Refactor filter configuration with regex validation

- Add regex validation utility for query strings
- Update filter_config_service to use regex validation
- Add comprehensive test coverage for regex validator
- Update exception handling for validation errors
- Update documentation for tasks

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

Docs/Tasks.md

@@ -1,39 +1,3 @@
-## [MEDIUM] No CORS configuration
-
-**Where found**
-
-- `backend/app/main.py` — no CORS middleware added
-
-**Why this is needed**
-
-If frontend on different origin, cross-origin requests blocked without CORS configuration.
-
-**Goal**
-
-Add CORS middleware with proper origin whitelisting.
-
-**What to do**
-
-1. Add CORS middleware with specific origin whitelist
-2. Make configurable via environment variable
-3. Default to localhost for development
-
-**Possible traps and issues**
-
-- `allow_origins=["*"]` defeats CORS security
-- Credentials require specific origins, not wildcard
-- Missing config silently fails in browser
-
-**Docs changes needed**
-
-- Update `Docs/Deployment.md` § CORS Configuration
-
-**Doc references**
-
-- `Docs/Deployment.md`
-
----
-
 ## [MEDIUM] Input validation missing for regex patterns (ReDoS)
 
 **Where found**

backend/app/exceptions.py

@@ -256,6 +256,56 @@ class FilterInvalidRegexError(BadRequestError):
         return {"pattern": self.pattern, "error": self.error}
 
 
+class FilterRegexTooLongError(BadRequestError):
+    """Raised when a regex pattern exceeds the maximum length."""
+
+    error_code: str = "filter_regex_too_long"
+
+    def __init__(self, pattern: str, max_length: int) -> None:
+        """Initialize with the pattern and maximum allowed length.
+
+        Args:
+            pattern: The regex pattern that is too long.
+            max_length: The maximum allowed length.
+        """
+        self.pattern = pattern
+        self.max_length = max_length
+        self.actual_length = len(pattern)
+        super().__init__(
+            f"Regex pattern exceeds maximum length of {max_length} characters: "
+            f"{self.actual_length} provided"
+        )
+
+    def get_error_metadata(self) -> dict[str, str | int | float | bool | None]:
+        return {
+            "pattern_length": self.actual_length,
+            "max_length": self.max_length,
+        }
+
+
+class FilterRegexTimeoutError(BadRequestError):
+    """Raised when a regex pattern compilation times out (possible ReDoS attack)."""
+
+    error_code: str = "filter_regex_timeout"
+
+    def __init__(self, pattern: str, timeout_seconds: int) -> None:
+        """Initialize with the pattern and timeout value.
+
+        Args:
+            pattern: The regex pattern that timed out.
+            timeout_seconds: The timeout value in seconds.
+        """
+        self.pattern = pattern
+        self.timeout_seconds = timeout_seconds
+        super().__init__(
+            f"Regex pattern compilation timed out after {timeout_seconds}s "
+            f"(possible ReDoS attack). Pattern is too complex or causes catastrophic backtracking."
+        )
+
+    def get_error_metadata(self) -> dict[str, str | int | float | bool | None]:
+        return {"timeout_seconds": self.timeout_seconds}
+
+
 class JailNotFoundInConfigError(NotFoundError):
     """Raised when the requested jail name is not defined in any config file."""
 

backend/app/routers/filter_config.py

@@ -118,9 +118,15 @@ async def update_filter(
 ) -> FilterConfig:
     """Update a filter's ``[Definition]`` fields by writing a ``.local`` override.
 
-    All regex patterns are validated before writing.  The original ``.conf``
+    All regex patterns are validated before writing. Validation includes:
-    file is never modified.  Fields left as ``null`` in the request body are
+
-    kept at their current values.
+    - **Length limit**: Patterns must not exceed 1000 characters (prevents DoS)
+    - **Compilation timeout**: Pattern compilation must complete within 2 seconds
+      (prevents ReDoS attacks via catastrophic backtracking)
+    - **Syntax validation**: Patterns must be valid Python regex
+
+    The original ``.conf`` file is never modified. Fields left as ``null`` in the
+    request body are kept at their current values.
 
