Summary

A Remote OOM (Out-of-Memory) vulnerability exists in the Sliver C2 server's mTLS and WireGuard C2 transport layer. The socketReadEnvelope and socketWGReadEnvelope functions trust an attacker-controlled 4-byte length prefix to allocate memory, with ServerMaxMessageSize allowing single allocations of up to ~2 GiB. A compromised implant or an attacker with valid credentials can exploit this by sending fabricated length prefixes over concurrent yamux streams (up to 128 per connection), forcing the server to attempt allocating ~256 GiB of memory and triggering an OS OOM kill. This crashes the Sliver server, disrupts all active implant sessions, and may degrade or kill other processes sharing the same host. The same pattern also affects all implant-side readers, which have no upper-bound check at all.

Proof of Concept

PoC Links: mtls_poc.go or Gist Version

1. Establish mTLS connection: Complete a valid TLS 1.3 handshake presenting a valid implant client certificate.
2. Negotiate yamux: Send the MUX/1 preface to enter multiplexed stream mode.
3. Open concurrent streams: Open multiple yamux streams (up to 128).
4. Send malicious length prefix: On each stream, send a 74-byte raw signature buffer followed by a 4-byte length prefix claiming 0x7FFFFFFF (2,147,483,647 bytes ≈ 2 GiB). No actual data needs to follow.
5. Result: Each stream triggers a make([]byte, 0x7FFFFFFF) allocation. With 128 concurrent streams, the server process attempts to allocate up to ~256 GiB of memory, causing the OS OOM killer to terminate the process.

Impact

• Server availability: The Sliver server process is killed. Active implant sessions are disrupted until the operator manually restarts the server.
• Host degradation: On hosts with swap enabled, the OOM event may cause swap thrashing and degrade other services sharing the same host before the process is killed.