nisesaito de ninsyoukoodo wo nyuuryokusi, akaunto no nottori ga okori kanenai.

Here で marks the place where the action happens.

• 偽サイトで認証コードを入力し = enter an authentication code on a fake site
• The action 入力する happens on/at the fake site, so で is the natural particle.

If you used に, it would not fit as well here, because に usually marks things like a destination, time, or target, not the location where 入力する is performed.

入力し is the continuative / connective form of 入力する in a more formal written style.

So:

• 入力して、... = common, neutral way to connect clauses
• 入力し、... = more formal, written, compact

In this sentence, it links the two ideas:

1. entering the authentication code on a fake site
2. account takeover may result

So it has a sense like entering the code ..., and as a result ...

〜かねない means might happen or could easily happen, usually with a negative or undesirable result.

So 起こりかねない does not just mean a neutral may happen. It has a warning nuance:

• there is a real risk that it could happen
• it might well happen, unfortunately

That is why this phrase is common in warnings, caution notices, and formal explanations.

Both can be translated as might, but the nuance is different.

• 〜かもしれない = simple possibility, fairly neutral
• 〜かねない = a worrying possibility, often used for bad outcomes

So:

• 起こるかもしれない = it may happen
• 起こりかねない = it could very well happen, and that would be bad

In a security warning like this, 〜かねない sounds more appropriate because it emphasizes danger.

Here 乗っ取り is a noun meaning takeover / hijacking.

So:

• アカウントの乗っ取り = account takeover
• literally, the takeover of an account

The particle の links the two nouns, just like of in English.

A more verb-like version would be something like:

• アカウントを乗っ取られる
• アカウントを乗っ取ること

But アカウントの乗っ取り is very compact and very common in formal writing.

Because 起こる means to occur / to happen, and the thing that occurs is アカウントの乗っ取り.

So:

• アカウントの乗っ取り = the event
• が = marks that event as the subject of 起こる
• 起こりかねない = may occur

In other words:

• What may occur?
  アカウントの乗っ取り

That is why が is used.

乗っ取り here is a noun, but it comes from the verb 乗っ取る.

• 乗っ取る = to seize, hijack, take over
• 乗っ取り = takeover, hijacking

Japanese often turns a verb stem into a noun in this way, especially in formal or technical language.

So in this sentence, 乗っ取り is not acting as a verb. It is the noun takeover.

Good catch. In standard modern Japanese, learners will usually see and write:

• 起こりかねない

The spelling 起り is a shortened okurigana variant. It is not the usual form most learners are taught first.

So for study purposes, it is safest to think:

• 起りかねない = a variant of 起こりかねない

If you are writing Japanese yourself, 起こりかねない is the better choice in most cases.

Japanese often leaves the subject unstated when it is clear from context.

In this sentence, the understood subject changes by clause:

• In 偽サイトで認証コードを入力し, the person doing the entering is the implied user/victim.
• In アカウントの乗っ取りが起こりかねない, the subject is explicitly アカウントの乗っ取り.

So English often needs to make things more explicit, but Japanese does not.

The comma helps separate the two parts of the sentence and makes the warning easier to read.

Japanese commas are often used more flexibly than English commas. In formal notices, they are very common after a connected clause like this:

• 入力し、アカウントの乗っ取りが起こりかねない。

Also, the spaces in your example are not normal Japanese spacing. They were probably added just to help learners read the sentence. In ordinary Japanese writing, it would usually appear without spaces.

It is a formal written warning style, the kind you might see in:

• security notices
• phishing warnings
• help pages
• official announcements

Signs of that style include:

• 認証コード instead of a simpler everyday wording
• 入力し instead of 入力して
• 乗っ取り
• 起こりかねない

So this is not casual conversation Japanese. It sounds like written explanatory or cautionary language.