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…oint FuzzEtreeRoundTripIdempotent asserted a one-round idempotence fixed point (f(f(x)) == f(x)). beevik/etree genuinely violates that for one infoset-invariant class: an empty CDATA section `<a><![CDATA[]]></a>` is the only XML construct that parses to a zero-length CharData node, which etree first serialises as `<a></a>` and then, on re-parse (the empty text is dropped), as `<a/>`. Both are the identical empty element, so this is a serialisation-spelling difference, not a parse divergence. Verified true positive, low severity: etree is transitive-only here (inside crewjam/saml + goxmldsig), and signature bytes on the SAML path are canonicalised by goxmldsig C14N — which always emits `<x></x>` and never self-closing — not by etree.WriteToBytes, so this flip-flop cannot reach canonicalised signature bytes. (See claude/2026-09-17-FINDING-etree-empty-cdata-nonidempotent-roundtrip.) Relax the property to a fixed point within TWO normalisation rounds (f(f(f(x))) == f(f(x))): s2 must be stable under one further round. This keeps the XSW-relevant strength — output etree cannot re-parse still fails (now checked on both rounds), and a serialiser that never reaches a fixed point (oscillation, or an infoset not stable under round-trip) still fails — while the benign spellings that settle on round 2 no longer false-positive. Adds the empty-CDATA and nested empty-CDATA inputs as permanent regression seeds. Verified: gofmt + go vet clean; seed corpus PASS (incl. the new seeds); 45s -fuzz burst PASS (143k execs, no crash). Signed-off-by: Andrei Beshkov <andrey.beshkov@gmail.com>
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What
Relaxes
FuzzEtreeRoundTripIdempotent's idempotence property from a one-round fixedpoint (
f(f(x)) == f(x)) to a two-round fixed point (f(f(f(x))) == f(f(x))), and addsthe reproducer class as permanent regression seeds.
f = serialise∘parse.Why
The rig recorded a crash on
<a><![CDATA[]]></a>. Verified true positive, low severity:node. etree serialises the element holding it as
<a></a>, but re-parsing<a></a>dropsthe empty text →
<a/>. Both are the identical empty element — a serialisation-spellingdifference, not a parse divergence. It stabilises at
<a/>from round 2 on.crewjam/saml+goxmldsig); keyorix's SAMLwrapper never calls
etree.WriteToByteson a verify path. Signature bytes are canonicalisedby goxmldsig C14N, which always emits
<x></x>and never self-closing — so thisflip-flop cannot reach canonicalised signature bytes. No XSW/signature-bypass exposure.
Full triage:
claude/2026-09-17-FINDING-etree-empty-cdata-nonidempotent-roundtrip.md.Strength preserved
The two-round form still fails on what matters for XSW:
infoset not stable under round-trip) still fails.
Only the benign infoset-invariant spellings that settle on round 2 stop false-positing.
Verification
gofmt+go vetclean.<a><![CDATA[]]></a>and nested regression seeds.-fuzzburst PASS (143k execs, no crash) against the live corpus under the new assertion.