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Sliver: Nil Pointer Dereference in tunnelCloseHandler causes panic when a reverse tunnel (rportfwd) close is attempted

High severity GitHub Reviewed Published Mar 27, 2026 in BishopFox/sliver

Package

gomod github.com/bishopfox/sliver (Go)

Affected versions

<= 1.7.3

Patched versions

None

Description

Summary

A nil pointer dereference in tunnelCloseHandler causes the handler goroutine to panic whenever a reverse tunnel (rportfwd) close is attempted. Both the legitimate close path AND the unauthorized close path dereference tunnel.SessionID where tunnel is guaranteed nil. This means rportfwd tunnels can never be cleanly closed, and any authenticated implant can trigger repeated goroutine panics.

Details

File: server/handlers/sessions.go lines 172 and 175

The function enters an else block precisely because core.Tunnels.Get(tunnelData.TunnelID) returned nil. Both conditions inside that else block then dereference tunnel.SessionID instead of rtunnel.SessionID:

} else {
    rtunnel := rtunnels.GetRTunnel(tunnelData.TunnelID)

    if rtunnel != nil && session.ID == tunnel.SessionID {      // LINE 172 — nil deref
        rtunnel.Close()
        rtunnels.RemoveRTunnel(rtunnel.ID)
    } else if rtunnel != nil && session.ID != tunnel.SessionID { // LINE 175 — nil deref
        sessionHandlerLog.Warnf("...")
    }
}

Note: The identical bug was already fixed in tunnelDataHandler at lines 124/126 (correctly uses rtunnel.SessionID), but the fix was
not applied to tunnelCloseHandler.

PoC

tunnel := GetTunnel(999)   // returns nil — no normal tunnel with this ID
// tunnel is nil here

rtunnel := GetRTunnel(999) // returns valid rtunnel owned by session-AAAA

// Both lines below panic with:
// runtime error: invalid memory address or nil pointer dereference
if rtunnel != nil && sessionID == tunnel.SessionID { ... }      // line 172
} else if rtunnel != nil && sessionID != tunnel.SessionID { ... } // line 175

Confirmed on master commit 7ac4db3fa with standalone reproducer.
Output:

PANIC on line 172 (legitimate close): runtime error: invalid memory address or nil pointer dereference
PANIC on line 175 (unauthorized close): runtime error: invalid memory address or nil pointer dereference

1
2
3

Impact

  • rportfwd tunnels cannot be closed — functional regression
  • Any authenticated implant can trigger repeated handler goroutine panics
  • rtunnel map entries leak (never cleaned up on close failure)
  • recoverAndLogPanic() prevents full server crash but silently drops the close operation

Fix

Replace tunnel.SessionID with rtunnel.SessionID on both lines:

-  if rtunnel != nil && session.ID == tunnel.SessionID {
+  if rtunnel != nil && session.ID == rtunnel.SessionID {
       rtunnel.Close()
       rtunnels.RemoveRTunnel(rtunnel.ID)
-  } else if rtunnel != nil && session.ID != tunnel.SessionID {
+  } else if rtunnel != nil && session.ID != rtunnel.SessionID {

References

@moloch-- moloch-- published to BishopFox/sliver Mar 27, 2026
Published to the GitHub Advisory Database Mar 29, 2026
Reviewed Mar 29, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

EPSS score

Weaknesses

NULL Pointer Dereference

The product dereferences a pointer that it expects to be valid but is NULL. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-c279-989m-238f

Source code

Credits

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