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82_yaml-billion-laughs.md

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id 82
title YAML billion-laughs mitigation
status ✅
summary Reject YAML anchor/alias syntax in user-supplied content before unmarshalling to prevent exponential memory expansion.

YAML billion-laughs mitigation

Goal

Prevent OOM from YAML front matter or directive bodies that use anchor/alias expansion. These create exponential memory growth during yaml.Unmarshal.

Background

gopkg.in/yaml.v3 has no alias-expansion limit. A 1 KB YAML with 8 levels of nested aliases can expand to 10^8 strings. Byte-length caps on input do not prevent this — the attack uses small input.

Prettier and remark-lint use eemeli/yaml which defaults to maxAliasCount: 100. Vale uses yaml.v2 v2.4.0 which back-ported alias-depth fixes. markdownlint avoids the issue by not parsing YAML at all. textlint (js-yaml v4) is vulnerable like mdsmith.

Legitimate Markdown front matter virtually never uses YAML anchors or aliases.

Design

Pre-scan approach

Before any yaml.Unmarshal call on user-supplied content, scan the raw bytes for YAML anchor (&) or alias (*) characters. If found, return an error diagnostic rather than proceeding to unmarshal.

func RejectYAMLAliases(data []byte) error {
    if bytes.ContainsAny(data, "&*") {
        return fmt.Errorf(
            "YAML anchors/aliases are not permitted")
    }
    return nil
}

Refinement: context-aware scan

A bare & or * can appear in YAML string values (e.g., title: "Q&A"). To reduce false positives, only reject when & or * appears in a YAML structural position:

  • & followed by a YAML identifier (anchor definition): pattern &\w
  • * followed by a YAML identifier (alias reference): pattern \*\w at the start of a value

Use a simple regex or byte scan, not a full YAML parser.

Call sites to guard

All yaml.Unmarshal sites processing user-supplied content (13 total):

  1. internal/archetype/gensection/parse.go:189 (directive YAML body)
  2. internal/rules/catalog/rule.go:415 (per-file front matter)
  3. internal/rules/requiredstructure/rule.go:220,233,556,924 (schema front matter, require directives)
  4. cmd/mdsmith/main.go:352 (query subcommand front matter)
  5. internal/config/load.go:22,35 (config file — lower risk, operator-controlled)
  6. internal/corpus/config.go:35,82 (corpus config — internal tooling)

Sites 1–4 are high priority (user-supplied .md content). Sites 5–6 are lower priority (operator- controlled config files) but should be guarded for defense in depth.

Alternative considered

Use goccy/go-yaml with yaml.WithMaxAliasesNum(100) to mirror eemeli/yaml. This is a larger dependency change. Defer unless the pre-scan produces false positives.

Tasks

  1. Add internal/lint/yamlsafe.go with RejectYAMLAliases(data []byte) error
  2. Add internal/lint/yamlsafe_test.go with tests for clean YAML, anchor YAML, alias YAML, Q&A in string values (false positive check)
  3. Guard all 11 yaml.Unmarshal call sites with a RejectYAMLAliases check before unmarshalling
  4. Add integration test: .md file with YAML anchor front matter produces a clear error diagnostic

Acceptance Criteria

  • YAML with &anchor / *alias is rejected before unmarshalling
  • Legitimate front matter with & in string values (e.g., "Q&A") is accepted
  • All 11 yaml.Unmarshal sites are guarded
  • Error message clearly states anchors/aliases are not permitted
  • All tests pass: go test ./...
  • go tool golangci-lint run reports no issues