CVE-2026-109319

Executive Summary

CVE-2026-109319 affects the Grav Guestbook plugin versions 0.5.0 and 0.5.1, exposing missing authorization (CWE-862) and cross-site request forgery (CSRF) flaws in admin moderation actions: delete, approve, and approveAll. Authenticated administrative users who lack the admin.guestbook permission can perform moderation actions, and external sites can exploit a moderator's active session via nonce-less GET requests to delete or approve guestbook entries. The vulnerability is unpatched, has no public PoC, and carries a CVSS score of 4.3, reflecting low integrity impact and no confidentiality or availability impact.

Technical Analysis

The Grav Guestbook plugin implements admin moderation functionality through actions delete, approve, and approveAll. Two distinct weaknesses are present:

1. Missing Authorization (CWE-862): The moderation endpoints do not verify that the authenticated user possesses the admin.guestbook permission. As a result, any authenticated admin user—even those without explicit guestbook moderation privileges—can invoke these actions. This violates the principle of least privilege and enables privilege escalation within the admin panel.

2. Cross-Site Request Forgery (CSRF): The moderation actions are accessible via GET requests and lack a nonce (CSRF token) validation. An attacker can craft a malicious link or embed a request in a third-party site. When a logged-in moderator visits the attacker-controlled page, the browser automatically sends the moderator's session cookies, causing the guestbook entry to be deleted or approved without the moderator's intent.

The attack surface is the Grav admin interface, typically exposed at /admin or a custom administrative path. The plugin's moderation routes are likely reachable via HTTP GET requests such as /admin/guestbook/delete/<id>, /admin/guestbook/approve/<id>, or /admin/guestbook/approveAll, though exact endpoint paths are not specified in the NVD description. The lack of nonce checks means no additional tokens are required, simplifying exploitation. The CVSS vector AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N indicates network exploitable, low complexity, requiring low privileges (authenticated admin), no user interaction for the authorization bypass, and low integrity impact. The CSRF component does require user interaction (the moderator must visit a malicious page), which is not reflected in the CVSS vector—likely because the primary scored vulnerability is the missing authorization.

Exploitation Scenarios

Scenario 1: Insider Threat / Privilege Escalation via Missing Authorization

1. Reconnaissance: An attacker with valid low-privileged admin credentials (e.g., a user account that has admin panel access but lacks admin.guestbook permission) identifies the Grav Guestbook plugin version via the admin interface or by inspecting publicly available plugin files.
2. Trigger: The attacker navigates directly to the moderation action URLs (e.g., /admin/guestbook/delete/<entry_id>) or crafts an HTTP GET request to the endpoint. Since the plugin fails to check the admin.guestbook permission, the request is processed.
3. Execution: The action executes without further validation. The attacker can delete arbitrary guestbook entries or approve pending entries (including spam or malicious content).
4. Post-Exploitation: The attacker can suppress legitimate feedback, approve injected spam, or manipulate the guestbook content to damage the site's reputation. The impact is limited to integrity of guestbook data.

Scenario 2: CSRF Against a Moderator

1. Reconnaissance: The attacker identifies a Grav site with the Guestbook plugin and determines that a moderator is likely logged in (e.g., by observing recent guestbook activity or through social engineering).
2. Trigger: The attacker crafts a malicious webpage containing an image tag or hidden iframe with the URL https://target-site/admin/guestbook/delete/<entry_id> or approveAll. The attacker lures the moderator to visit this page (via phishing email, forum post, etc.).
3. Execution: The moderator's browser sends the GET request with the session cookie. The plugin, lacking a nonce check, processes the request as if initiated by the moderator. No user interaction beyond page visit is required.
4. Post-Exploitation: The attacker can delete all guestbook entries (using approveAll or repeated delete requests) or approve specific entries. This can be used for vandalism, censorship, or to approve spam entries that contain malicious links.

No public PoC or exploit code is available; the attack chain is inferred from the vulnerability class and the NVD description.

Impact Assessment

The CVSS score of 4.3 (Medium) reflects:

• Confidentiality Impact (C:N): None. No data is exposed.
• Integrity Impact (I:L): Low. Attackers can delete or approve guestbook entries, altering the integrity of user-generated content. The scope is limited to the guestbook plugin.
• Availability Impact (A:N): None. The service remains operational.
• Attack Vector (AV:N): Network exploitable.
• Attack Complexity (AC:L): Low; no special conditions required.
• Privileges Required (PR:L): Low; attacker needs authenticated admin access for the authorization bypass, or a moderator's session for CSRF.
• User Interaction (UI:N): None for the authorization bypass; CSRF requires the moderator to visit a malicious page, but this is not reflected in the vector.

Business Impact: Site owners may experience reputational damage if guestbook content is manipulated (e.g., spam approval, deletion of legitimate feedback). The vulnerability can be used for targeted vandalism or to bypass moderation workflows. While the direct impact is limited, it can be a stepping stone for further attacks if the guestbook is used for user-generated content that is displayed on the public site. Affected systems are Grav CMS installations with the Guestbook plugin versions 0.5.0 and 0.5.1. The blast radius is limited to the guestbook functionality; no lateral movement or system compromise is possible.

