Sanitize Untrusted Boundaries

Treat every externally sourced value (API results, URLs, cached artifacts, filesystem-derived error strings, CLI/config inputs) as untrusted at the boundary, and apply context-specific hardening before it can be rendered, logged, persisted, or used in a subprocess.

Practical rules:

• Output encoding/sanitization (context-specific): If untrusted data may become active syntax (Markdown links, HTML, reports), sanitize to an inert form first—e.g., strip embedded newlines and block characters that would corrupt the syntax.
• Sensitive data minimization: Never emit absolute local paths (or other environment-specific secrets) in notes/details destined for user-visible reports or internal evidence. Prefer root-relative/basename-only, and when formatting errors, prefer exception class/strerror over raw exception strings that may embed paths.
• Allowlist/gate propagation: When propagating cached/derived context (e.g., subreddit context during a drill), only pass it if the current operation’s allowed sources include it.
• Secure subprocess + auth wiring: Ensure availability checks use the same auth mode as runtime, and construct subprocess commands defensively (quoting and option terminators like -- to prevent argument-injection regressions).
• Destructive actions require explicit ownership signals: Don’t delete based on filename patterns alone; require a marker in the content/metadata.

Example (Markdown-link hardening):

import re

_SAFE_MARKDOWN_LINK_SCHEMES = ("http://", "https://")
_MARKDOWN_LINK_UNSAFE_CHARS = ("(", ")", "[", "]", "\\")

def _sanitize_url_for_single_line_output(url: str) -> str:
    return re.sub(r"[\r\n]+", " ", url).strip()

def render_untrusted_url(url: str) -> str:
    if not url:
        return ""
    url = _sanitize_url_for_single_line_output(url)
    if not url.startswith(_SAFE_MARKDOWN_LINK_SCHEMES):
        return url  # inert plain text
    if any(ch in url for ch in _MARKDOWN_LINK_UNSAFE_CHARS):
        return url  # inert plain text
    return f"[{url}]({url})"  # safe HTTP(S) only

When implementing changes, add regression tests for: delimiter/active-syntax payloads, newline injection, path-leak checks, allowlist gating, and subprocess argument/host edge cases.