A possible effect of malicious code is data theft, and it is only the start — malicious code can also encrypt your files and demand ransom, hand an attacker remote control of the machine, quietly drain computing resources, corrupt or delete data, and spread itself across an entire network. When people ask what is a possible effect of malicious code, they are usually looking for a single answer for a quiz; the honest answer is that the effects span from silent, months-long spying to instant, business-halting destruction. Understanding the range is what lets you size your defenses to the actual risk.
Data theft and exfiltration
The most common goal of malicious code is to steal information. Keyloggers record every keystroke, capturing passwords and messages as you type them. Infostealers scrape saved browser credentials, session cookies, cryptocurrency wallet keys, and files matching patterns the attacker cares about. Spyware watches activity over time.
What makes exfiltration dangerous is that it is quiet. Well-written malicious code does not announce itself — it siphons data slowly to an attacker-controlled server and leaves everything working normally, so the victim has no reason to suspect anything. Breaches discovered months after the initial compromise are almost always this pattern: the effect was ongoing theft, not a visible crash.
File encryption and ransom
Ransomware produces the most visible effect: it encrypts files, or locks the entire system, and demands payment for the decryption key. For an organization, the impact cascades beyond the ransom itself — operations stop, and modern ransomware groups add a second threat by stealing the data first and threatening to publish it if you refuse to pay ("double extortion"). Even victims who pay do not always recover their data, and paying funds the next attack. This is why offline, tested backups are the single most effective ransomware defense: they turn a catastrophe into a restore.
Remote control and system hijacking
Some malicious code exists to give an attacker a foothold rather than to cause immediate damage. A remote access trojan (RAT) or a backdoor opens a channel the attacker uses to issue commands, move to other systems, and escalate privileges at their own pace. Compromised machines conscripted this way become part of a botnet — a network of hijacked devices the attacker directs collectively to send spam, mine cryptocurrency, or launch distributed denial-of-service attacks against third parties. In that case, the effect of the malicious code on your machine is that it is now attacking someone else.
Resource drain and degraded performance
Cryptojacking — malicious code that mines cryptocurrency using your hardware — produces a subtler effect: rising CPU usage, slower systems, higher cloud bills, and shortened hardware life. It rarely destroys anything, which is precisely why it can run undetected for a long time. In cloud environments the effect shows up as a surprise invoice, because auto-scaling happily provisions more compute to feed the miner.
Data corruption and destruction
Not all malicious code wants profit. Wipers exist purely to destroy — overwriting files, corrupting master boot records, and rendering systems unbootable. Logic bombs detonate a destructive payload when a trigger condition is met, often planted by an insider. The NotPetya attack in 2017, disguised as ransomware, was effectively a wiper: even paying could not recover the data, and it caused billions in damage worldwide. The effect here is permanent loss.
Propagation across a network
A defining effect of many types of malicious code is that they spread. Worms self-propagate to every reachable vulnerable machine; some infostealers spread laterally to harvest more credentials. One compromised laptop becomes a foothold, and from there the malicious code moves to servers, backups, and connected systems. This is why the effect of malicious code is rarely confined to the first machine — containment and network segmentation exist precisely to break that chain.
The supply-chain dimension
The effects above describe what malicious code does once it runs. Where it increasingly comes from is the software supply chain — a compromised dependency, package, or build tool. The effect is amplified because the code runs automatically in trusted environments. When malicious code lands in a package that thousands of teams install, it can exfiltrate the credentials sitting in every one of their CI runners, inject a backdoor into the software each of them ships, and reach far more victims than any phishing campaign. The 2021 ua-parser-js compromise installed a password stealer and cryptominer on every machine that ran the poisoned release; the effect was simultaneous data theft and resource drain across the package's entire install base.
How to limit the effects
You cannot prevent every infection, so mature defense assumes some malicious code will get through and focuses on limiting the blast radius:
- Least privilege: code running with limited permissions can do limited damage. An infostealer in a low-privilege process cannot reach system-wide secrets.
- Network segmentation: isolating systems stops propagation from turning one compromise into a whole-network event.
- Offline, tested backups: the antidote to ransomware and wipers. If you can restore, encryption and deletion lose their leverage.
- Monitoring and anomaly detection: unusual outbound traffic, CPU spikes, or new persistence mechanisms are how you catch the quiet effects — exfiltration and cryptojacking — before they run for months.
- Dependency scanning: to catch malicious code arriving through the supply chain, continuously scan your dependency tree. Our SCA overview explains how software composition analysis inventories packages and flags known-bad or anomalous versions. An SCA tool such as Safeguard can flag a package version exhibiting the behaviors — surprise network calls, obfuscated payloads — that precede data theft and backdoor injection.
For the taxonomy of the code that produces these effects, see our companion post on which of the following is an example of malicious code.
FAQ
What is the most common effect of malicious code?
Data theft. Most malicious code aims to steal credentials, financial data, or intellectual property, and it does so quietly to avoid detection. Ransomware's file encryption is the most visible effect, but silent exfiltration is the most common.
Can malicious code affect more than one computer?
Yes. Worms self-propagate across networks with no user action, and other malware spreads laterally to harvest more access. Supply-chain malicious code reaches thousands of machines at once by hiding in a package that many teams install. Network segmentation exists to contain this spread.
Does malicious code always damage files?
No. Many effects are non-destructive by design — spyware and cryptojacking keep systems working normally so they can run undetected for as long as possible. Destruction (wipers, some ransomware) is one category of effect, not the whole picture.
How do I know if malicious code is running?
Watch for anomalies: unexplained outbound network traffic, sustained high CPU usage, new unknown processes or startup entries, unexpected cloud costs, or files becoming inaccessible. Endpoint detection and network monitoring surface these signs, and dependency scanning catches supply-chain malicious code before it ever executes.