Impact

A ReDoS vulnerability in the UriTemplate class allows attackers to cause denial of service. The partToRegExp() function generates a regex pattern with nested quantifiers (([^/]+(?:,[^/]+))) for exploded template variables (e.g., {/id}, {?tags}), causing catastrophic backtracking on malicious input.

Who is affected: MCP servers that register resource templates with exploded array patterns and accept requests from untrusted clients.

Attack result: An attacker sends a crafted URI via resources/read request, causing 100% CPU utilization, server hang/crash, and denial of service for all clients.

Affected Versions

All versions of @modelcontextprotocol/sdk prior to the patched release.

Patches

v1.25.2 contains b392f02ffcf37c088dbd114fedf25026ec3913d3 the fix modifies the regex pattern to prevent backtracking.

Workarounds

- Avoid using exploded patterns ({/id}, {?tags}) in resource templates - Implement request timeouts and rate limiting - Validate URIs before processing to reject suspicious patterns