Security hardening: sanitize MCP App CSP domains - #17192
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Filter out domain values containing whitespace, ";" or "," before interpolating them into the generated Content-Security-Policy so a malicious value cannot inject additional sources, directives, or policies. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Validate each server-supplied CSP domain by parsing it as an absolute URL, keeping only https/wss origins, and re-checking the decoded origin. This closes an encoding bypass where a percent-encoded separator (e.g. "%3B") passes a raw-string filter but decodes back into ";" downstream, and additionally rejects non-https/wss schemes and bare keyword sources. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Lock base-uri and form-action to 'none' so a sandboxed app cannot retarget <base href> or submit forms to an arbitrary destination. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
…P sources Drop a bare "*" host (which allows every origin, defeating the scheme allowlist) and reject quote/backtick characters that survive URL parsing and could break out of an HTML attribute the policy is embedded in. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
gr2m
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Jul 14, 2026
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getMCPAppCSPinpackages/react/src/mcp-apps/sandbox.tsinterpolated server-supplied domain values (connectDomains,resourceDomains,frameDomains) straight into the Content-Security-Policy string. Since CSP directives are;-separated and sources are whitespace-separated, a value could break out of its directive and inject arbitrary sources, directives, or policies.This adds a
sanitizeCSPSourcesfilter that drops any value containing whitespace,;, or,before it reaches the policy string.Behavior
Happy path:
https://api.example.com) pass through unchanged into their directives.Unhappy path:
https://api.example.test; script-src-elem https://attacker.exampleis dropped entirely, so no extra directive appears.,(which could split into a second policy) is dropped.Tests
Added
sandbox.test.tscovering valid passthrough and injection-attempt rejection.🤖 Generated with Claude Code