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baserCMS CVE-2026-93464

| EUVDEUVD-2026-89663 MEDIUM
Cross-site Scripting (XSS) (CWE-79)
2026-09-30 jpcert
5.1
CVSS 4.0 · Vendor: jpcert
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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Access
    Authenticate as low-privileged CMS user
  2. Delivery
    Inject script into custom content description
  3. Exploit
    Payload stored in baserCMS
  4. Execution
    Victim views affected content
  5. Persist
    Script executes in victim browser session
  6. Impact
    Session hijack or action-on-behalf

Vulnerability Assessment

Weakness
CWE
OWASP Top 10:2025
A05:2025 - Injection
ASVS 5.0
V1 - Encoding and Sanitization Output encoding, context-aware escaping
MITRE ATT&CK

2 techniques across 2 tactics: Initial Access, Execution

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

Exploitation Conditions

Exploitation requires an authenticated account with sufficient privilege to create or edit content via the 'custom content' feature (PR:L), specifically the content description field that is stored and rendered unsanitized. Execution also requires victim user interaction (UI:R) - another user must view the malicious content in their browser for the injected script to run. This limits exploitation to attackers who already possess a low-privileged baserCMS editorial account and can lure or wait for a higher-value user (e.g. an administrator) to view the poisoned content; it is not exploitable pre-authentication or against a user who never renders the affected content.

Impact

The single available severity signal is a vendor/JVN-sourced CVSS 3.0 base score of 5.4 (AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N), a medium rating consistent with a typical stored XSS. No EPSS probability, no CISA KEV listing, and no public POC are present in the provided data, so exploitation status is 'no public exploit identified at time of analysis.' No SSVC decision data was supplied. The vector shows network reach (AV:N) and low complexity (AC:L) but is meaningfully constrained: PR:L means the attacker must already hold a low-privileged authenticated account to inject the payload, and UI:R means a victim must load the affected content for the script to fire. The scope change (S:C) with C:L/I:L reflects that script executes in the victim's browser session against the CMS origin rather than compromising the server directly. This is a real but moderate issue - a genuine stored XSS worth patching, not a critical unauthenticated RCE. Realistic risk is bounded by the authentication requirement and the limited (Low) confidentiality/integrity impact; there is no availability impact. No conflicting signals were detected between the vector and the description; both describe an authenticated, browser-side stored XSS.

Exploit ScenarioDerived

An authenticated low-privileged baserCMS content editor injects a malicious script into a custom content description field, which is stored and later rendered without proper sanitization. When an administrator or other user views that content in the CMS, the script executes in their browser session, potentially stealing session cookies or performing actions as the victim. No public proof-of-concept exploit was identified in the provided data.

This Month Plan within 30 days. Why: MEDIUM severity (CVSS 4.0-6.9).
Patch available 0.1% exploitation probability (EPSS)

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AnalysisAIv1 – Sep 30, 2026

Stored cross-site scripting in baserCMS lets an authenticated user with a low-privileged editorial account inject arbitrary script through the 'custom content' description field, which is stored and later rendered without sanitisation in other users' browsers. Exploitation requires victim interaction (the poisoned page must be viewed) and the vulnerable endpoint is reachable remotely, giving the medium-severity rating of CVSS 5.4 (CVSS:3.0/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N). No public exploit code and no confirmed active exploitation were identified at time of analysis; the issue was reported through JPCERT/CC coordination and published via JVN.

