modelcontextprotocol / modelcontextprotocol/php-sdk
[Schema] SEP-2106: Harden SchemaValidator against external $ref SSRF and composition DoS
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- Dominant language
- PHP
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Description
Implements the security hardening portion of SEP-2106 for the MCP Spec 2026-07-28 release.
Tracked by umbrella #337.
Spec rationale
Once
$refand rich composition land ininputSchema/outputSchema, naive validators are exposed to: SSRF via external$refURIs, and DoS via pathological compositions / deep nesting / large subschema counts.
PHP SDK changes
SchemaValidatorMUST NOT auto-dereference external$refURIs (block any non-same-document reference by default).- Bound: schema depth, subschema count, validation time.
- Configurable limits with sensible defaults (mirror existing session-store DoS limit ergonomics).
- Add regression tests with adversarial schemas.
Related
- Umbrella #337
- Vocabulary sub-issue (introduces
$refsupport that this hardens)
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start at SchemaValidator and inspect the existing session-store DoS limit configuration for the intended limit ergonomics and defaults. Review current $ref handling, then add adversarial regression coverage for external references, deep nesting, large subschema counts, and validation time; done means external non-same-document references are blocked by default and the limits are configurable.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- php
- Domain
- backend-api-design, security
- Issue type
- Feature
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 48/100