A self-referencing YAML anchor in frontmatter crashes the process with StackOverflow; nested aliases expand exponentially

[matt-edmondson]

What's wrong

YamlDotNet resolves each alias to the same object reference, and an alias can point back at its own anchor. That means a deserialized frontmatter value can be a cyclic List<object> / Dictionary<object, object>.

YamlSerializer.ConvertValue (Frontmatter/YamlSerializer.cs:158-170) and DeepCloneValue (:139-153) copy this graph recursively, with no visited set, depth limit or node budget. TryParseYamlObject (:72-90) reaches them for every block.

• With a cycle: the recursion runs until the stack overflows. .NET cannot catch a StackOverflowException, so the whole host process dies. The existing catch (YamlException/InvalidOperationException/ArgumentException) blocks never get a chance to run.
• Without a cycle: each alias is expanded in full at every reference, so nested aliases grow exponentially ("billion laughs"). Serialization then writes the fully expanded tree back out.

Reproduction

Cycle:

Frontmatter.CombineFrontmatter("---\na: &x [1, *x]\n---\nbody\n",
    FrontmatterNaming.AsIs, FrontmatterOrder.AsIs, FrontmatterMergeStrategy.None);
// or
Frontmatter.ExtractFrontmatter("---\na: &x {b: *x}\n---\nbody\n");

The test host exited with code 134 and Stack overflow.. The stack frames alternate between YamlSerializer.ConvertValue and Enumerable.ToList / ToDictionary.

Expansion: I used a block of about 350 bytes with 6 alias levels:

a: &a [x, x, x, x, x, x, x, x, x, x]
b: &b [*a, *a, *a, *a, *a, *a, *a, *a, *a, *a]
# … up to f

• ExtractFrontmatter took about 150 ms and built 1,000,000 list nodes.
• CombineFrontmatter took about 2.3 s and returned a 15.3M-character string.
• Each extra level multiplies both figures by about 10, so 8–9 levels runs out of memory.

Why it matters

The library is built for processing markdown files in bulk, such as site generators and vault tools. One malicious or accidental file can kill the host process or exhaust its memory, and the caller has no way to catch the failure.

Suggested fix

• Reject cycles. Track the containers on the current recursion path with a ReferenceEqualityComparer set in ConvertValue / DeepCloneValue. If a cycle is found, TryParseYamlObject should return false.
• Cap expansion. Enforce a total node budget, e.g. 100k. Alternatively, reject aliases outright, since frontmatter almost never uses them. One way is to scan the IParser event stream for AnchorAlias before deserializing.
• Add regression tests for a self-referencing sequence, a self-referencing mapping, and the 6-level alias bomb. Each should return quickly, leave the block unchanged or report it as unreadable, and not crash.

[matt-edmondson]

Triage

• Category: Bug (denial of service on untrusted input)
• Priority: High. A single frontmatter block such as a: &x [1, *x] kills the host process with a StackOverflowException, which cannot be caught. Nested aliases also expand exponentially and exhaust memory. The library is meant for bulk-processing markdown files, so one hostile or accidental file takes down a site generator or vault tool.
• Area / suggested assignee: ConvertValue, DeepCloneValue and TryParseYamlObject in Frontmatter/YamlSerializer.cs. Owner: @matt-edmondson
• Duplicates / in progress: not a duplicate, and no open PR covers it. It is related to Process-wide caches keep every processed document forever, so long-running hosts leak memory #148 (process-wide caches keep documents forever), since both are about resource use in long-running hosts.
• Next step: the simplest option is to reject aliases outright by scanning the IParser event stream for AnchorAlias, because frontmatter almost never uses them. Otherwise, add a ReferenceEqualityComparer visited set to catch cycles plus a node budget. Add regression tests for a self-referencing sequence, a self-referencing mapping and the 6-level alias bomb.

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