RemediationAI

The provided intelligence does not include a confirmed vendor patched version, so no vendor-released patch can be cited; consult https://basercms.net/security/JVN_14353754 and https://jvn.jp/en/jp/JVN14353754 for the vendor-advised fixed release and upgrade immediately once that version is identified. Until then, the following compensating controls are actionable and each carries a trade-off: restrict the ability to create or edit custom content to a minimal set of fully trusted accounts and remove editorial permissions from anyone who does not need them (reduces attack surface but does not eliminate the flaw if a trusted account is compromised); if the custom content feature is not in active use, disable or uninstall it or block access to its administrative endpoints at the web server/WAF layer (effective, but breaks any legitimate use and may cause broken pages referencing existing custom content); apply output encoding/HTML-escaping to the custom content description field at render time via a template-level fix if you control the theme (mitigates script execution but is a local code change that may be overwritten by upgrades); deploy a strict Content-Security-Policy without 'unsafe-inline' on administrative pages to blunt inline script execution (may break legitimate inline scripts and requires testing); and set session and authentication cookies to HttpOnly and SameSite=Lax/Strict to reduce the value of stolen session material for the low-impact C:L/I:L outcomes assessed here. Because exploitation requires an authenticated low-privileged account plus victim interaction, monitoring for unexpected edits to custom content descriptions and for admin users browsing newly modified content is a practical detective measure.

Affected VersionsENISA

basercms 0 <5.3.1
basercms 0 <5.4.1

Affected Packages

euvd basercms fix: 5.3.1, 5.4.1

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 1 package in euvd

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

Technical ContextAI · show details

The root cause is CWE-79 (Improper Neutralisation of Input During Web Page Generation, i.e. cross-site scripting) in the baserCMS custom content feature: a description field is persisted by the application and subsequently emitted into HTML without adequate output encoding or tag filtering. Because the payload is stored server-side rather than reflected, the malicious script survives across requests and executes for every user who renders the affected content, including higher-privileged administrators. The CVSS vector's PR:L reflects that the attacker needs an authenticated account able to create or edit custom content, and UI:R reflects that a second user must view the page; the S:C (scope change) component is typical of XSS, where the script executes in the origin and trust context of the victim's browser rather than the vulnerable component itself, enabling session/token theft, credential phishing, CSRF-style privileged actions, or CMS content manipulation on behalf of the victim. The CPE entry cpe:2.3:a:basercms_user_community:basercms identifies the affected product lineage; the vulnerability class is the well-understood persistent XSS pattern where the missing control is context-aware escaping (or CSP) at render time plus validation on input.

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Severity by source

Vendor (jpcert) PRIMARY 5.1 MEDIUM
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
vuln.today AI 5.4 MEDIUM

Authenticated content-editor injection (PR:L) needing a victim to view the payload (UI:R); stored XSS crosses into the browser origin (S:C) with limited C/I and no availability impact.

3.1 AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
4.0 AV:N/AC:L/AT:N/PR:L/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N

Primary rating from Vendor (jpcert).

CVSS VectorVendor: jpcert

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
None
Privileges Required
Low
User Interaction
Passive
Vulnerable system: Confidentiality
None
Vulnerable system: Integrity
None
Vulnerable system: Availability
None
Subsequent system: Confidentiality
Low
Subsequent system: Integrity
Low
Subsequent system: Availability
None

Lifecycle Timeline

2
Patch available
Sep 30, 2026 - 09:01 EUVD
CVSS changed
Sep 30, 2026 - 08:22 NVD
5.4 (MEDIUM) 5.1 (MEDIUM)

DescriptionCVE.org

Stored Cross-Site Scripting via custom content descriptions vulnerability exists in baserCMS . If this vulnerability is exploited, an arbitrary script may be executed in the user's web browser may be caused.

Threat Intelligence Overview

EPSS Score
0.1%
Top 97% exploitable
Prevalence
Low
niche open-source CMS with primarily Japanese-market deployment footprint
Data Quality
4 sources
Confidence: Medium

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity MEDIUM
CISA KEV No
Public POC No
EPSS 0.1%
Patch available Yes
Active threat No
Deciding rule
MEDIUM severity (CVSS 4.0-6.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived
Scheduled
Exploitation
None
System Exposure· derived
Open
Automatable
No
Human Impact· derived
Medium
Technical Impact
Partial

Priority Score

26
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +26
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Cross-Site Scripting occurs when an application accepts untrusted data and sends it to a web browser without proper validation or encoding.

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