llm-security-commons/docs/lexicon-port-divergence.md

Lexicon port divergence — commons vs the Python guard

Status: informative. Nothing here is normative and nothing here changes a data file. It records a measured disagreement between two ports of one source table, so the decision can be taken where each table is tested. Under this repository's behaviour-preservation invariant, a divergence found here is reported, not fixed.

Produced 2026-08-09. Every number below came from a command; the scripts live in the session scratchpad rather than in this repository, because executable code here would breach the charter. They are reproducible from the method column.

What was compared

Side	Artefact	Version
commons	lexicon/injection-lexicon.json	file version as committed
guard	llm-ingestion-pipeline-security src/llm_ingestion_guard/injection_lexicon.json	lexicon version 1.0, repo v0.3.4, commit 0bf0729

Both are ports of the same file: llm-security/scanners/lib/injection-patterns.mjs. The guard's JSON says so in its own note field — "Injection lexicon ported from llm-security injection-patterns.mjs. Single source of truth." Commons extracted the same table from an operator dump of that module.

That is what makes the comparison worth running. These are not two different detectors that happen to overlap; they are two transcriptions of one table, and where they disagree, they disagree about what the same source says.

Result

Measure	Method	Result
Pattern count, both sides	count entries	83 and 83
Label correspondence	match commons label to guard desc, em-dash normalised to hyphen	83/83
Regex source byte-identical	string compare	64/83
Differing regex text	string compare	19
— of those, provably equivalent	unescape commons' JS-isms (\/ → /, \uXXXX → literal) and compare for string identity	6/6 identical
— of those, behaviourally divergent	differential match-set comparison (offsets + matched text), targeted corpus per pattern family	13, each with a concrete witness input
Total input comparisons	count	401
Flags	compare declared flags	0 differences
Severity / family	commons family vs guard severity	8 differences (all hybrid)

64 identical + 6 escaping-only + 13 divergent = 83.

The 6 escaping-only pairs are equivalent by string identity after unescaping, not merely by absence of a counter-example: <\/system> vs </system>, text\/html vs text/html, (?:https?:\/\/…) vs (?:https?://…), (?:~\/)? vs (?:~/)?, and two \uXXXX-vs-literal character classes. Commons writes code points as escapes by convention; the guard's note records the same choice for non-ASCII.

The 13, and which side misses what

Divergence is not one-directional. Each side has inputs the other's pattern does not match.

Pattern	Direction	Witness input
html-obfuscation: display:none element with content	commons misses; span differs	attribute run padded past 256 chars
html-obfuscation: visibility:hidden element	span only	<div data-x="<" style="visibility:hidden">x</div>
html-obfuscation: off-screen positioned element	span only	same shape, position:absolute
html-obfuscation: zero font-size element	span only	same shape, font-size:0
html-obfuscation: zero opacity element	span only	same shape, opacity:0
html-obfuscation: zero-size overflow-hidden element	span only	same shape, height:0;overflow:hidden
html-obfuscation: injection in aria-label attribute	commons misses	<a aria-label=" + 300 × a + ignore">x</a>
markdown: suspicious instruction in link-reference comment	guard misses	[//]: # (a (b) ignore)
markdown: injection payload in link anchor text	guard misses	[click ignore this](http://e.com/a(b))
sub-agent: spawn instruction with dangerous capability keywords	guard misses	create an agent that + 20 filler words + execute
sub-agent: delegation with safety bypass instruction	guard misses	delegate to a new agent + 200 chars + bypass
hybrid-xss: <script> tag in content (agent context XSS)	commons misses; span differs	<script>alert(1) (unclosed), <script src=x.js>
hybrid-xss: iframe with executable src (agent context XSS)	guard misses	<iframe data-x="<" src="javascript:alert(1)">

"Span only" means both sides produce a match on the same input but over different extents — the guard's match starts at an interior <. Whether that matters depends on whether a consumer reports offsets or evidence text; it does not change whether a finding is raised.

The commons-side misses were confirmed in a real JS engine (Node v25.8.2, RegExp built from the committed JSON), not only in the Python harness used for the differential.

Why they diverge: two different ReDoS mitigations of one table

This is not drift, and framing it as a bug in either repository would be wrong.

Both ports have been hardened against catastrophic backtracking, by different strategies:

• The Node side bounds the run. [^"]{0,256}, [^>]{1,256}. Cost: an attacker who pads the attribute past 256 characters falls out of the pattern.
• The guard excludes the anchor character. [^><], [^\]\[], [^)(]. Cost: content that legitimately contains that character stops matching.

