Big-O notation explained: time and space complexity for interviews
Big-O describes how an algorithm's cost grows as input grows. Learn to read and estimate it without heavy maths.
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Big-O describes how an algorithm's cost grows as input grows. Learn to read and estimate it without heavy maths.
Tests let you change code without fear. Learn which kinds to write, how many of each and how to keep them useful.
Code is read far more often than it is written. Learn the habits that make code easy to understand, change and review.
A good API is predictable, consistent and hard to misuse. These conventions make yours easy to use and easy to evolve.
JavaScript runs in every browser and on servers. Learn the language properly, then add TypeScript to catch bugs early.
Python is readable, versatile and in demand. Follow this path from first script to useful projects in web, data and automation.
Developers spend much of their time reading docs and fixing bugs. A method for both makes you faster than most beginners.
Frontend is everything users see and touch. Learn the layers in the right order and build something at each step.
Backend work is the logic, data and APIs behind apps. Here is a clear order to learn it, with a project at every stage.
Git saves the history of your code and lets teams work together. Learn the few commands and habits you will use every day.
Starting is easier than staying on track. This roadmap shows what to learn first, what to build and how to avoid the common traps.
Open-weight models can run on your own hardware for privacy and control. Learn what you need and when a hosted API is still the better choice.
LLM bills grow with tokens and traffic. A handful of habits can cut cost and speed up responses without hurting quality.
Tools let a model act, not just talk. Learn the loop behind agents and the design rules that keep them reliable and safe.
Without tests, every prompt change is a guess. Learn to build a small evaluation set and use it to improve safely.
Three tools improve an LLM app, and they solve different problems. Pick the cheapest one that fixes your actual failure.
A language model cannot reliably tell instructions from data. Design your app so a tricked model cannot do real harm.
RAG gives a language model the right documents at question time, so answers are grounded in your data instead of guesswork.
Embeddings turn text into numbers that capture meaning, so you can search by idea instead of exact words.
Good prompts are clear instructions with the right context. Learn the habits and templates that improve results quickly.
LLMs predict the next piece of text, and that simple idea explains both their power and their mistakes. Learn the concepts every builder needs.
Serverless lets you run code without managing servers and pay only when it runs. Learn the trade-offs before you commit.
Object storage holds files at huge scale for very little money. Learn how to use it safely, without leaking private data.
Most cloud breaches start with stolen or over-powerful credentials. Learn how to give each person and service only the access it needs.
A VPC is your private network in the cloud. A clear layout of public and private subnets keeps databases off the internet.
The cloud is rented computing. Learn the service models, how regions work and which security jobs stay with you.
A CDN keeps copies of your content close to users. Learn the caching headers that control it and the mistakes to avoid.
A load balancer spreads traffic over several servers and removes the broken ones. Learn the types, algorithms and settings that matter.
Type a URL and a lot happens in under a second. Follow the whole journey and learn the parts you will meet every day.
DNS is the phone book of the internet, and many outages are really DNS problems. Learn how a lookup works and what each record does.
Every device on a network has an address. Learn how IP addresses, subnets and CIDR notation fit together, with the maths made easy.
A new server on the internet is probed within minutes. This checklist covers the steps that remove the most risk first.
A backup you have never restored is only a hope. Plan what to save, where to keep it and how to prove it works.
You cannot fix what you cannot see. Learn what to measure, how Prometheus and Grafana fit together and which alerts are worth waking up for.
CI/CD runs your tests and deploys automatically on every change. Here is a clear first workflow and the safety rules that matter.
Kubernetes runs containers across many machines and keeps them healthy. Learn the few core objects that explain most of it.
SSH is the door to your server, and bots knock on it all day. A few settings make it far harder to break in.
Docker packages an app with everything it needs so it runs the same everywhere. Learn the core ideas and the commands you will use first.
Nginx sits in front of your app, handles HTTPS and passes requests along. Here is a small, safe configuration you can adapt.
systemd keeps your app running, restarts it on failure and schedules jobs. Learn a minimal service file, timers and the commands you will use daily.
Permissions decide who can read, write and run a file. Learn to read them, change them and choose safe defaults.
Most serious API bugs are about who may do what. Use a simple matrix and two accounts to test it carefully and prove it cleanly.
Sites change, but the past is archived. Learn how public archives and open sources help you understand a target, and where to draw the line.
Burp Suite lets you see and edit the requests your browser sends. Learn the core tools and a calm workflow that keeps you in scope.