     Args:
         request: FastAPI request object.
@@ -135,8 +141,10 @@ async def update_filter(
 
     Raises:
         HTTPException: 400 if *name* contains invalid characters.
-        HTTPException: 404 if the filter does not exist.
+        HTTPException: 400 if any regex pattern exceeds 1000 characters.
+        HTTPException: 400 if any regex pattern times out during compilation (ReDoS).
         HTTPException: 422 if any regex pattern fails to compile.
+        HTTPException: 404 if the filter does not exist.
         HTTPException: 500 if writing the ``.local`` file fails.
     """
     return await filter_config_service.update_filter(config_dir, socket_path, name, body, do_reload=reload)
@@ -164,6 +172,13 @@ async def create_filter(
     shipped ``.conf`` files.  Returns 409 if a ``.conf`` or ``.local`` for
     the requested name already exists.
 
+    All regex patterns are validated before writing. Validation includes:
+
+    - **Length limit**: Patterns must not exceed 1000 characters (prevents DoS)
+    - **Compilation timeout**: Pattern compilation must complete within 2 seconds
+      (prevents ReDoS attacks via catastrophic backtracking)
+    - **Syntax validation**: Patterns must be valid Python regex
+
     Args:
         request: FastAPI request object.
         _auth: Validated session.
@@ -175,6 +190,8 @@ async def create_filter(
 
     Raises:
         HTTPException: 400 if the name contains invalid characters.
+        HTTPException: 400 if any regex pattern exceeds 1000 characters.
+        HTTPException: 400 if any regex pattern times out during compilation (ReDoS).
         HTTPException: 409 if the filter already exists.
         HTTPException: 422 if any regex pattern is invalid.
         HTTPException: 500 if writing fails.

backend/app/services/filter_config_service.py

@@ -21,6 +21,8 @@ from app.exceptions import (
     FilterInvalidRegexError,
     FilterNotFoundError,
     FilterReadonlyError,
+    FilterRegexTimeoutError,
+    FilterRegexTooLongError,
     JailNotFoundInConfigError,
 )
 from app.models.config import (
@@ -45,6 +47,7 @@ from app.utils.config_file_utils import (
     set_jail_local_key_sync,
 )
 from app.utils.jail_socket import reload_all
+from app.utils.regex_validator import RegexTimeoutError, validate_regex_pattern
 
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
 
@@ -231,16 +234,30 @@ def _parse_filters_sync(
 def _validate_regex_patterns(patterns: list[str]) -> None:
     """Validate each pattern in *patterns* using Python's ``re`` module.
 
+    Checks each pattern for:
+    - Length limit (max 1000 characters)
+    - Compilation timeout (2 seconds) to prevent ReDoS attacks
+    - Syntax validity
+
     Args:
         patterns: List of regex strings to validate.
 
     Raises:
+        FilterRegexTooLongError: If any pattern exceeds 1000 characters.
+        FilterRegexTimeoutError: If compilation times out (possible ReDoS).
         FilterInvalidRegexError: If any pattern fails to compile.
     """
     for pattern in patterns:
         try:
-            re.compile(pattern)
+            validate_regex_pattern(pattern)
+        except ValueError as exc:
+            # Pattern length exceeded
+            raise FilterRegexTooLongError(pattern, max_length=1000) from exc
+        except RegexTimeoutError as exc:
+            # Pattern compilation timed out
+            raise FilterRegexTimeoutError(pattern, timeout_seconds=2) from exc
         except re.error as exc:
+            # Pattern syntax error
             raise FilterInvalidRegexError(pattern, str(exc)) from exc
 
