["CRITICAL", "ERROR", "WARNING", "NOTICE", "INFO", "DEBUG"]`.
-3. Apply the same restrictions in `get_global_config` responses for consistency.
-
-### Possible traps and issues
-- The allowlist for `log_target` paths must be consistent with TASK-014 (`BANGUI_ALLOWED_LOG_DIRS`).
-- Existing deployments using non-standard `log_target` values (e.g., `/var/log/fail2ban.log`) must still work — ensure `/var/log` is in the default allowlist.
-
-### Docs changes needed
-- `Features.md` — document valid values for `log_target` and `log_level`.
-- `Backend-Development.md` — Pydantic Literal types for constrained string fields.
-
-### Doc references
-- [Features.md](Features.md) — global fail2ban configuration
-- [Backend-Development.md](Backend-Development.md) — model validation
-
----
-
 ## TASK-016 — `delete_log_path` query parameter unvalidated
 
 **Severity:** Medium

backend/app/models/config.py

@@ -10,6 +10,7 @@ from typing import Literal
 from pydantic import BaseModel, ConfigDict, Field, field_validator
 
 from app.config import get_settings
+from app.utils.path_utils import validate_log_path
 
 DNSMode = Literal["yes", "warn", "no", "raw"]
 LogEncoding = Literal["auto", "ascii", "utf-8", "UTF-8", "latin-1"]
@@ -295,12 +296,9 @@ class AddLogPathRequest(BaseModel):
 
     @field_validator("log_path", mode="after")
     @classmethod
-    def validate_log_path(cls, value: str) -> str:
+    def validate_log_path_field(cls, value: str) -> str:
         """Validate that the log path is within allowed directories.
 
-        Resolves the path to its canonical form (resolving symlinks) and checks
-        that it is relative to one of the allowed log directories from settings.
-
         Args:
             value: The log path to validate.
 
@@ -310,24 +308,7 @@ class AddLogPathRequest(BaseModel):
         Raises:
             ValueError: If the path is outside allowed log directories.
         """
-        settings = get_settings()
-        try:
-            resolved_path = Path(value).resolve()
-        except (OSError, RuntimeError) as e:
-            raise ValueError(f"Cannot resolve path {value!r}: {e}") from e
-
-        for allowed_dir in settings.allowed_log_dirs:
-            allowed_path = Path(allowed_dir).resolve()
-            try:
-                resolved_path.relative_to(allowed_path)
-                return value
-            except ValueError:
-                continue
-
-        allowed_dirs_str = ", ".join(settings.allowed_log_dirs)
-        raise ValueError(
-            f"Log path {value!r} is outside allowed directories: {allowed_dirs_str}"
-        )
+        return validate_log_path(value)
 
 
 

backend/app/routers/jail_config.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import shlex
 from typing import Annotated
 
-from fastapi import APIRouter, Path, Query, Request, status
+from fastapi import APIRouter, HTTPException, Path, Query, Request, status
 
 from app.dependencies import (
     AppDep,
@@ -33,6 +33,7 @@ from app.services import (
     filter_config_service,
     jail_config_service,
 )
+from app.utils.path_utils import validate_log_path
 from app.utils.runtime_state import (
     clear_activation_record,
     clear_pending_recovery,
@@ -248,10 +249,19 @@ async def delete_log_path(
         log_path: Absolute path to the log file to remove (query parameter).
 
     Raises:
+        HTTPException: 422 when the log path is outside allowed directories.
         HTTPException: 404 when the jail does not exist.
         HTTPException: 400 when the command is rejected.
         HTTPException: 502 when fail2ban is unreachable.
     """
+    try:
+        validate_log_path(log_path)
+    except ValueError as e:
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail=str(e),
+        ) from e
+
     await config_service.delete_log_path(socket_path, name, log_path)
 
