Sliver has DNS C2 OTP Bypass that Allows Unauthenticated Session Flooding and Denial of Service

Summary

The DNS C2 listener accepts unauthenticated TOTP bootstrap messages and allocates server-side DNS sessions without validating OTP values, even when EnforceOTP is enabled. Because sessions are stored without a cleanup/expiry path in this flow, an unauthenticated remote actor can repeatedly create sessions and drive memory exhaustion.

Vulnerable Component

• server/c2/dns.go:84-90 (EnforceOTP stored but not enforced in bootstrap)
• server/c2/dns.go:378-390 (TOTP requests routed directly to bootstrap)
• server/c2/dns.go:490-521 (handleHello allocates session without OTP validation)
• server/c2/dns.go:495 (sessions.Store with no lifecycle control in this path)
• client/command/jobs/dns.go:46-52 (operator-facing EnforceOTP control implies auth gate)
• implant/sliver/transports/dnsclient/dnsclient.go:896-900 (otpMsg sends TOTP with ID=0)
• protobuf/dnspb/dns.proto:22 (documents TOTP in ID field)

Attack Vector

• Network-accessible DNS listener
• No authentication required
• Low-complexity repeated DNS query loop
• Trigger path: DNSMessageType_TOTP bootstrap handling

Proof of Concept

Preconditions

• DNS listener is reachable
• DNS C2 job is active

Reproduction Steps

1. Send repeated DNS queries with a minimal protobuf message of type TOTP.
2. Observe repeated session allocation/issuance behavior.
3. Continue requests to increase active in-memory session state.

Example

while true; do
  dig +short @<DNS_C2_IP> baa8.<parent-domain> A >/dev/null
done

baa8 is a base32 payload for a minimal TOTP-type protobuf message.

Observable Indicators

• Repeated bootstrap/session-allocation log entries from handleHello
• Rising memory usage in the Sliver server process
• Service slowdown or instability under sustained request volume

Impact

• Unauthenticated remote denial of service (availability)
• Resource exhaustion through unbounded session growth in DNS bootstrap path
• Estimated CVSS v3.1: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (7.5 High)