 

backend/app/utils/regex_validator.py

@@ -0,0 +1,123 @@
+"""Regex pattern validation with security checks against ReDoS attacks.
+
+Provides timeout and complexity limits to prevent catastrophic backtracking
+(ReDoS - Regular Expression Denial of Service).
+"""
+
+from __future__ import annotations
+
+import re
+import signal
+from contextlib import contextmanager
+from typing import TYPE_CHECKING
+
+import structlog
+
+if TYPE_CHECKING:
+    from collections.abc import Generator
+
+logger = structlog.get_logger()
+
+# Constants for regex validation
+MAX_REGEX_LENGTH = 1000
+REGEX_COMPILE_TIMEOUT_SECONDS = 2
+
+
+class RegexTimeoutError(Exception):
+    """Raised when regex compilation exceeds the timeout limit."""
+
+    def __init__(self, pattern: str, timeout_seconds: int) -> None:
+        """Initialize with the pattern and timeout value.
+
+        Args:
+            pattern: The regex pattern that timed out.
+            timeout_seconds: The timeout value in seconds.
+        """
+        self.pattern = pattern
+        self.timeout_seconds = timeout_seconds
+        super().__init__(
+            f"Regex pattern compilation timed out after {timeout_seconds}s "
+            f"(possible ReDoS attack): {pattern!r}"
+        )
+
+
+def validate_regex_pattern(pattern: str) -> None:
+    """Validate a regex pattern with length and timeout checks.
+
+    Validates a regex pattern by:
+    1. Checking length does not exceed MAX_REGEX_LENGTH characters
+    2. Attempting compilation with a timeout to prevent ReDoS attacks
+
+    Args:
+        pattern: The regex pattern string to validate.
+
+    Raises:
+        ValueError: If the pattern exceeds maximum length.
+        RegexTimeoutError: If compilation exceeds the timeout.
+        re.error: If the pattern is syntactically invalid.
+
+    Example:
+        >>> validate_regex_pattern(r'^[a-z]+$')  # OK
+        >>> validate_regex_pattern('a' * 1001)   # Raises ValueError
+        >>> validate_regex_pattern(r'(a+)+b')    # May raise RegexTimeoutError
+    """
+    # Check length first (fast, no timeout needed)
+    if len(pattern) > MAX_REGEX_LENGTH:
+        msg = f"Regex pattern exceeds maximum length of {MAX_REGEX_LENGTH} characters: {len(pattern)} provided"
+        logger.warning("regex_validation_length_exceeded", max_length=MAX_REGEX_LENGTH, actual_length=len(pattern))
+        raise ValueError(msg)
+
+    # Attempt compilation with timeout
+    try:
+        with _timeout_context(REGEX_COMPILE_TIMEOUT_SECONDS):
+            re.compile(pattern)
+    except TimeoutError as exc:
+        logger.warning(
+            "regex_compilation_timeout",
+            timeout_seconds=REGEX_COMPILE_TIMEOUT_SECONDS,
+            pattern_preview=pattern[:100],
+        )
+        raise RegexTimeoutError(pattern, REGEX_COMPILE_TIMEOUT_SECONDS) from exc
+
+
+@contextmanager
+def _timeout_context(timeout_seconds: int) -> Generator[None, None, None]:
+    """Context manager to enforce a timeout using signal.alarm().
+
+    Works on Unix-like systems (Linux, macOS, etc.). On Windows or other
+    platforms where signal.SIGALRM is unavailable, compilation proceeds
+    without timeout (not ideal, but graceful degradation).
+
+    Args:
+        timeout_seconds: Timeout duration in seconds.
+
+    Yields:
+        None.
+
+    Raises:
+        TimeoutError: If the timeout is exceeded.
+
+    Note:
+        This uses signal.alarm() which is only available on Unix. On Windows,
+        timeouts are not enforced (limitation of the platform).
+    """
+    # Check if signal.SIGALRM is available (Unix-like systems)
+    if not hasattr(signal, "SIGALRM"):
+        # Windows or other platforms without SIGALRM
+        # Just proceed without timeout (not ideal, but prevents crashes)
+        yield
+        return
+
+    def _timeout_handler(signum: int, frame: object) -> None:
+        raise TimeoutError("Timeout exceeded")
+
+    # Set up signal handler
+    old_handler = signal.signal(signal.SIGALRM, _timeout_handler)
+    signal.alarm(timeout_seconds)
+
+    try:
+        yield
+    finally:
+        # Always disable the alarm, even if an exception occurred
+        signal.alarm(0)
+        signal.signal(signal.SIGALRM, old_handler)

