fix(api): RFC1918 CORS reflection opt-in via STEM_CORS_ALLOW_PRIVATE (security) - #399
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The CORS middleware reflected ANY RFC1918 private-network origin with Allow-Credentials: true — a cross-origin CSRF-bypass vector on a shared or hostile LAN (a malicious LAN page could drive credentialed requests with the victim admin's cookies). Default is now localhost + same-origin only (normal UI access is same-origin, so no impact); RFC1918 reflection is opt-in via STEM_CORS_ALLOW_PRIVATE (logs a warning when enabled). Validated RFC1918 helpers + their unit tests are retained; switched the RFC1918-allowed test to the opt-in path and added TestCorsMiddleware_RFC1918BlockedByDefault. Full internal/api suite green.
krisarmstrong
enabled auto-merge (squash)
June 7, 2026 05:19
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This was referenced Jul 8, 2026
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stem security hardening (follow-up to #398).
Problem
corsMiddlewarereflected any RFC1918 private-network origin (192.168/16,10/8,172.16-31) and setAccess-Control-Allow-Credentials: true. On a shared/hostile LAN, a malicious origin could make credentialed cross-origin requests with the victim admin's cookies — a CSRF-bypass vector (the attacker origin can also read the CSRF-token response).Fix
Secure by default: allow only localhost + same-origin (normal UI access is same-origin, so zero impact). RFC1918 reflection is now opt-in via
STEM_CORS_ALLOW_PRIVATE(env), which logs a warning when enabled — for operators who genuinely need cross-origin LAN access.Kept all the validated RFC1918 helper functions + their unit tests; just gated their use. Switched
TestCorsMiddleware_RFC1918Originto the opt-in path and addedTestCorsMiddleware_RFC1918BlockedByDefault. Fullinternal/apisuite passes.