[Security] Enforce untrusted-input path, resource, and rendering boundaries

[Dyu20705]

Problem statement

RunSift parses attacker-controlled log and metadata content. The process must remain bounded and non-executable even when inputs contain traversal paths, symlinks, oversized lines/files, compressed bombs, terminal control sequences, or active Markdown/HTML payloads.

Scope

• input/output root isolation;
• path traversal and unsafe symlink rejection;
• bounded file, line, decompression, evidence, and output sizes;
• bounded processing time/retry behavior where applicable;
• safe terminal and Markdown rendering;
• prohibition on shell execution/interpolation;
• minimal local retention and cleanup behavior.

Checklist

• Define configurable limits with safe defaults
• Validate canonical input/output paths remain within configured roots
• Reject unsafe symlinks and traversal attempts
• Reject or safely classify binary-like and unsupported encodings
• Enforce file-size, line-length, decompression-ratio, evidence-count, and output-size limits
• Neutralize ANSI/control sequences and active Markdown/HTML constructs
• Ensure log text is never executed, evaluated, or passed through a shell
• Use atomic writes and bounded temporary storage
• Document cleanup and retention behavior
• Add adversarial, boundary, timeout, and regression tests

Acceptance criteria

• Traversal and symlink escape attempts fail safely without reading/writing outside configured roots
• Oversized, compressed, malformed, and binary-like inputs use bounded memory/disk/time
• Untrusted text cannot alter terminal state or render active content in Markdown reports
• No code path executes or shell-interpolates input content
• Partial writes do not appear as valid completed bundles/reports
• Security boundary tests run locally and in CI
• Limits and failure behavior are documented in CLI help or user documentation

Dependencies

• Complements redaction issue [Security] Implement pre-persistence secret redaction and redaction reporting #33
• Blocks parser/report MVP completion
• Related epics: [Epic] CLI and Project Foundation #2, [Epic] Log Parsing and Failure Extraction #4, [Epic] Evidence-Based Triage Report #18, [Epic] Testing and Fixtures #20
• Release gate: [Release] Verify and publish RunSift CLI v0.1.0 MVP #35