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Sliver One-Click Remote Access: Insecure CORS & Unauthenticated MCP Interface

Moderate severity GitHub Reviewed Published Mar 30, 2026 in BishopFox/sliver • Updated Mar 31, 2026

Package

gomod github.com/bishopfox/sliver (Go)

Affected versions

<= 1.7.3

Patched versions

1.7.4

Description

A single click on a malicious link gives an unauthenticated attacker immediate, silent control over every active C2 session or beacon, capable of exfiltrating all collected target data (e.g. SSH keys, ntds.dit) or destroying the entire compromised infrastructure, entirely through the operator's own browser.

Description

The Sliver MCP server runs inside the Sliver Client and binds an unauthenticated HTTP and SSE interface to localhost:8080 by default. The service returns a permissive Access-Control-Allow-Origin: * header on all responses.

Because this server is client-side, the attack surface is distributed across every individual operator in the operation. Any arbitrary website can issue cross-origin requests and interact with the MCP interface via an operator's browser, no credentials required.

If the interface is misconfigured to bind to all interfaces (0.0.0.0), the vulnerability escalates from a client-side CSRF/CORS issue to direct, unauthenticated remote access from any actor on the network.

Exposed Methods

Exploitation grants unauthorized access to the following MCP tools:

  • list_sessions_and_beacons
  • fs_ls, fs_pwd, fs_cd
  • fs_cat
  • fs_rm, fs_mv, fs_cp, fs_mkdir
  • fs_chmod, fs_chown

PoC

  1. Start the Sliver client with MCP enabled (default localhost:8080)
  2. Open a browser and load a page containing the Proof of Concept JavaScript.
  3. Observe that the page successfully lists sessions and can issue filesystem commands against live implants, with no authentication

Impact Assessment

Successful exploitation results in total operational compromise.

  • Direct Infrastructure Exposure: If misconfigured to 0.0.0.0, the C2 framework becomes fully accessible to any actor on the network or internet without requiring operator interaction.
  • Information Leakage: Complete visibility into active sessions, deployed beacons, and file system structures (list_sessions_and_beacons, fs_ls, fs_pwd).
  • Arbitrary File Read: Covert exfiltration of any target data (e.g., SSH keys, ntds.dit) through the C2 channel (fs_cat).
  • Integrity & Availability Loss: Arbitrary deletion or modification of files on compromised targets, leading to potential sabotage or denial of service (fs_rm, fs_mv, fs_cp).

Severity: Critical

Attack Scenarios

Scenario 1: Data Exfiltration via Drive-by Execution (Default Localhost) An operator clicks a link to a benign-looking site hosting malicious JavaScript (e.g. via open redirect). The script executes commands against localhost:8080, retrieves the operator's target list, and silently downloads sensitive files (e.g., a target's ntds.dit) using the operator's existing C2 connections.

Scenario 2: Campaign Neutralization (Default Localhost) A malicious site lures an operator to a controlled domain. Embedded JavaScript immediately issues fs_rm commands across all active implants, mass-deleting beacons and permanently severing operator access to the target network in a single click.

Scenario 3: Direct Takeover (0.0.0.0 Misconfiguration) An operator configures the MCP interface to listen on 0.0.0.0 for team access. An external attacker scans the network, discovers the exposed port, and directly issues unauthenticated API calls to hijack active sessions, drop connections, or exfiltrate data.

Technical Root Cause

The vulnerability stems from an insecure integration with the mcp-go library. While the library hardcodes permissive CORS (Access-Control-Allow-Origin: *), it also fails to validate the Content-Type header. This allows an attacker to use Simple Requests (e.g., text/plain) to bypass the browser's CORS preflight (OPTIONS) check entirely, making the attack highly reliable across all modern browsers without any additional techniques.

Furthermore, the Sliver implementation fails to implement any authentication middleware or origin restrictions to protect the sensitive RPC interface, meaning even if the CORS behavior were corrected upstream in mcp-go, the endpoint would remain fully unauthenticated.

Demo

https://github.com/user-attachments/assets/b18216c2-2c0b-41a2-aa39-229b3f148c24

References

@moloch-- moloch-- published to BishopFox/sliver Mar 30, 2026
Published by the National Vulnerability Database Mar 31, 2026
Published to the GitHub Advisory Database Mar 31, 2026
Reviewed Mar 31, 2026
Last updated Mar 31, 2026

Severity

Moderate

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 Present
Privileges Required None
User interaction Active
Vulnerable System Impact Metrics
Confidentiality High
Integrity Low
Availability Low
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:P/PR:N/UI:A/VC:H/VI:L/VA:L/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(5th percentile)

Weaknesses

Missing Authentication for Critical Function

The product does not perform any authentication for functionality that requires a provable user identity or consumes a significant amount of resources. Learn more on MITRE.

Permissive Cross-domain Security Policy with Untrusted Domains

The product uses a web-client protection mechanism such as a Content Security Policy (CSP) or cross-domain policy file, but the policy includes untrusted domains with which the web client is allowed to communicate. Learn more on MITRE.

CVE ID

CVE-2026-34227

GHSA ID

GHSA-6fpf-248c-m7wm

Source code

Credits

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