CVE-2026-77999 Vulnerability Analysis & Exploit Details

CVE-2026-77999 Vulnerability Summary

CVE-2026-77999: Joomla Extension - j2commerce.com - Unauthenticated PayPal callback forgery leading to order confirmation fraud in J2Store 1.0.0-3.3.21, 4.0.0-4.0.21, 4.1.0-4.1.6 - The PayPal IPN listener's signature check (`_validateIPN()`) accepted `UNVERIFIED` and any non-`INVALID` response as valid, made its verification request with `CURLOPT_SSL_VERIFYPEER` disabled, and stored its verdict in a field nothing downstream ever checked — so processing continued regardless of the outcome. Separately, the paid-amount comparison only ran when `mc_gross` was a positive number; omitting the field from the POST body (`floatval(null) == 0`) skipped the check entirely. Combined with a merchant-configured `receiver_email` and a sequential, enumerable order id read from the `custom` field, an anonymous POST was enough to move a pending order straight to `CONFIRMED` with no payment, or force another customer's pending order to `FAILED`. `paypalv2.php` performed no amount check under any circumstances.

CVE-2026-77999 References

External References

• https://www.j2commerce.com/

CWE Common Weakness Enumeration

CAPEC Common Attack Pattern Enumeration and Classification

• Manipulating Hidden Fields CAPEC-162 An adversary exploits a weakness in the server's trust of client-side processing by modifying data on the client-side, such as price information, and then submitting this data to the server, which processes the modified data. For example, eShoplifting is a data manipulation attack against an on-line merchant during a purchasing transaction. The manipulation of price, discount or quantity fields in the transaction message allows the adversary to acquire items at a lower cost than the merchant intended. The adversary performs a normal purchasing transaction but edits hidden fields within the HTML form response that store price or other information to give themselves a better deal. The merchant then uses the modified pricing information in calculating the cost of the selected items.
• Create Malicious Client CAPEC-202 An adversary creates a client application to interface with a target service where the client violates assumptions the service makes about clients. Services that have designated client applications (as opposed to services that use general client applications, such as IMAP or POP mail servers which can interact with any IMAP or POP client) may assume that the client will follow specific procedures.
• Removing Important Client Functionality CAPEC-207 An adversary removes or disables functionality on the client that the server assumes to be present and trustworthy.
• Removing/short-circuiting 'Purse' logic: removing/mutating 'cash' decrements CAPEC-208 An attacker removes or modifies the logic on a client associated with monetary calculations resulting in incorrect information being sent to the server. A server may rely on a client to correctly compute monetary information. For example, a server might supply a price for an item and then rely on the client to correctly compute the total cost of a purchase given the number of items the user is buying. If the attacker can remove or modify the logic that controls these calculations, they can return incorrect values to the server. The attacker can use this to make purchases for a fraction of the legitimate cost or otherwise avoid correct billing for activities.
• Exploitation of Trusted Identifiers CAPEC-21 An adversary guesses, obtains, or "rides" a trusted identifier (e.g. session ID, resource ID, cookie, etc.) to perform authorized actions under the guise of an authenticated user or service.
• Accessing/Intercepting/Modifying HTTP Cookies CAPEC-31 This attack relies on the use of HTTP Cookies to store credentials, state information and other critical data on client systems. There are several different forms of this attack. The first form of this attack involves accessing HTTP Cookies to mine for potentially sensitive data contained therein. The second form involves intercepting this data as it is transmitted from client to server. This intercepted information is then used by the adversary to impersonate the remote user/session. The third form is when the cookie's content is modified by the adversary before it is sent back to the server. Here the adversary seeks to convince the target server to operate on this falsified information.
• Harvesting Information via API Event Monitoring CAPEC-383 An adversary hosts an event within an application framework and then monitors the data exchanged during the course of the event for the purpose of harvesting any important data leaked during the transactions. One example could be harvesting lists of usernames or userIDs for the purpose of sending spam messages to those users. One example of this type of attack involves the adversary creating an event within the sub-application. Assume the adversary hosts a "virtual sale" of rare items. As other users enter the event, the attacker records via AiTM (CAPEC-94) proxy the user_ids and usernames of everyone who attends. The adversary would then be able to spam those users within the application using an automated script.
• Application API Message Manipulation via Man-in-the-Middle CAPEC-384 An attacker manipulates either egress or ingress data from a client within an application framework in order to change the content of messages. Performing this attack can allow the attacker to gain unauthorized privileges within the application, or conduct attacks such as phishing, deceptive strategies to spread malware, or traditional web-application attacks. The techniques require use of specialized software that allow the attacker to perform adversary-in-the-middle (CAPEC-94) communications between the web browser and the remote system. Despite the use of AiTH software, the attack is actually directed at the server, as the client is one node in a series of content brokers that pass information along to the application framework. Additionally, it is not true "Adversary-in-the-Middle" attack at the network layer, but an application-layer attack the root cause of which is the master applications trust in the integrity of code supplied by the client.
• Transaction or Event Tampering via Application API Manipulation CAPEC-385 An attacker hosts or joins an event or transaction within an application framework in order to change the content of messages or items that are being exchanged. Performing this attack allows the attacker to manipulate content in such a way as to produce messages or content that look authentic but may contain deceptive links, substitute one item or another, spoof an existing item and conduct a false exchange, or otherwise change the amounts or identity of what is being exchanged. The techniques require use of specialized software that allow the attacker to man-in-the-middle communications between the web browser and the remote system in order to change the content of various application elements. Often, items exchanged in game can be monetized via sales for coin, virtual dollars, etc. The purpose of the attack is for the attack to scam the victim by trapping the data packets involved the exchange and altering the integrity of the transfer process.
• Application API Navigation Remapping CAPEC-386 An attacker manipulates either egress or ingress data from a client within an application framework in order to change the destination and/or content of links/buttons displayed to a user within API messages. Performing this attack allows the attacker to manipulate content in such a way as to produce messages or content that looks authentic but contains links/buttons that point to an attacker controlled destination. Some applications make navigation remapping more difficult to detect because the actual HREF values of images, profile elements, and links/buttons are masked. One example would be to place an image in a user's photo gallery that when clicked upon redirected the user to an off-site location. Also, traditional web vulnerabilities (such as CSRF) can be constructed with remapped buttons or links. In some cases navigation remapping can be used for Phishing attacks or even means to artificially boost the page view, user site reputation, or click-fraud.
• Navigation Remapping To Propagate Malicious Content CAPEC-387 An adversary manipulates either egress or ingress data from a client within an application framework in order to change the content of messages and thereby circumvent the expected application logic.
• Application API Button Hijacking CAPEC-388 An attacker manipulates either egress or ingress data from a client within an application framework in order to change the destination and/or content of buttons displayed to a user within API messages. Performing this attack allows the attacker to manipulate content in such a way as to produce messages or content that looks authentic but contains buttons that point to an attacker controlled destination.

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