Decoder accepts indefinite-length items

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#230 1 comentario 0 reacciones 0 asignados Ver en GitHub

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Evaluación

Dificultad
3/5
Tiempo estimado
1-2 días
Aptitud para principiantes
58/100
Tipo de issue
Error
Claridad
Bien especificado
Estado de actividad
Tranquilo
Stack tecnológico
javascript, node.js
Área
backend, security

Línea de trabajo

Start by tracing the decoder path used by decodeFirstSync and how it handles indefinite-length items; compare that behavior with the encoder’s output and the CBOR specification linked in the issue. Add coverage for the supplied byte sequence and verify that decoding rejects it in strict or canonical mode, while checking existing tests for any intended lenient behavior.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Summary

RFC 8949 section 4.2.1 forbids indefinite-length items in the deterministic encoding profile, and DAG-CBOR forbids them outright, because the same value then has more than one valid encoding.

Specification: https://ipld.io/specs/codecs/dag-cbor/spec/

Problem

node-cbor accepts an input that its own encoder would
never produce. Decoding the witness below succeeds, and re-encoding the
decoded value with the same library yields different bytes. The library is
therefore not the inverse of itself on this input, so two peers that disagree
about whether to accept it will disagree about the value's encoding, and any
system that treats the encoding as identifying (content addressing, signature
preimages, deduplication) can be made to see two encodings of one value.

Version tested: cbor 10.0.12

Reproduction

import cbor from 'cbor';
const buf = Buffer.from('bf617007617007ff', 'hex');
const v = cbor.decodeFirstSync(buf);
const re = Buffer.from(cbor.encodeOne(v));
console.log('input   :', buf.toString('hex'));
console.log('decoded :', JSON.stringify(v, (_k, x) => typeof x === 'bigint' ? x.toString() : x));
console.log('re-encode:', re.toString('hex'));
console.log(re.equals(buf) ? 'ROUNDTRIP-STABLE' : 'REENCODE-DIFFERS');

Observed output:

input   : bf617007617007ff
decoded : {"p":7}
re-encode: a1617007
REENCODE-DIFFERS

The re-encoding differs from the input, which is the defect.

Expected behavior

The decoder should reject this input rather than accept it, because the
encoding is not the canonical one for the decoded value. A canonical encoding of the same value is 07, which round-trips byte-identically.

Suggested fix

Reject the indefinite-length initial byte outright when decoding, rather than
assembling the chunks into a definite-length value. A strict/canonical decode
mode is the usual place for this if the lenient behaviour must stay available.

Lenguaje dominante
JavaScript
Estrellas
379
Forks
73
Merge medio
7 min
PR fusionados (30 d)
1

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