Threat Intelligence Report: CVE-2026-76586

Executive Summary

CVE-2026-76586 is a business logic vulnerability in the Appointment Booking Calendar Plugin and Scheduling Plugin for WordPress, affecting versions before 1.6.3. The flaw allows unauthenticated attackers to confirm a paid appointment at a fraction of the server-side staged price, with the plugin failing to validate the actual amount paid against the expected booking price at the point of payment confirmation. Successful exploitation results in financial loss to the booking operator, bypass of payment enforcement mechanisms, and potential service abuse — appointments can be secured without legitimate payment.

Technical Analysis

The vulnerability resides in the payment confirmation flow of the Appointment Booking Calendar Plugin and Scheduling Plugin for WordPress (versions prior to 1.6.3). The NVD description indicates the plugin does not verify the amount actually paid against the server-side price staged for a booking during the payment confirmation step. This is a classic price/amount manipulation vulnerability belonging to the CWE-840 (Business Logic Errors) class.

The attack mechanism is as follows:

1. Payment Staging: The plugin generates a booking record with a server-side price stored in the database (e.g., in a session, post meta, or a transient value). This price is the authoritative amount that should be charged for the appointment.

2. Payment Gateway Flow: The booking proceeds through an online payment gateway. At some point, the gateway returns a confirmation callback (webhook or postback) indicating a payment has been made.

3. Verification Gap: During the confirmation handling, the plugin fails to compare the amount transmitted by the payment gateway (i.e., the actual amount charged/paid) against the stored server-side price for that booking. Instead, it likely trusts the booking status or some client-supplied confirmation flag.

4. Result: An attacker can intercept or manipulate the payment flow — for example by altering the amount parameter sent to the gateway, modifying the return/confirmation request, or replaying a confirmation response with a low amount — to set the booking to "paid" without paying the full price.

The attack surface is the WordPress front-end booking interface and the payment callback/confirmation endpoint. Because the vulnerability is triggered during payment confirmation, no authentication is required; the attacker is a regular unauthenticated visitor to the site.

Note on technical specifics: The NVD description does not disclose function names, exact endpoints, or the payment gateway integration details. The description is deliberately generic, and no further implementation-specific information is available from this CVE record.

Exploitation Scenarios

Reconnaissance / Target Identification:
An attacker scans for WordPress sites running the vulnerable plugin. Shodan dorks such as "appointment booking calendar" wp-content/plugins can identify installations. An alternative approach is crawling WordPress sites and looking for booking-enabled pages. Once identified, the attacker determines the plugin version (e.g., via readme.txt, plugin version meta tags, or asset fingerprints).

Triggering the Flaw Class:
The attacker navigates to the public booking form and goes through the normal booking flow to select a slot/time for an appointment. During the checkout/payment step, the attacker intercepts the payment request before it reaches the payment gateway.

Payload / Execution at Vulnerability-Class Level:
The specific exploitation technique depends on how the payment confirmation is processed. Two plausible attack patterns (at the class level, not implementation-specific) are:

• Amount Parameter Modification: The attacker modifies the amount or price parameter in the payment initialization/redirect request, submitting a negligible sum (e.g., $0.01) instead of the legitimate booking fee.
• Callback Forgery/Interception: The attacker intercepts or forges the payment gateway's confirmation callback, sending a success status along with a bogus transaction ID while omitting or falsifying the actual amount, causing the plugin to mark the booking as fully paid.

Because the plugin does not cross-verify the paid amount against the server-side staged price, the booking is approved as "paid" despite the insufficient funds.

Post-Exploitation Impact:

• The attacker obtains a confirmed, paid-status appointment for a fraction of the real price.
• The site operator suffers direct financial loss equal to the difference between the legitimate price and the paid amount.
• The booking slot is consumed, potentially denying service to legitimate paying customers.
• If the appointment involves a resource that is limited (e.g., a clinic slot, consultation hour, or equipment rental), the attacker may create scarcity for other users.

