CVE-2026-25791

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

• 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)

A critical operation is accessible without requiring any authentication. Typical impact: any user can invoke the privileged function.

CVE-2026-25791 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.6.12); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-25791? CVE-2026-25791 is a high-severity missing authentication for critical function vulnerability in github.com/bishopfox/sliver (go), affecting versions <= 1.6.11. It is fixed in 1.6.12. A critical operation is accessible without requiring any authentication.
2. How severe is CVE-2026-25791? CVE-2026-25791 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/bishopfox/sliver are affected by CVE-2026-25791? github.com/bishopfox/sliver (go) versions <= 1.6.11 is affected.
4. Is there a fix for CVE-2026-25791? Yes. CVE-2026-25791 is fixed in 1.6.12. Upgrade to this version or later.
5. Is CVE-2026-25791 exploitable, and should I be worried? Whether CVE-2026-25791 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-25791 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-25791? Upgrade github.com/bishopfox/sliver to 1.6.12 or later.