CVE-2026-41887
MediumCVSS 4.9Exploitation Probability (EPSS)
Low risk41th percentile - higher than 41% of all known CVEs
Summary
In Flarum prior to versions 1.8.16 and 2.0.0-rc.1, the patch for CVE-2023-27577 restricted LESS @import and data-uri() features only in the custom_less setting, but not in other settings registered as LESS config variables (e.g., theme_primary_color, theme_secondary_color, and keys registered via Extend\Settings::registerLessConfigVar()). These values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to inject an arbitrary @import directive into the compiled forum.css. Since the LESS parser honors @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (LFI) or trigger outbound HTTP(S) requests (SSRF). The issue is patched in versions 1.8.16 and 2.0.0-rc.1.
Risk Assessment
This can lead to disclosure of sensitive files or use of the server for SSRF attacks against internal resources. The risk affects organizations using Flarum with administrators able to modify theme settings.
Recommendation
Upgrade Flarum to version 1.8.16 or 2.0.0-rc.1 (or later). Restrict administrative permissions to the minimum necessary.
Other vulnerabilities in Flarum
See all- CVE-2026-39924Medium
Flarum before 1.8.16 contains an improper session invalidation vulnerability that allows attackers who hold a valid session token to retain full account access after a victim changes their password, because the access_tokens table is never cleared on password change events. The TokensClearer::clearPasswordTokens() function only removes rows from the password_tokens table while leaving all active session cookies and API bearer tokens intact, including long-lived RememberAccessToken entries, and administrator-forced password resets via the user update endpoint are equally ineffective at revoking attacker-held sessions.
- CVE-2026-39923High
Flarum before 1.8.16 contains a password reset token expiry bypass vulnerability that allows unauthenticated attackers to reuse expired password reset tokens by submitting them directly to the reset processing endpoint. The SavePasswordController::handle() method calls PasswordToken::findOrFail() without performing any expiry validation, allowing attackers to bypass the 24-hour token lifetime enforced only during form rendering and change any account's password to gain an authenticated session.
- CVE-2023-40033High
Flarum, an open source forum software, is vulnerable to a Blind Server-Side Request Forgery (SSRF) attack and file disclosure on the server, even with a basic user account. By exploiting the vulnerability, an attacker can manipulate the application to execute unintended actions.
Original NVD description (English source)
Flarum is open-source forum software. Prior to versions 1.8.16 and 2.0.0-rc.1, Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()). Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery). This issue has been patched in versions 1.8.16 and 2.0.0-rc.1.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS