umputun remark42 up to 1.15.x Header Content-Type cross site scripting

Summaryinfo

A vulnerability, which was classified as problematic, was found in umputun remark42 up to 1.15.x. Affected by this vulnerability is an unknown functionality of the component Header Handler. Such manipulation of the argument Content-Type leads to cross site scripting. This vulnerability is uniquely identified as CVE-2026-48788. The attack can be launched remotely. No exploit exists. You should upgrade the affected component.

Detailsinfo

A vulnerability was found in umputun remark42 up to 1.15.x. It has been rated as problematic. Affected by this issue is some unknown functionality of the component Header Handler. The manipulation of the argument Content-Type with an unknown input leads to a cross site scripting vulnerability. Using CWE to declare the problem leads to CWE-79. 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. Impacted is integrity. CVE summarizes:

Remark42 is a self-hosted comment engine for blogs, articles, or any other place where readers can add comments. Versions 1.6.0 through 1.15.0 contain a Cross-Site Scripting (XSS) vulnerability exploitable through content-type spoofing. The Remark42 image proxy fetches an arbitrary remote URL and re-serves the response from Remark42's own origin. During the download phase, the proxy determines whether the resource is an image by inspecting only the Content-Type header advertised by the remote server, never examining the actual bytes; during the serving phase, it instead derives the response Content-Type by sniffing those bytes with http.DetectContentType. An attacker can exploit this inconsistency by hosting a URL that advertises Content-Type: image/png while returning an HTML/JavaScript body: the download check accepts it as an image, the serving path sniffs the body and emits Content-Type: text/html, and the browser renders the attacker-controlled HTML/JavaScript as a document within Remark42's origin. Exploitation requires no Remark42 account on the target instance; the attacker only needs to host the malicious upstream URL and deliver the proxy link to a victim by any means, such as email, direct message, or a link on another website. This issue has been fixed in version 1.16.0.

The advisory is shared for download at github.com. This vulnerability is handled as CVE-2026-48788 since 05/22/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. Successful exploitation requires user interaction by the victim. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1059.007.

Upgrading to version 1.16.0 eliminates this vulnerability.

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Productinfo

Vendor

• umputun

Name

• remark42

Version

• 1.0
• 1.1
• 1.2
• 1.3
• 1.4
• 1.5
• 1.6
• 1.7
• 1.8
• 1.9
• 1.10
• 1.11
• 1.12
• 1.13
• 1.14
• 1.15

Website

• Product: https://github.com/umputun/remark42/

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