backend/tests/test_utils/test_regex_validator.py

@@ -0,0 +1,155 @@
+"""Tests for regex pattern validation with ReDoS protection."""
+
+from __future__ import annotations
+
+import re
+
+import pytest
+
+from app.utils.regex_validator import (
+    MAX_REGEX_LENGTH,
+    REGEX_COMPILE_TIMEOUT_SECONDS,
+    RegexTimeoutError,
+    validate_regex_pattern,
+)
+
+
+class TestValidateRegexPattern:
+    """Tests for validate_regex_pattern function."""
+
+    def test_valid_simple_pattern(self) -> None:
+        """Valid simple patterns should compile without error."""
+        validate_regex_pattern(r"^[a-z]+$")
+        validate_regex_pattern(r"\d{3}-\d{3}-\d{4}")
+        validate_regex_pattern(r"[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}")
+
+    def test_valid_complex_pattern(self) -> None:
+        """Valid complex patterns should compile without error."""
+        validate_regex_pattern(r"^(?:[0-9]{1,3}\.){3}[0-9]{1,3}$")
+        validate_regex_pattern(r"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$")
+        validate_regex_pattern(r"^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$")
+
+    def test_pattern_exceeds_max_length(self) -> None:
+        """Patterns exceeding MAX_REGEX_LENGTH should raise ValueError."""
+        # Create pattern exactly at max length (should work)
+        max_pattern = "a" * MAX_REGEX_LENGTH
+        validate_regex_pattern(max_pattern)
+
+        # Create pattern exceeding max length
+        too_long = "a" * (MAX_REGEX_LENGTH + 1)
+        with pytest.raises(ValueError, match="exceeds maximum length"):
+            validate_regex_pattern(too_long)
+
+    def test_pattern_far_exceeds_max_length(self) -> None:
+        """Patterns far exceeding max length should raise ValueError."""
+        too_long = "a" * (MAX_REGEX_LENGTH * 2)
+        with pytest.raises(ValueError, match="exceeds maximum length"):
+            validate_regex_pattern(too_long)
+
+    def test_invalid_syntax_raises_error(self) -> None:
+        """Patterns with invalid syntax should raise re.error."""
+        with pytest.raises(re.error):
+            validate_regex_pattern(r"[unclosed")
+
+        with pytest.raises(re.error):
+            validate_regex_pattern(r"(?P<incomplete")
+
+        with pytest.raises(re.error):
+            validate_regex_pattern(r"(unclosed")
+
+    def test_empty_pattern(self) -> None:
+        """Empty patterns should compile without error."""
+        validate_regex_pattern("")
+
+    def test_special_characters_allowed(self) -> None:
+        """Patterns with special regex characters should work."""
+        validate_regex_pattern(r"\b\w+\b")
+        validate_regex_pattern(r"(?:foo|bar|baz)")
+        validate_regex_pattern(r"(?P<name>\w+)")
+
+    def test_pattern_length_validation_is_first(self) -> None:
+        """Length validation should happen before compilation (faster)."""
+        # Create a pattern that is too long and also invalid
+        # Should raise ValueError for length, not re.error for syntax
+        too_long_and_invalid = "a" * (MAX_REGEX_LENGTH + 1) + r"[unclosed"
+        with pytest.raises(ValueError, match="exceeds maximum length"):
+            validate_regex_pattern(too_long_and_invalid)
+
+    def test_common_fail2ban_patterns(self) -> None:
+        """Common fail2ban regex patterns should validate correctly."""
+        # These are real patterns from fail2ban filters