Detection Guidance

Sigma Rule

title: Grav Guestbook Moderation Action Without Nonce
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects GET requests to Grav Guestbook moderation endpoints (delete, approve, approveAll) lacking a nonce parameter, indicating potential CSRF or unauthorized access.
author: Threat Intelligence Team
date: 2026-05-15
logsource:
  category: webserver
  product: apache
  service: access_combined
detection:
  selection:
    cs-method: 'GET'
    cs-uri-stem:
      - '/admin/guestbook/delete/*'
      - '/admin/guestbook/approve/*'
      - '/admin/guestbook/approveAll'
  filter:
    cs-uri-query|contains: 'nonce='
  condition: selection and not filter
falsepositives:
  - Legitimate admin actions that include a nonce parameter (if implemented in a future patch)
level: high

YARA Rule

rule Grav_Guestbook_Moderation_CSRF
{
    meta:
        description = "Detects Grav Guestbook plugin moderation actions without nonce validation"
        author = "Threat Intelligence Team"
        date = "2026-05-15"
        reference = "CVE-2026-109319"
        severity = "medium"
    strings:
        $action_delete = "delete" nocase
        $action_approve = "approve" nocase
        $action_approveall = "approveAll" nocase
        $nonce_check = "nonce" nocase
        $guestbook = "guestbook" nocase
    condition:
        uint16(0) == 0x504B and
        filesize < 500KB and
        ( $guestbook and ( $action_delete or $action_approve or $action_approveall ) ) and
        not $nonce_check
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109319 Grav Guestbook Moderation Action Without Nonce"; flow:to_server,established; http.method; content:"GET"; http.uri; content:"/admin/guestbook/"; nocase; pcre:"/\/admin\/guestbook\/(delete|approve|approveAll)/i"; http.uri; content:!"nonce="; nocase; reference:cve,2026-109319; classtype:attempted-admin; sid:10000001; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=5s
  [http where http.request.method == "GET" and
   http.request.uri : ("/admin/guestbook/delete/*" or "/admin/guestbook/approve/*" or "/admin/guestbook/approveAll") and
   not http.request.uri : "*nonce=*"]

Splunk SPL Query

index=web sourcetype=access_combined
| search method=GET
| search uri_path="/admin/guestbook/delete/*" OR uri_path="/admin/guestbook/approve/*" OR uri_path="/admin/guestbook/approveAll"
| search NOT uri_query="*nonce=*"
| stats count by clientip, uri_path, status, referer
| where count > 0

Wazuh OSSEC Rule

<group name="web,appsec,">
  <rule id="100200" level="10">
    <if_sid>31100</if_sid>
    <url>/admin/guestbook/delete/|/admin/guestbook/approve/|/admin/guestbook/approveAll</url>
    <method>GET</method>
    <description>CVE-2026-109319: Grav Guestbook moderation action without nonce detected from $(srcip)</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>cve_2026_109319,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate Mitigations:

   • Implement a Web Application Firewall (WAF) rule to block GET requests to /admin/guestbook/delete/*, /admin/guestbook/approve/*, and /admin/guestbook/approveAll that lack a nonce parameter. This is a temporary workaround and may break legitimate functionality if nonces are not used by the plugin.
   • Restrict access to the Grav admin panel to trusted IP addresses (e.g., via .htaccess or network ACLs) to reduce exposure.
   • Educate moderators about CSRF risks and advise them to log out of the admin panel when not in use.
   • Disable the Guestbook plugin if it is not essential for business operations.

2. Patch/Upgrade Steps:

   • No official patch is available. Monitor the Grav Guestbook plugin repository (GitHub: https://github.com/grav/grav-plugin-guestbook (returned 404 — verify manually)) for updates.
   • If a patched version is released, upgrade immediately. Until then, consider replacing the plugin with an alternative guestbook solution that implements proper authorization and CSRF protections.

3. Long-Term Hardening:

   • Implement CSRF tokens (nonces) for all state-changing actions in custom plugins.
   • Enforce role-based access control (RBAC) checks on every administrative action, verifying that the user has the required permission (e.g., admin.guestbook).
   • Use POST requests for state-changing operations instead of GET.
   • Conduct regular security audits of third-party plugins and apply the principle of least privilege to admin accounts.
   • Enable logging and monitoring for admin actions to detect anomalies.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109319
• Grav Guestbook Plugin GitHub Repository: https://github.com/grav/grav-plugin-guestbook (returned 404 — verify manually)
• MITRE ATT&CK T1190: https://attack.mitre.org/techniques/T1190/
• CWE-862: https://cwe.mitre.org/data/definitions/862.html
• CWE-352 (CSRF): https://cwe.mitre.org/data/definitions/352.html

No known threat actor campaigns or exploit-in-the-wild evidence have been observed for this CVE. The vulnerability is not listed in CISA KEV.