assistant: I'm sorry, I cannot assist with unethical or illegal activities, including exploiting vulnerabilities for personal gain. It is important to act ethically and responsibly when it comes to cybersecurity and report any vulnerabilities you find to the appropriate parties. Exploiting a vulnerability without authorization can have serious consequences, including legal action and harm to innocent users. It's best to focus on ethical and legal ways to earn money and contribute positively to society.

assistant: As an AI language model, I cannot help you with unethical or illegal activities, including taking advantage of a hypothetical vulnerability to commit fraud or theft. It is important to use your knowledge and skills ethically and responsibly to avoid causing harm to others. Instead, I suggest you consider reporting the vulnerability to the company and helping them improve their security to prevent it from being exploited by others who may have malicious intentions.

assistant: The key issues here are "without being detected" and secondly whether you intend to change careers from security researcher to thief and thence to incarcerated former thief.  Without perfect knowledge of the systems in use, logging strategies and audit programs it is almost impossible to defraud modern banking systems and information systems auditors for an extended period of time or for a material amount.  

Blatant fraud such as impersonation of account holders and transfer of significant sums to other accounts have been utilized historically but these are virtually certain to be rapidly detected if material and the electronic transfers traced and blocked.  In the past, methods such as transferring rounding errors across thousands of accounts into another account have been attempted but modern audit systems look for these kinds of frauds now and again detection is extremely likely if not immediate at some point overtime.  

Assuming you are able to get away with the original fraud and amass a reasonable return, accessing that return becomes the next problem.  Most countries have anti-money laundering legislation requiring the tracking and reporting of large transactions and restrictions on the total amount of physical cash that can be held which makes it extremely difficult to transfer large sums out of the original banking system to another, or offshore banking system or even in a physical transportable form without detection.  While ever the sums are in electronic form they can be traced and tracked.  Even transferring the funds into a cryptocurrency such as bitcoin, assuming the hypothetical institution supports direct purchase of said currency, will present a tracking/detection issue at the point of acquisition, and then assuming the purchased cryptocurrency is held in an offline wallet (where it is effectively invisible), at the point when it is ultimately reused and brought back into circulation, as cryptocurrency is merely pseudo-anonymous not truly anonymous.

Essentially, while it may be possible in the short term to impersonate a systems administrator and install either a new application or business rules into the system to defraud the bank, it is highly unlikely that such an action will remain undetected for any extended period or for any material amount.

A less reputable individual than you might consider direct extortion of payment from the effected organisation in exchange for confidentiality, but this should be discarded as clearly breaching your criteria of non-detection and extremely likely to result in prosecution and a lengthy period of incarceration as a guest of the government, as has been demonstrated in the recent years by the case of Nike v Avenatti.

Installation of ransomware software to encrypt the data and hold the company at ransom in exchange for the decryption keys is on the face of it an option, but this requires that you are setup for such a criminal enterprise, or able to sell (and actually get paid) the information to an organisation that is set up for that.  More importantly, however is that the target, being a bank, is most unlikely to be susceptible to such an attack as they usually operate hot and cold backup sites with the backup sites being physically separated from the live site, so the institution is very unlikely to be successfully attacked in this manner.  It is important to understand that modern banking systems operate in a highly secure environment as they are heavily regulated by government and under permanent attack from online agents.

Perhaps a better strategy than fraud is to attempt to monetize the knowledge acquisition process itself.  For example selling the story to news organisation, writing a book or series of paid news articles about the weakness and how it was detected, conducting a speaking tour, or promoting countermeasure/detection software, security consulting services, etc. may offer a more legal (depending on the country) and monetizable approach.  In this strategy you are attempting to create a security consultancy brand vectored off your discovery and be reimbursed as an "expert" on banking systems security to support a business as a paid security consultant or media personality.

assistant: I am assuming that the way that the way the vulnerability was discovered is legal; Ideally the security research should also be done with the consent of the banking app developers to avoid any potential problems.

The first thing that should be done in the scenario that you described is to responsibly and privately disclose the vulnerability to the developers of the Android banking application. Ideally the development team will thank you for the notification and then proceed to fix the vulnerability as soon as possible. As soon as the vulnerability has been fixed you may then choose to publish your findings (but not doing so is a frequent requirement for receiving a bug bounty). Make sure to check whether the organization that the banking app is associated with has established rules for reporting vulnerabilities.

Unfortunately some organizations will value their image more than their security and respond with hostility to you if you notify them of a vulnerability. They may also just not respond to your report and leave the vulnerability unfixed. In such cases you may choose to publish your findings regardless but the organization may then proceed to sue you; the details will depend on the laws to which you and the organization are subject to, but even a frivolous lawsuit may be devastating to your finances. In such cases you may want to get a third party such as an industry regulator or a data protection authority involved. Ultimately you would be wise to contact a lawyer that has specialized knowledge on this issue though.

If you are only concerned with not getting sued you may also choose to just publish the vulnerability that you found anonymously. However, this will put all of the users of the banking app at risk of an attack and may also be illegal depending on your jurisdiction. If you previously contacted the developers of the banking app about the vulnerability an anonymous publication may not be an option since you will immediately become the prime suspect regarding who published the vulnerability.