+        common_patterns = [
+            r"^%(__prefix_line)s(?:error|warn).*Failed (?:password|publickey) for invalid user (?:[^ ]+) from <HOST>",
+            r"^%(__prefix_line)s(?:error|warn).*Connection (?:closed|reset) by (?:authenticating )?user .* <HOST>",
+            r"^%(__prefix_line)s(?:error|warn).*Invalid user (?:[^ ]+) from <HOST>",
+            r"^%(__prefix_line)s(?:error|warn).*Received disconnect from <HOST>",
+        ]
+        for pattern in common_patterns:
+            validate_regex_pattern(pattern)
+
+    def test_pattern_with_lookahead_lookbehind(self) -> None:
+        """Patterns with lookahead/lookbehind should work."""
+        validate_regex_pattern(r"(?<!abc)def")  # Negative lookbehind
+        validate_regex_pattern(r"abc(?!def)")   # Negative lookahead
+        validate_regex_pattern(r"(?<=abc)def")  # Positive lookbehind
+        validate_regex_pattern(r"abc(?=def)")   # Positive lookahead
+
+
+class TestRegexTimeoutError:
+    """Tests for RegexTimeoutError exception class."""
+
+    def test_regex_timeout_error_message(self) -> None:
+        """RegexTimeoutError should have a descriptive message."""
+        pattern = r"(a+)+b"
+        timeout_seconds = 2
+        exc = RegexTimeoutError(pattern, timeout_seconds)
+        assert f"{timeout_seconds}s" in str(exc)
+        assert "ReDoS" in str(exc)
+        assert pattern in str(exc)
+
+    def test_regex_timeout_error_attributes(self) -> None:
+        """RegexTimeoutError should store pattern and timeout."""
+        pattern = r"(a+)+b"
+        timeout_seconds = 2
+        exc = RegexTimeoutError(pattern, timeout_seconds)
+        assert exc.pattern == pattern
+        assert exc.timeout_seconds == timeout_seconds
+
+
+class TestValidateRegexPatternEdgeCases:
+    """Test edge cases and boundary conditions."""
+
+    def test_pattern_with_null_bytes(self) -> None:
+        """Patterns with null bytes should still validate (if compilable)."""
+        # This may or may not work depending on Python version
+        # but shouldn't cause a crash
+        try:
+            validate_regex_pattern("a\x00b")
+        except (ValueError, re.error):
+            pass  # Either error is acceptable
+
+    def test_pattern_with_unicode(self) -> None:
+        """Patterns with Unicode characters should work."""
+        validate_regex_pattern(r"[α-ω]+")
+        validate_regex_pattern(r"café")
+        validate_regex_pattern(r"日本語")
+
+    def test_unicode_pattern_within_length_limit(self) -> None:
+        """Unicode patterns should count by character, not bytes."""
+        # Create a long unicode pattern that's under character limit
+        unicode_pattern = "ア" * (MAX_REGEX_LENGTH - 1)
+        validate_regex_pattern(unicode_pattern)
+
+        # Exceeding character limit should fail
+        unicode_pattern_long = "ア" * (MAX_REGEX_LENGTH + 1)
+        with pytest.raises(ValueError, match="exceeds maximum length"):
+            validate_regex_pattern(unicode_pattern_long)
+
+    def test_pattern_repeated_at_boundary(self) -> None:
+        """Pattern at exact boundary should work."""
+        boundary_pattern = "a" * MAX_REGEX_LENGTH
+        validate_regex_pattern(boundary_pattern)
+
+        just_over = "a" * (MAX_REGEX_LENGTH + 1)
+        with pytest.raises(ValueError):
+            validate_regex_pattern(just_over)