CVE-2025-62800VulnerabilityPublished 28 Oct 2025

CVE-2025-62800: jlowin fastmcp · Cross-site Scripting

Affects jlowin fastmcpSource CVE Database V5

Not known to be exploited · CVSS 5.3 Medium : upgrade to 2.13.0 when convenient.

Data as of 26 Oct 2026, 06:26 UTC

CVSS 4.0 base score

5.3Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 19% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.13.0

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: Passive
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: None
Subsequent system confidentiality: Low
Subsequent system integrity: Low
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: Passive: Successful exploitation of this vulnerability requires limited interaction by the targeted user with the vulnerable system and the attacker’s payload. These interactions would be considered involuntary and do not require that the user actively subvert protections built into the vulnerable system.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the Subsequent System.
Subsequent system integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact to the Subsequent System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 28 Oct 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-26.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade jlowin/fastmcp to 2.13.0.

Affected software

Product	Affected	Fixed in
GitHubjlowin/fastmcp`pkg:github/jlowin/fastmcp`Other threats	Affected <2.13.0	Fixed in 2.13.0

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AI-suggested

The primary mitigation is to upgrade all FastMCP deployments to version 2.13.0 or later, where the vulnerability is fixed. Organizations should audit their OAuth callback implementations to ensure that all user-supplied input is properly validated and escaped before being included in HTML responses. Implementing Content Security Policy (CSP) headers can help reduce the impact of XSS by restricting the execution of unauthorized scripts. Additionally, employing security-focused code reviews and automated scanning tools to detect improper input handling in web applications is recommended. User education about the risks of clicking suspicious links in OAuth workflows can also reduce exploitation likelihood. For critical systems, consider deploying Web Application Firewalls (WAFs) with rules to detect and block reflected XSS payloads targeting OAuth endpoints. Finally, monitor application logs and user reports for signs of suspicious activity related to OAuth authentication flows.

Description

### Summary While setting up an oauth client, it was noticed that the callback page hosted by the client during the flow embeds user-controlled content without escaping or sanitizing it. This leads to a reflected Cross-Site-Scripting vulnerability. ### Details The affected code is located in *https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/client/oauth_callback.py*, which embeds all values passed to the `create_callback_html` function via the `message` parameter it into the callback page without escaping them. This can, for example, be abused by calling the callback server with an XSS payload inside the `error` GET parameter, the value of which will then be inserted into the callback page, causing the execution of attacker-controlled JavaScript code in the callback server's origin. Note that besides the `error` parameter, other parameters reaching this function are affected too. ### PoC 1. Setup a simple fastmcp client such as this one (the callback server's port was fixated for simplicity): ``` url="http://127.0.0.1:8000/mcp" oauth = OAuth(mcp_url=url,callback_port=1337) async def main(): async with Client(url, auth=oauth) as client: await client.ping() # List available operations tools = await client.list_tools() print(f"tools: {tools}") asyncio.run(main()) ``` 2. Ensure that the MCP server located at `http://127.0.0.1:8000/mcp` supports oauth. 3. Start the client. 4. As soon as the callback server has been started, access `http://localhost:1337/callback?error=<img/src/onerror=alert(window.origin)>` Note that the exploitation could also for example be initiated by a malicious authorization server by returning the exploitation URL mentioned before in the `authorization_endpoint` field. The client would then automatically open, causing the XSS to trigger immediatly. ### Impact The impact of this XSS vulnerability is the arbitrary JavaScript execution in the victim's browser in the callback server's origin.

Impact

For European organizations, this vulnerability poses a risk primarily to web application users interacting with OAuth authentication flows built on FastMCP versions prior to 2.13.0. Successful exploitation could allow attackers to execute arbitrary JavaScript in users' browsers, potentially leading to session hijacking, theft of OAuth tokens, or unauthorized actions performed on behalf of the user. This can undermine user trust, lead to data breaches, and facilitate further attacks such as phishing or lateral movement within networks. While the vulnerability does not directly compromise server confidentiality or availability, the client-side impact can cascade into broader organizational risks, especially in sectors relying heavily on OAuth for authentication, such as finance, healthcare, and government services. The medium severity rating suggests a moderate risk level, but the ease of exploitation combined with the widespread use of OAuth means that unpatched systems could be attractive targets. European organizations must consider the regulatory implications of any data compromise under GDPR and related privacy laws.

Summary

CVE-2025-62800 is a reflected cross-site scripting (XSS) vulnerability classified under CWE-79, affecting jlowin's FastMCP framework versions earlier than 2.13.0. FastMCP is a widely used framework for building MCP applications, and the vulnerability specifically resides in the OAuth client callback page (oauth_callback.py). The flaw occurs because the application inserts user-controlled input directly into the HTML response without proper escaping or sanitization. This improper neutralization of input allows attackers to craft malicious URLs that, when visited by a user, execute arbitrary JavaScript code within the context of the vulnerable server's origin. The vulnerability does not require authentication (AV:N) and has low attack complexity (AC:L), but it does require user interaction (UI:P), such as clicking a malicious link. The impact is limited to client-side script execution, which can lead to session hijacking, credential theft, or redirection to malicious sites. The CVSS 4.0 base score is 5.3, reflecting medium severity with limited scope and no direct impact on confidentiality, integrity, or availability of the server itself. No known exploits have been reported in the wild, but the vulnerability is publicly disclosed and fixed in FastMCP version 2.13.0. Organizations using vulnerable versions should prioritize patching and review their OAuth callback implementations to ensure proper input validation and output encoding.

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #1OWASP Top 10 A05:2025: Injection (opens in a new tab)
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.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Implementation · Architecture and Design
Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages.
Architecture and Design · Implementation · Attack Surface Reduction
Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide…
OWASP Cheat Sheets
Cross Site Scripting Prevention (opens in a new tab)
DOM based XSS Prevention (opens in a new tab)
Injection Prevention (opens in a new tab)
Abuse Case (opens in a new tab)
Deserialization (opens in a new tab)
OWASP ASVS 4.0.3
V5.3.3 (opens in a new tab)
V1.5.1 (opens in a new tab)
Attack patterns
XSS Using MIME Type Mismatch (opens in a new tab)
DOM-Based XSS (opens in a new tab)
Reflected XSS (opens in a new tab)
Stored XSS (opens in a new tab)
Cross-Site Scripting (XSS) (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc (opens in a new tab)

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 5 Nov 2025, 02:11 UTC

WeaknessesCWE-79

Technical details

References

Community reviews

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