CVE-2026-56866
EPSS 0.5%Vendor advisories dominate; no PoC, KEV, or real-world signals; recycled lists.
What: Go http.Transport HTTP/1 CONNECT request handling flaw allowing request body injection and connection desynchronization, affecting reverse proxies and shared transports. EPSS 0.22%.
Why it matters: Affects Go standard library with potential for cross-user response poisoning in reverse proxies. Not KEV-listed; EPSS extremely low. Chatter shows Google bundling fix across multiple Apigee components (watcher, udca, redis, operators, cassandra-client, asm-istiod), indicating vendor patching underway but no public PoC or active exploitation signals.
Where it's seen: Repetitive automated security advisory posts listing CVE-2026-56866 alongside 10+ others in Apigee releases. No journalist coverage, defender questions, or exploitation reports—pure advisory noise.
RISK: MODERATE — Go stdlib impact; low EPSS; vendor patching but no active exploitation confirmed.
Description
When http.Transport sends an HTTP/1 CONNECT request with a non-empty Request.Body, it writes the body directly to the connection without framing after the request headers. If the server rejects the CONNECT request with a non-2xx keep-alive response, Transport returns the connection to the idle pool. Because CONNECT requests do not have a request body, the server may interpret the trailing body bytes as a subsequent pipelined HTTP/1.1 request on the connection, leaving the pooled connection desynchronized and causing the next caller that reuses it to read the response to the injected request. In reverse proxies (including httputil.ReverseProxy) that forward CONNECT requests through a shared Transport, this can lead to cross-user response poisoning.