 

backend/app/utils/path_utils.py

@@ -0,0 +1,40 @@
+"""Path validation utilities."""
+
+from pathlib import Path
+
+from app.config import get_settings
+
+
+def validate_log_path(log_path: str) -> str:
+    """Validate that a log path is within allowed directories.
+
+    Resolves the path to its canonical form (resolving symlinks) and checks
+    that it is relative to one of the allowed log directories from settings.
+
+    Args:
+        log_path: The log path to validate.
+
+    Returns:
+        The validated log path (unchanged).
+
+    Raises:
+        ValueError: If the path is outside allowed log directories.
+    """
+    settings = get_settings()
+    try:
+        resolved_path = Path(log_path).resolve()
+    except (OSError, RuntimeError) as e:
+        raise ValueError(f"Cannot resolve path {log_path!r}: {e}") from e
+
+    for allowed_dir in settings.allowed_log_dirs:
+        allowed_path = Path(allowed_dir).resolve()
+        try:
+            resolved_path.relative_to(allowed_path)
+            return log_path
+        except ValueError:
+            continue
+
+    allowed_dirs_str = ", ".join(settings.allowed_log_dirs)
+    raise ValueError(
+        f"Log path {log_path!r} is outside allowed directories: {allowed_dirs_str}"
+    )

backend/tests/test_models.py

@@ -22,6 +22,7 @@ def _mock_allowed_dirs(monkeypatch: pytest.MonkeyPatch) -> None:
         )
 
     monkeypatch.setattr("app.models.config.get_settings", mock_get_settings)
+    monkeypatch.setattr("app.utils.path_utils.get_settings", mock_get_settings)
 
 
 def test_add_log_path_request_valid_in_var_log(_mock_allowed_dirs: None) -> None:
@@ -90,6 +91,7 @@ def test_add_log_path_request_rejects_symlink_escape(monkeypatch: pytest.MonkeyP
             )
 
         monkeypatch.setattr("app.models.config.get_settings", mock_get_settings)
+        monkeypatch.setattr("app.utils.path_utils.get_settings", mock_get_settings)
 
         with pytest.raises(ValidationError) as exc_info:
             AddLogPathRequest(log_path=str(symlink / "evil.log"), tail=True)
@@ -132,6 +134,7 @@ def test_add_log_path_request_custom_allowed_dirs(monkeypatch: pytest.MonkeyPatc
         )
 
     monkeypatch.setattr("app.models.config.get_settings", mock_get_settings)
+    monkeypatch.setattr("app.utils.path_utils.get_settings", mock_get_settings)
 
     req = AddLogPathRequest(log_path="/custom/logs/app.log", tail=True)
     assert req.log_path == "/custom/logs/app.log"

