Remark42: Cross-Site Scripting (XSS) on /api/v1/img via content-type spoofing

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Summary

CVECVE-2026-48788
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-06-17 13:20:43 UTC
Updated2026-06-17 16:28:34 UTC
DescriptionRemark42 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.

Risk And Classification

Primary CVSS: v3.0 8.2 HIGH from [email protected]

CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

Problem Types: CWE-79 | CWE-436 | CWE-79 CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') | CWE-436 CWE-436: Interpretation Conflict

VersionSourceTypeScoreSeverityVector
3.0[email protected]Secondary8.2HIGHCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N
3.0CNADECLARED8.2HIGHCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

CVSS v3.0 Breakdown

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

CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Umputun Remark42 affected >= 1.6.0, < 1.16.0 Not specified

References

ReferenceSourceLinkTags
github.com/umputun/remark42/security/advisories/GHSA-4c8j-mgm4-qqvp 134c704f-9b21-4f2e-91b3-4a467353bcc0 github.com
github.com/umputun/remark42/releases/tag/v1.16.0 [email protected] github.com
github.com/umputun/remark42/commit/78d6de6bce1e961f023969da3ec8a00dd80c9ae8 [email protected] github.com
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