Note on exploitation certainty: No public PoC or exploit code is available for this CVE at the time of writing. The attack chain above is inferred strictly from the vulnerability class described in the NVD entry.

Proof of Concept Intelligence

Why exploitation is still possible despite no public PoC:

1. Vulnerability Class Simplicity: Price manipulation flaws are conceptually simple to exploit. Any reasonably skilled attacker familiar with web proxying tools (e.g., Burp Suite, OWASP ZAP) can test for this flaw within minutes by intercepting and altering payment parameters.

2. No Patch Exists: With a patch confidence of 0/4 and no vendor fix announced, every running instance of the plugin before version 1.6.3 remains exploitable indefinitely.

3. WordPress Plugin Ecosystem: Automatic update behavior for WordPress plugins varies. Many site owners do not update plugins immediately, leaving a large window of vulnerable installations reachable via organic search and Shodan-style scanning.

Resources defenders should monitor for future PoC disclosure:

• NVD/MITRE CVE updates — any new details could surface as the CVE record matures.
• ExploitDB (exploit-db.com) and Packet Storm Security (packetstormsecurity.com) — common venues for WordPress plugin exploit disclosure.
• WPScan Vulnerability Database (wpscan.com) — the WordPress-specific vulnerability disclosure platform.
• GitHub code search — research PoCs are frequently posted in public repositories. Search for CVE-2026-76586 or the plugin name.
• WordPress plugin changelog — if and when version 1.6.3 is actually released, the changelog may reference security fixes, confirming the vulnerability scope.
• Exploit-in-the-wild telemetry — monitor web application firewall (WAF) logs and managed WordPress hosting telemetry for anomalous payment parameter tampering attempts.

Impact Assessment

Business Impact:
The primary impact is financial loss to booking operators. For businesses that rely on online booking for revenue (medical clinics, salons, consultancies, rental services), this vulnerability allows an attacker to obtain services at a heavily discounted rate — or entirely free if they can set the amount to zero. The severity is high for businesses with high-value appointment fees (e.g., specialist medical consultations, legal consultations) where a single stolen appointment represents significant revenue.

Affected Systems:
All WordPress installations running the Appointment Booking Calendar Plugin and Scheduling Plugin versions below 1.6.3. Given the plugin's purpose, typical deployments include healthcare providers, education institutions, professional service firms, and appointment-based retail businesses.

Blast Radius:
Unauthenticated exploitation means no account takeover or privilege escalation is required. The attack is network-based and can be automated at scale. The vulnerability compromises data integrity (financial records inaccurately reflect payment) and has a direct availability impact on business resources (consumed appointment slots). Confidentiality is not directly impacted.

CVSS Context:
The CVE record does not provide a CVSS vector or score (N/A). However, based on the class characteristics: unauthenticated access (AV:N, PR:N, UI:N), integrity impact (booking status falsified), and availability impact (slot consumption), a reasonable severity would likely fall in the 7.5–8.5 range under CVSS v3.1 if scored. This estimate is analytical and not from an authoritative source.

Detection Guidance

Sigma Rule

title: Suspicious Booking Payment AMOUNT Modification Attempt
id: 4f7c3a2d-9b8e-4f1a-8e3c-7d2b5a9c1e0f
status: experimental
description: Detects HTTP requests targeting WordPress booking plugin endpoints where the payment amount parameter is suspiciously low or absent during checkout/confirmation.
author: Valters IT Threat Intel Team
date: 2026/01/15
logsource:
  category: webserver
  product: apache
  detection:
    selection:
      cs-method: 'POST'
      cs-uri-query|contains:
        - 'booking'
        - 'checkout'
        - 'payment'
        - 'confirm'
        - 'appointment'
      sc-status: 200
    filter_legit:
      cs-uri-query|contains:
        - 'amount=0.0'
        - 'price=0'
        - 'amount=0'
        - 'total=0'
        - 'amount=1'
        - 'amount=0.01'
        - 'price=1'
        - 'price=0.01'
    condition: selection and filter_legit
  falsepositives:
    - Legitimate free trials or zero-cost bookings if the business offers promotional free appointments
    - Price display errors from the site itself
  level: high
  tags:
    - attack.t1190
    - cve.2026-76586
    - cwe.840