backend/tests/test_path_utils.py

@@ -0,0 +1,149 @@
+"""Unit tests for path validation utilities."""
+
+import tempfile
+from pathlib import Path
+
+import pytest
+
+from app.config import Settings
+from app.utils.path_utils import validate_log_path
+
+
+@pytest.fixture
+def _mock_settings(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Mock get_settings to return test settings with default allowed directories."""
+    def mock_get_settings() -> Settings:
+        return Settings(
+            database_path=":memory:",
+            fail2ban_socket="/tmp/fake.sock",
+            fail2ban_config_dir="/tmp/fail2ban",
+            session_secret="test-secret-key-do-not-use",
+        )
+
+    monkeypatch.setattr("app.utils.path_utils.get_settings", mock_get_settings)
+
+
+def test_validate_log_path_valid_in_var_log(_mock_settings: None) -> None:
+    """Valid log paths in /var/log are accepted."""
+    result = validate_log_path("/var/log/auth.log")
+    assert result == "/var/log/auth.log"
+
+
+def test_validate_log_path_valid_in_config_log(_mock_settings: None) -> None:
+    """Valid log paths in /config/log are accepted."""
+    result = validate_log_path("/config/log/app.log")
+    assert result == "/config/log/app.log"
+
+
+def test_validate_log_path_valid_with_subdirectory(_mock_settings: None) -> None:
+    """Log paths in subdirectories of allowed paths are accepted."""
+    result = validate_log_path("/var/log/syslog/auth.log")
+    assert result == "/var/log/syslog/auth.log"
+
+
+def test_validate_log_path_rejects_path_outside_allowed(_mock_settings: None) -> None:
+    """Paths outside allowed directories are rejected."""
+    with pytest.raises(ValueError) as exc_info:
+        validate_log_path("/etc/passwd")
+    error_msg = str(exc_info.value)
+    assert "outside allowed directories" in error_msg
+    assert "/etc/passwd" in error_msg
+
+
+def test_validate_log_path_rejects_home_directory(_mock_settings: None) -> None:
+    """Paths in home directories are rejected."""
+    with pytest.raises(ValueError) as exc_info:
+        validate_log_path("/home/user/app.log")
+    error_msg = str(exc_info.value)
+    assert "outside allowed directories" in error_msg
+
+
+def test_validate_log_path_rejects_shadow_file(_mock_settings: None) -> None:
+    """Paths to sensitive files like /etc/shadow are rejected."""
+    with pytest.raises(ValueError) as exc_info:
+        validate_log_path("/etc/shadow")
+    error_msg = str(exc_info.value)
+    assert "outside allowed directories" in error_msg
+
+
+def test_validate_log_path_rejects_symlink_escape(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Symlinks that escape allowed directories are rejected."""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        allowed_dir = Path(tmpdir) / "allowed"
+        escape_dir = Path(tmpdir) / "escape"
+        allowed_dir.mkdir()
+        escape_dir.mkdir()
+
+        symlink = allowed_dir / "escape_link"
+        symlink.symlink_to(escape_dir)
+
+        def mock_get_settings() -> Settings:
+            return Settings(
+                database_path=":memory:",
+                fail2ban_socket="/tmp/fake.sock",
+                fail2ban_config_dir="/tmp/fail2ban",
+                session_secret="test-secret-key-do-not-use",
+                allowed_log_dirs=[str(allowed_dir)],
+            )
+
+        monkeypatch.setattr("app.utils.path_utils.get_settings", mock_get_settings)
+
+        # Accessing the symlink resolves it to the escape_dir, which is outside the allowed_dir.
+        with pytest.raises(ValueError) as exc_info:
+            validate_log_path(str(symlink / "evil.log"))
+        error_msg = str(exc_info.value)
+        assert "outside allowed directories" in error_msg
+
+
+def test_validate_log_path_rejects_prefix_bypass(_mock_settings: None) -> None:
+    """Paths that are similar to allowed paths but outside (e.g., /var/log_evil) are rejected."""
+    with pytest.raises(ValueError) as exc_info:
+        validate_log_path("/var/log_evil/somefile.log")
+    error_msg = str(exc_info.value)
+    assert "outside allowed directories" in error_msg
+
+
+def test_validate_log_path_returns_unchanged_value(_mock_settings: None) -> None:
+    """The returned value is exactly what was passed in."""
+    path = "/var/log/app.log"
+    result = validate_log_path(path)
+    assert result is path or result == path
+
+
+def test_validate_log_path_rejects_custom_allowed_dir_outside(
+    _mock_settings: None, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """Paths outside custom allowed directories are rejected."""
+    def mock_get_settings() -> Settings:
+        return Settings(
+            database_path=":memory:",
+            fail2ban_socket="/tmp/fake.sock",
+            fail2ban_config_dir="/tmp/fail2ban",
+            session_secret="test-secret-key-do-not-use",
+            allowed_log_dirs=["/custom/logs"],
+        )
+
+    monkeypatch.setattr("app.utils.path_utils.get_settings", mock_get_settings)
+
+    with pytest.raises(ValueError) as exc_info:
+        validate_log_path("/var/log/app.log")
+    error_msg = str(exc_info.value)
+    assert "outside allowed directories" in error_msg
+    assert "/custom/logs" in error_msg
+
+
+def test_validate_log_path_accepts_custom_allowed_dir(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Paths within custom allowed directories are accepted."""
+    def mock_get_settings() -> Settings:
+        return Settings(
+            database_path=":memory:",
+            fail2ban_socket="/tmp/fake.sock",
+            fail2ban_config_dir="/tmp/fail2ban",
+            session_secret="test-secret-key-do-not-use",
+            allowed_log_dirs=["/custom/logs"],
+        )
+
+    monkeypatch.setattr("app.utils.path_utils.get_settings", mock_get_settings)
+
+    result = validate_log_path("/custom/logs/app.log")
+    assert result == "/custom/logs/app.log"