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fix(active-content): raw-html graded two inert shapes HIGH, and the fix moved a second surface

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`is_active_tag`'s URL-attribute branch was a presence test: any element carrying
`href=`/`src=`/`action=` graded HIGH regardless of where the URL pointed. An MDX
`<Card href="/en/agent-sdk/quickstart">` reaches no attacker-controlled host, and
neither does APIM policy XML's `<set-header>`. It now requires an external target
-- the rule the markdown paths have applied since 0.3.1. `<base>` left the active
name set in the same change: HTML's `<base>` has its whole affordance in an `href`
the attribute branch still catches, and APIM's attribute-less `<base />` is inert.

Measured before and after in ONE session against one corpus state, because two of
the three corpora are living and a split would mix this with re-harvest drift:

  reference-corpus  389 docs   133 -> 108   (ceiling 107)
  vendor-harvest    187 docs   100 ->  98   (ceiling  62)
  generated-notes   550 docs    90 ->  88   (ceiling  49)

96% of the achievable reduction in reference-corpus, 5% in the wiki corpora. The
two classes had to be measured TOGETHER -- alone they free 3 and 13 documents,
together 25, because a document carrying one usually carries the other.

The second surface: `neutralize` imported `is_active_tag` by name, so this would
have silently narrowed the opt-in mutator too -- and no test discriminated the two
halves, since every `neutralize:raw-html` payload stays active under any narrowing
considered. That test is written first here. The predicates are now separate
symbols; the mutator keeps defanging anything, because over-defanging is auditable
and blocks nothing while under-defanging hands a human a live construct.

Behaviour change: a document whose only finding was one of these classes now WARNs
instead of holding. Detection is unchanged -- 128/128 classes, 6/6 gaps hold.

Self-safety: reading an attribute VALUE needs a pattern the presence test lacks. It
reuses the same literal alternation so no new run shape enters the table; its
`_REDOS_PAYLOADS` row denies the `=` the pattern requires, since a unit supplying it
matches at once and never exercises the run (the lexicon's `script-tag` row is the
cautionary case). 0.031-0.046s across five attack shapes at 100_000 chars against a
2.0s bound; `docs/redos-sweep.py` reports 0 candidates of 152. An attribute the
presence test saw but the value parser cannot read counts as external -- fail secure.

`docs/rawhtml-census.py` gains a PRODUCTION row that re-measures the shipped
predicate rather than a hypothesis, so a published number and the code cannot drift
apart unnoticed. README's limitation count moves 34 -> 33.

727 passed (was 717).
This commit is contained in:
Kjell Tore Guttormsen 2026-08-11 16:56:31 +02:00
commit 736f370cfb
9 changed files with 287 additions and 72 deletions
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README.md
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@ -241,8 +241,12 @@ a green scan means safe content. The highest-impact items:
other than WARN. Technical documentation is the expensive case: it is dense in the
exact constructs the gate grades. Budget human review, or run a source you actually
trust as trusted. Re-run it yourself with [`docs/fp-sweep.py`](docs/fp-sweep.py).
Those three are the published pre-0.6.0 numbers; the raw-HTML narrowing in 0.6.0
moves them to **108**, **98** and **88** measured against current corpus state —
two of the three corpora are living, so the cells are not rewritten in place.
Method and before/after: [`docs/rawhtml-census.py`](docs/rawhtml-census.py).
**Full list — 34 items, each with the mechanism, plus the out-of-scope boundary:**
**Full list — 33 items, each with the mechanism, plus the out-of-scope boundary:**
[`docs/LIMITATIONS.md`](docs/LIMITATIONS.md). Several carry field measurements from
consumer corpora, including the false positives the URL-shape rule actually produces.