Sliver Vulnerable to Pre-Auth Memory Exhaustion via NoEncoder Bypass

Summary

### Summary A specially crafted nonce routes unauthenticated requests through the NoEncoder path, where `startSessionHandler()` reads the entire request body without limits, allowing attacker-driven memory exhaustion and process crash.

### Details - `server/encoders/encoders.go`: `EncoderFromNonce()` returns NoEncoder when `nonce % 65537 == 0` (lines 254-264); NoEncoder is a passthrough (`util/encoders/nop.go:22-32`). - `server/c2/http.go`: `anonymousHandler()` routes requests with any encoder (including NoEncoder) to `startSessionHandler()` (lines 551-562). - `server/c2/http.go`: `startSessionHandler()` uses `io.ReadAll(req.Body)` without a size cap (lines 564-643), unlike the authenticated path that uses `io.LimitedReader` (`readReqBody()`, lines 708-732).

### PoC An attacker could send an HTTP POST with a nonce that is a multiple of 65537 (e.g., ?q=65537) so it is handled by startSessionHandler() with a NoEncoder, and advertise a very large Content-Length while streaming data. Because this handler uses io.ReadAll(req.Body) without a size limit, the server is expected to allocate large amounts of memory and may exhaust available RAM, leading to process termination on typical deployments.

### Impact Unauthenticated remote DoS: attacker can crash the Sliver HTTP listener, dropping all active sessions and locking out operators until restart. No credentials or non-default config required.