Every divergence on the guard's side is documented at source, and traceable to the commit that introduced it. An earlier draft of this file claimed the two sub-agent bounds were not; that was wrong, and it was wrong because the search behind it never looked outside the CHANGELOG. Both mechanisms are named in lexicon.py's own module docstring:

Bounded token gaps — the two sub-agent patterns whose seed form nested .*? are ported with (?:\S+\s+){0,N}?.

Anchor exclusion — […] Measured across all 83 patterns arm by arm, two markdown patterns had this defect; both now exclude the anchor character from the run.

git log -S separates the two: the eight [^><] patterns (six html-obfuscation, two hybrid-xss) arrived with cff0437, "fix(output): 19 quadratic regex runs on the output path" — the v0.3.2 campaign, whose CHANGELOG describes exactly this remedy ("exclude the character that opens the pattern's own anchor ([ for markdown, < for tags)") across a sweep of "150 patterns across 11 tables". The {0,12} / {0,120} sub-agent bounds are older still: they arrived with the original port commit f397cd9, so they were never a divergence introduced later — they are how that table was transcribed in the first place.

The v0.3.4 entry also states the measured recall cost, naming precisely the two exceptions this comparison rediscovered: "URLs containing a literal ( inside a markdown link target and comment bodies containing a literal ( before the keyword."

Worth recording, because it anticipates the criticism the Node side invites: the guard considered bounding those runs and rejected it, on the grounds that "the content is attacker-controlled, so padding past a bound would be a one-line bypass." That is the same objection the {0,256} witness above demonstrates against the Node table. The two projects reached opposite conclusions from the same reasoning, which is the substance of the disagreement — not an oversight on either side.

Neither strategy is free, and neither is obviously right. That is the decision the two owning repositories have to take, and it is not commons' to take for them.

Severity: the 8 hybrid patterns

Commons records the hybrid family with severity: null and a note that the seed dump did not supply it, so a consumer MUST NOT assume one. The guard's port assigns all eight high.

This has deliberately not been copied into commons. The guard's port is a second-hand transcription, not the producing module; adopting its value would convert a documented gap into an unverified claim, which is the defect class this repository's changelog already records twice. It is reported instead, as a strong hint that the Node source assigns high, for llm-security to confirm from the module.

What this does not show

• Not that either runtime misses an attack. All witness payloads were re-tested against every pattern table commons holds (111 compiled rules: 83 lexicon, 28 from active-content.json and secret-egress.json). All of them still produce a finding, via active-content: constructs.raw-html. The divergence is in the finding set — which labels are raised — not in whether anything is raised at all. That still matters, because a finding set is exactly what a conformance/expected.json would encode.
• Not that llm-security behaves as described here. Only its extracted pattern table was available. Whether the Node engine runs an active-content table that compensates cannot be determined from this repository.
• Not dump-to-module fidelity. Every check above proves the two ports agree or disagree. That either matches injection-patterns.mjs remains llm-security's assertion, reproducible only in a session with read access to it.
• Not exhaustive. The corpus is targeted per pattern family, 401 comparisons. Absence of a witness proves nothing except for the 6 escaping-only pairs, which are settled by string identity rather than by the corpus.

Consequence for conformance/

An expected.json names findings. Naming a finding needs a stable id, and the two runtimes do not have one: the same pattern is override:ignore-previous in the guard and override: ignore previous instructions in the Node table. The guard's JSON happens to carry both — id and desc — which is evidence that a commons-owned id is achievable rather than speculative.

So the id question is a prerequisite for the corpus, not a parallel task: until commons owns a pattern id both ports map to, no fixture can be written, including for the 64 patterns that are byte-identical. And for the 13 divergent patterns a fixture cannot be authored at all without first deciding whose recall cost is the contract.

Resolved for the first half, 2026-08-09 (operator decision). lexicon/injection-lexicon.json 0.2.0 now carries a commons-owned id per pattern, plus an aliases object naming what each seeding runtime calls it. The id was adopted verbatim from the guard's port, not invented here — that port already carried both names, so the mapping came from source data. The detection data is provably unmoved: stripping id, aliases and the new pattern_id_space block reproduces the previous committed file byte for byte (23 566 bytes, identical).

Neither runtime has agreed to it yet — both were asked by coord the same day — so the file records it as this repository's proposal rather than a ratified contract. The second half of the blocker stands: the 13 divergent patterns still have no agreed expected behaviour, so their fixtures cannot be authored even now that they can be named.