How to fix CVE-2026-40487?

Within 24 hours: Identify all Postiz deployments and confirm current version numbers. Within 7 days: Upgrade all Postiz instances to version 2.21.6 or later per vendor patch release. Verify patch deployment across all affected systems. Within 30 days: Conduct audit of file upload logs since deployment to identify any suspicious uploads with mismatched Content-Type headers; reset session tokens for all users as a precautionary measure.

Is Nginx affected by CVE-2026-40487?

Postiz social media scheduler versions before 2.21.6 contain a file upload validation bypass that allows authenticated users to upload malicious files disguised as benign content types, leading to stored cross-site scripting (XSS) attacks that can compromise other user accounts and hijack sessions.

CVE-2026-40487

EUVD-2026-23634 HIGH

2026-04-18 GitHub_M

XSS File Upload Nginx

8.9

CVSS 3.1

CVSS VectorNVD

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

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

Required

Scope

Changed

Confidentiality

High

Integrity

High

Availability

Low

Lifecycle Timeline

Analysis Updated

Apr 18, 2026 - 02:27 vuln.today

v2 (cvss_changed)

Re-analysis Queued

Apr 18, 2026 - 02:22 vuln.today

cvss_changed

patch_available

Apr 18, 2026 - 02:01 EUVD

Analysis Generated

Apr 18, 2026 - 01:53 vuln.today

DescriptionNVD

Postiz is an AI social media scheduling tool. Prior to version 2.21.6, a file upload validation bypass allows any authenticated user to upload arbitrary HTML, SVG, or other executable file types to the server by spoofing the Content-Type header. The uploaded files are then served by nginx with a Content-Type derived from their original extension (text/html, image/svg+xml), enabling Stored Cross-Site Scripting (XSS) in the context of the application's origin. This can lead to session riding, account takeover, and full compromise of other users' accounts. Version 2.21.6 contains a fix.

AnalysisAI

File upload validation bypass in Postiz social media scheduler (versions before 2.21.6) allows authenticated users to upload executable file types (HTML, SVG) with spoofed Content-Type headers, achieving stored XSS when nginx serves files using their original extensions. Attackers can hijack sessions and take over other user accounts. …

RemediationAI

Within 24 hours: Identify all Postiz deployments and confirm current version numbers. Within 7 days: Upgrade all Postiz instances to version 2.21.6 or later per vendor patch release. …

Analysis

Detailed AI-generated intelligence summary covering attack vector, impact assessment, affected versions, remediation steps, and indicators of compromise.

POChttps://••••••••••.com/exploit

PATCHhttps://••••••••••.com/patch

ADVISORYhttps://••••••••••.com/advisory

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CVE-2026-40487 vulnerability details – vuln.today

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