Skip to content

SillyTavern has a reflected XSS vulnerability in the CORS proxy middleware

Moderate severity GitHub Reviewed Published May 11, 2026 in SillyTavern/SillyTavern • Updated May 12, 2026

Package

npm sillytavern (npm)

Affected versions

<= 1.17.0

Patched versions

1.18.0

Description

Resolution

Fixed in SillyTavern 1.18.0: a user-provided URL is no longer reflected in the HTTP response body.

Overview

  • Vulnerability Type: XSS
  • Affected Location: src/middleware/corsProxy.js:40
  • Trigger Scenario: reflected XSS in CORS proxy error response

Root Cause

When fetch(url) throws, the code sends:
res.status(500).send('Error occurred while trying to proxy to: ' + url + ' ' + error).
The url value is attacker-controlled (req.params.url) and is not HTML-escaped before rendering.

Source-to-Sink Chain

  1. Source (user-controlled input)
  • Entry point: GET /proxy/:url(*)
  1. Data flow
  • Code analysis shows concrete propagation into this sink:
    • vulnerability title: Reflected XSS in CORS proxy error response
    • sink location reached by attacker-controlled input: src/middleware/corsProxy.js:40
  • The same sink behavior is confirmed by controlled execution observations.
  1. Sink (dangerous operation)
  • Sink location: src/middleware/corsProxy.js:40
  • Vulnerable behavior: reflected XSS in CORS proxy error response

Exploitation Preconditions

  1. The attacker can inject controllable content into a rendered response.
  2. The vulnerable rendering context does not apply strict output encoding/sanitization.
  3. A victim user opens the affected page or response.

Risk

This issue enables script execution in the victim context and can compromise session or data integrity.

Impact

An attacker may run arbitrary JavaScript in the victim context, steal tokens, and manipulate user-visible behavior.

Remediation

  1. Never concatenate raw user input into HTML error responses.
  2. If URL echo is required, HTML-escape it or force plain-text output.
  3. Re-enable/strengthen CSP to reduce reflected injection impact.

References

@Cohee1207 Cohee1207 published to SillyTavern/SillyTavern May 11, 2026
Published to the GitHub Advisory Database May 12, 2026
Reviewed May 12, 2026
Last updated May 12, 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 None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality Low
Integrity Low
Availability None
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:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N

EPSS score

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

CVE ID

CVE-2026-44651

GHSA ID

GHSA-xc4x-2452-5gc9

Source code

Credits

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.