CVE-2026-25791

## 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 ```bash 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**)

Sliver has DNS C2 OTP Bypass that Allows Unauthenticated Session Flooding and Denial of Service in github.com/bishopfox/sliver. NOTE: The source advisory for this report contains additional versions that could not be automatically mapped to standard Go module versions. (If this is causing false-positive reports from vulnerability scanners, please suggest an edit to the report.) The additional affected modules and versions are: github.com/bishopfox/sliver before v1.6.12.