CVE-2026-39903 in SMF
Summary
by MITRE • 07/10/2026
Simple Machines Forum 2.1 prior to 2.1.8 and 3.0 prior to 3.0 Alpha 5 contains an authorization bypass vulnerability in Sources/Actions/AttachmentApprove.php where a single-character operator error causes the permission check to always pass regardless of user permissions. An authenticated low-privileged user can approve, reject, or delete any pending attachments on any board without holding the required approve_posts permission, bypass moderation queues for their own uploads, and enumerate and delete other users' pending attachments.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 07/10/2026
This vulnerability exists in Simple Machines Forum versions prior to 218 and 30 Alpha 5 within the AttachmentApprove.php file where a critical authorization flaw allows unauthorized privilege escalation. The issue stems from a single-character operator error that fundamentally undermines the permission validation mechanism, causing the system to always evaluate the access control check as true regardless of actual user permissions. This flaw represents a classic case of improper access control where the security boundary has been completely bypassed through a syntactic mistake in the conditional logic.
The technical implementation of this vulnerability involves a logical operator error that causes the permission verification routine to fail silently, allowing any authenticated user regardless of their role or board-specific permissions to perform administrative actions on attachments. This includes approving, rejecting, or deleting pending attachments across all boards within the forum system. The flaw operates at the core level of the moderation queue functionality, where legitimate access controls that should restrict such operations to users with specific approve_posts permissions are completely circumvented through this coding error.
The operational impact of this vulnerability is severe and multifaceted, as it enables low-privileged users to bypass the entire moderation workflow for their own uploads while simultaneously gaining the ability to manipulate attachments belonging to other forum members. This creates both a privacy concern regarding user content and an abuse potential where malicious actors could delete or modify others' pending submissions. The enumeration capability further amplifies the risk by allowing attackers to discover and target specific user attachments, potentially leading to targeted harassment or content destruction.
From a cybersecurity perspective, this vulnerability aligns with CWE-284 which addresses improper access control issues, and represents a clear violation of the principle of least privilege. The flaw also maps to ATT&CK technique T1078 which covers valid accounts and privilege escalation through unauthorized access to administrative functions. Organizations running affected versions of Simple Machines Forum face significant risk of content manipulation, user privacy violations, and potential disruption of forum operations. The vulnerability demonstrates how seemingly minor coding errors can create substantial security implications that affect the entire platform's integrity.
The recommended mitigation strategy involves immediate upgrade to Simple Machines Forum 218 or 30 Alpha 5 where this authorization bypass has been patched. Administrators should also implement additional monitoring for unusual attachment approval activities and consider temporary restriction of attachment-related permissions until the upgrade is complete. The patch addresses the root cause by correcting the operator error in the permission validation logic, restoring proper access controls and ensuring that only users with legitimate approve_posts permissions can perform the affected actions. This vulnerability underscores the critical importance of thorough code review processes, particularly for security-sensitive operations within web applications.