YARA Rule

rule CVE_2026_76586_WordPress_Price_Manipulation_Indicator
{
    meta:
        author = "Valters IT Threat Intel"
        description = "Detects strings and parameter names associated with attempted price manipulation in WordPress booking plugin payment flows."
        cve = "CVE-2026-76586"
        date = "2026-01-15"
        severity = "high"
    strings:
        $amount_param = /amount[\"']?\s*[:=]\s*[\"']?(0|0\.0+|0\.01|1|0\.10)/ nocase
        $price_param  = /price[\"']?\s*[:=]\s*[\"']?(0|0\.0+|0\.01|1|0\.10)/ nocase
        $booking_keyword = "booking" nocase
        $appointment_keyword = "appointment" nocase
        $payment_keyword = "payment" nocase
        $confirm_keyword = "confirm" nocase
    condition:
        (uint16(0) == 0x4D5A or filesize < 5MB) and
        any of ($amount_param) and
        any of ($booking_keyword, $appointment_keyword) and
        any of ($payment_keyword, $confirm_keyword)
}

Note: This YARA rule is designed for scanning captured HTTP payloads, log files, or memory dumps where request bodies or URIs have been logged. It should be tuned to the site's legitimate request patterns.

Suricata IDS Rule

alert http any any -> $HOME_NET any (
    msg:"CVE-2026-76586 WordPress Appointment Booking Price Tampering Attempt";
    flow:to_server,established;
    http.method;
    content:"POST";
    pcre:"/(booking|appointment|payment|checkout|confirm)/Ui";
    pcre:"/(amount|price|total|cost)=[0-9]{1,2}(\.(0|00|01|10))?&?/Ui";
    pcre:"/(amount|price|total|cost)=(0(\.0{1,2})?|1|0\.01)/Ui";
    metadata:attack_target Web_Server, created_at 2026_01_15, cve CVE-2026-76586, tag wordpress, tag booking, tag business_logic;
    reference:url, https://nvd.nist.gov/vuln/detail/CVE-2026-76586;
    sid: 20260076586;
    rev: 1;
)

Fidelity note: This rule detects the broader pattern of an HTTP POST request to a booking-related endpoint carrying an unusually low amount, price, total, or cost value. False positives may occur from legitimate promotional pricing; tune the threshold values based on your business's price range.

Elastic Detection

// EQL query for detecting low-value payment submissions to booking endpoints
sequence by source.ip with maxspan=5m
  [network where process.name == "httpd" and destination.port == 80 or destination.port == 443 and
   url.path contains "booking" or url.path contains "appointment" or url.path contains "payment" or url.path contains "checkout" and
   http.request.method == "POST"]
  [network where process.name == "httpd" and
   (wildcard(http.request.body.content, "*amount=0*") or
    wildcard(http.request.body.content, "*amount=0.0*") or
    wildcard(http.request.body.content, "*amount=0.01*") or
    wildcard(http.request.body.content, "*price=0*") or
    wildcard(http.request.body.content, "*price=1*") or
    wildcard(http.request.body.content, "*total=0*") or
    wildcard(http.request.body.content, "*total=0.01*"))
]

Splunk SPL Query

index=web* OR index=proxy* OR sourcetype=iis OR sourcetype=apache
  (uri_path="*booking*" OR uri_path="*appointment*" OR uri_path="*payment*" OR uri_path="*checkout*" OR uri_path="*confirm*")
  AND method=POST
  AND (
    (form_data="*amount=0*") OR
    (form_data="*amount=0.0*") OR
    (form_data="*amount=0.01*") OR
    (form_data="*price=0*") OR
    (form_data="*price=1*") OR
    (form_data="*total=0*") OR
    (form_data="*total=0.01*") OR
    (form_data="*cost=0*") OR
    (form_data="*cost=1*")
  )
  | stats count by src_ip, uri_path, form_data, _time
  | sort - _time
  | table _time, src_ip, uri_path, form_data, count

