Replaces the WS-3 PR-B2a minimum precaution (`startsWith('/') &&
!startsWith('//')`) with a layered validator that rejects every input
that is not a strict relative path.
isSafeRelativePath rejects:
- Empty / null / undefined input
- Non-`/`-prefixed paths (including leading whitespace)
- Protocol-relative URLs (`//evil.com`)
- Backslash anywhere (browsers normalise `\` → `/` in some contexts;
`/\evil.com` parses as `//evil.com`)
- ASCII control characters `\x00`–`\x1F` and `\x7F` (NUL, tab, LF, CR,
DEL, etc. — header-injection vectors)
- Anything the URL constructor parses to a different origin than the
synthetic invalid origin used as the resolution base
The URL-constructor check is the authoritative guard; the prefix and
character checks are fast pre-filters that short-circuit common
attack shapes without paying the URL allocation.
Test coverage expands from 6 → 16 cases. New cases pin the
backslash, control-character, leading-whitespace, and positive-
character-set contracts. The URL-encoded-slash-in-query case
documents that we don't false-positive on `%2F` in query strings.
Closes A13-3 (open-redirect on post-login).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bert.hausmans
96cb1519de