VulnerabilityPublished 27 Oct 2026

CVE-2026-50571: w3c websub · Cross-site Scripting

Affects w3c websubSource CVE Database V5

Not known to be exploited · CVSS 5.7 Medium.

Data as of 27 Oct 2026, 23:12 UTC

CVSS 3.1 base score

5.7Medium

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

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: Required
Scope: Unchanged
Confidentiality: None
Integrity: High
Availability: None

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 27 Oct 2026
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 26 Oct 2026.

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 or the CVSS exploit maturity 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

Affected software

Product	Affected	Fixed in
w3cwebsub	Affected =REC-websub-20180123	Fixed in–

AI-suggested

Mitigation requires subscribers to enforce safe response media typing when echoing the hub.challenge value and to set the HTTP header X-Content-Type-Options to nosniff to prevent MIME sniffing. These measures prevent execution of malicious scripts embedded in the challenge response. Until such mitigations are implemented, the vulnerability remains exploitable. Monitor vendor advisories for any future official fixes.

Description

CVE-2026-50571 is a cross-site scripting (XSS) vulnerability in the W3C WebSub recommendation. The vulnerability arises because the intent-verification protocol requires echoing the hub.challenge value without enforcing safe response media typing or disabling MIME sniffing. This allows an attacker to craft verification requests that can execute malicious scripts in the subscriber's origin context, potentially exposing private data or enabling unauthorized actions. The vulnerability can be mitigated by enforcing safe media types and setting the X-Content-Type-Options header to nosniff.

Impact

Successful exploitation can lead to reflected cross-site scripting in the subscriber's origin, allowing an attacker to execute arbitrary scripts in the context of a victim's browser session. This can result in unauthorized actions, exposure of localStorage and sessionStorage data, and abuse of browser permissions granted to the origin. The CVSS score of 5.7 indicates a medium severity impact with no confidentiality loss but high integrity impact. There is no indication of availability impact or confirmed exploitation in the wild.

Summary

The W3C WebSub specification's intent-verification protocol requires subscribers to echo back a hub.challenge value during subscription verification. However, it does not mandate safe response media typing or disabling MIME sniffing. Attackers can exploit this by initiating subscriptions that cause the subscriber to return the challenge in a way that executes reflected HTML or JavaScript in the victim's browser context. This can lead to execution of attacker-controlled scripts using the victim's session, exposing localStorage, sessionStorage, and browser permissions. Additionally, the vulnerability can bypass script-src 'self' CSP policies if MIME sniffing is allowed. Safe media typing and the X-Content-Type-Options: nosniff header are necessary to prevent these attacks. The advisory notes a demonstration of reflection but does not confirm practical impact on the demonstration site.

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

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 27 Oct 2026, 22:58 UTC

WeaknessesCWE-79

Technical details

References

Community reviews

Related threats