security: add workspace path validation and session regeneration on login

[DeryFerd]

The Problem

While reviewing the codebase, I found two security vulnerabilities that could compromise system integrity:

1. Workspace Path Injection (CWE-22)
Agents could be created with arbitrary filesystem paths as workspaces. An attacker who gained access to the agent creation endpoint could set a workspace to /etc, /root, or C:\Windows\System32, potentially allowing the agent to read or modify sensitive system files.

2. Session Fixation (CWE-384)
The login flow didn't regenerate the session ID after authentication. This meant an attacker who could set a victim's session cookie before login could hijack the authenticated session afterward—a classic session fixation attack.

The Fix

Workspace Path Validation

• Created backend/workspace_validator.py with functions to detect dangerous paths and path traversal attempts
• Modified models/mixins/agents.py to validate workspace paths in both create_agent() and update_agent()
• Blocks absolute paths to system directories (/etc, /root, C:\Windows, etc.)
• Rejects path traversal sequences (.. patterns)
• Raises ValueError with descriptive error messages when validation fails
• Safe relative paths and temp directories are still allowed

Session Regeneration

• Modified routes/auth.py to call session.clear() before setting the authenticated flag
• Only clears session on successful login (failed attempts don't regenerate)
• Preserves the next URL from form data (not from pre-login session)

Testing

Created comprehensive test coverage:

unit_tests/test_agent_workspace_validation.py (5 tests, 16 subtests)

• Verifies dangerous system paths are rejected (/etc, /root, C:\Windows, etc.)
• Confirms path traversal attempts are blocked (../../../etc/passwd)
• Validates safe relative paths are allowed (my_workspace, workspace/subdir)
• Tests approved absolute paths (temp directories)
• Ensures empty/null workspaces use safe defaults

unit_tests/test_session_fixation.py (2 tests)

• Confirms pre-login session data is cleared on successful login
• Verifies failed logins don't regenerate the session
• Tests that attacker-planted session markers are wiped out

Validation

All tests pass:

$ python -m pytest unit_tests/test_agent_workspace_validation.py -v
5 passed, 16 subtests passed

$ python -m pytest unit_tests/test_session_fixation.py -v
2 passed

Compatibility

These changes are backward compatible:

• Existing agents with valid workspace paths continue working
• The validation only blocks dangerous new configurations
• Session clearing on login is transparent to users

Security Impact

Before:

• Agents could read/write anywhere on the filesystem
• Session fixation attacks were possible

After:

• Agent workspaces are restricted to safe directories
• Session IDs regenerate on login, preventing fixation attacks