Wazuh OSSEC Rule

<group name="wordpress,">
  <rule id="100865" level="10">
    <decoded_as>json</decoded_as>
    <field name="url">\.*booking\.*|\.*appointment\.*|\.*payment\.*|\.*checkout\.*|\.*confirm\.*</field>
    <field name="data">\.*(amount|price|total|cost)=(0|0\.0+|0\.01|1)\.*</field>
    <description>CVE-2026-76586: Potential WordPress Appointment Booking price manipulation attempt detected</description>
    <group>attack.t1190,cwe.840,cve.2026-76586,</group>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 0/4 — UNPATCHED.

No vendor advisory, no patch, and no fixed version beyond the reference to 1.6.3 exists in tracked sources. It is possible that version 1.6.3 resolves this issue, but the version's existence and release date are not independently confirmed in the provided data.

1. Immediate Mitigations (prior to patch availability)

• Disable online payment confirmation: Configure the plugin (if settings allow) to require manual/administrative approval of all bookings. This ensures a human verifies payment amounts before the booking is confirmed as paid.
• Harden the booking page with a WAF rule: Deploy a web application firewall rule that blocks any booking/payment HTTP request containing an amount, price, total, or cost parameter whose value is below the lowest legitimate price offered by the business. This acts as a compensating control.
• Reconcile manually: If manual approval is not feasible, audit all online bookings on a daily basis for any discrepancies between the expected server-side price and the actual payment record.
• Disable the plugin temporarily: For high-value bookings (e.g., consultations above a revenue threshold), consider pausing the plugin and switching to a manual/manual-email booking workflow until a patch is confirmed.

2. Patch / Upgrade Steps

• Check for plugin updates: Verify in the WordPress admin dashboard whether version 1.6.3 or later is available for the plugin. If so, test in a staging environment and update production immediately.
• Validate the changelog: Before updating, review the plugin's changelog for security-related entries referencing "payment validation" or "amount verification." Confirm the fix actually addresses CVE-2026-76586.
• Monitor the vendor: If version 1.6.3 is not yet available, subscribe to the plugin's WordPress.org page and the vendor website to be notified when the security release lands.

3. Long-Term Hardening

• Adopt a WordPress maintenance policy: Configure automatic updates for all plugins, or at minimum weekly manual updates, to reduce the window of exposure for future vulnerabilities.
• Enforce server-side validation as a development standard: Ensure any third-party plugin with payment functionality is reviewed to confirm price verification is performed server-side from the database staged value, never trusting client-supplied or gateway-pass-through amounts without comparison.
• Enable Web Application Firewall (WAF) for all booking pages and customize rules for business-logic anomalies.
• Register the CVE in your vulnerability management program and track remediation even after a patch is available, given the lag in WordPress plugin adoption.
• Consider offline payment methods as a fallback for critical bookings where skimming a payment cannot be automated.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-76586 — Official NVD record (primary reference).
2. CWE-840: Business Logic Errors — https://cwe.mitre.org/data/definitions/840.html — Vulnerability class reference for description.
3. MITRE ATT&CK T1190 — Exploit Public-Facing Application — https://attack.mitre.org/techniques/T1190/ — Associated attack technique.
4. WPScan Vulnerability Database — https://wpscan.com/ — Monitor for future WordPress-specific advisories on this plugin (search for the plugin name or CVE ID).
5. ExploitDB — https://www.exploit-db.com — Monitor for future proof-of-concept disclosure related to this CVE.

Threat Actor / Exploit-in-the-Wild Evidence: None reported at the time of writing. No CISA KEV (Known Exploited Vulnerabilities) listing, no public PoC, and no campaign references are available in the provided data.

Report generated by Valters IT Threat Intelligence. Distribution to authorized security personnel only.