The front end tells you how the back end works. Learn to read bundled JavaScript calmly, and what it can reveal about an application's design.
VEON, the parent of Banglalink, has proposed a merger with state-owned Teletalk, while the government says Teletalk is not for sale. What the reports say, what is missing, and what to watch next.
Recon is how you find what is in scope and worth testing. Learn passive and active discovery, and how to do it politely and in bounds.
Most hunters see many closed reports. Understanding why they happen is the fastest way to get better results.
Two researchers can find the same bug and get different outcomes because of the report. Use this template to be clear, fast to verify and professional.
Where you hunt matters as much as how. Learn how platforms differ, what a VDP is, and how to pick a first program that will not waste your time.
Bug bounty is a skill, not a lottery. Here is an honest roadmap from zero, with what to learn first and what to expect.
A few response headers close a whole class of attacks. Here is the short list to set, what each does and how to test them.
A feature of XML lets a document pull in outside content. If your parser allows it, an attacker can use it. The fix is a parser setting.
Turning bytes back into objects is risky when the bytes come from an attacker. Learn why and what to use instead.
File upload is one of the riskiest features. A careful design turns it from a common critical bug into a safe feature.
CORS is not a firewall. It controls which websites may read your API's responses in a browser. Learn it properly and avoid the risky settings.
JWTs are everywhere, and so are the mistakes. Learn what a JWT really guarantees and the checklist that keeps it safe.
Change an ID in the URL and see someone else's invoice. IDOR is simple, common and serious. Here is how it happens and how to stop it.
Browsers attach cookies automatically, and attackers abuse that. Learn how CSRF works and the modern defences.
XSS lets an attacker run their JavaScript in your users' browsers. Learn the three types and the layered defences that stop it.
SQL injection lets attacker input become part of your query. The fix is simple and well known: never build SQL from strings.
Redis is a Swiss army knife for backend work. Here are the patterns that solve real problems, and the mistakes to avoid.
A careless migration can lock a table and take a site down. Learn the expand and contract pattern and the checks that make schema changes boring.
All three are solid relational databases, but they suit different jobs. Here is a plain comparison and a decision guide.
Connections are expensive, and databases allow only a limited number. A pool reuses a small set. Learn to size it and avoid the traps.
One query for the list, then one more for each row. N+1 is the most common reason an API page is slow. Here is how to find and fix it.
EXPLAIN shows how the database will run your query. Learn to read scans, joins and costs, and fix the usual causes of slow queries.
Symantec captured the instructions of an autonomous attack agent built to run on other people's AI models. The model owner pays the bill. Here is what was found and how to protect your AI endpoints.
Isolation decides what one transaction can see of another. Learn the four levels and the concurrency bugs behind them.
A transaction groups several steps so they either all happen or none do. Learn ACID with a bank transfer example.
An index is why a query takes milliseconds instead of minutes. Learn how B-tree indexes work and when they help or hurt.
JOINs combine rows from two tables. Learn each type with a tiny example and the mistakes that cause wrong results.
A SaaS serves many customers (tenants) from one system. The design question is how strongly to separate them.
The old model trusted anything inside the office network. Zero trust assumes the network is hostile and checks every request.
A DDoS attack floods your site with fake traffic. You cannot out-scale it alone, but layered defences make it survivable.
An outage is only wasted if you learn nothing. Here is a template for blameless postmortems that lead to real fixes.
When one dependency fails, it can drag everything down with it. Circuit breakers and bulkheads contain the damage.
Retries fix small failures and create big ones. Learn timeouts, exponential backoff with jitter, retry budgets and why retries must be idempotent.
A free series that builds system design from the ground up. Start with the basics here, then follow the parts in order. New parts are added every week.
You cannot fix what you cannot see. We design how metrics are collected, stored and turned into dashboards and alerts.
A highly available key-value database combines consistent hashing, replication and quorum rules. We walk through the Dynamo-style design.
When one cache server is not enough, you need many. We design partitioning, replication, eviction and failure behaviour.
A single database counter does not scale across servers. Learn UUIDs, Snowflake-style 64-bit IDs and the clock traps.
Advertisers pay per click, so every count must be accurate and fast. We design a streaming pipeline with deduplication and reconciliation.
Running other people's code is dangerous. We design a judge with a queue, sandboxed workers and strict limits.
When a hot item goes on sale, thousands of users hit one product at once. We design for the spike without overselling or crashing.
Checkout is where money and stock meet. We design the cart, stock reservation, order flow and payment so nothing is oversold or charged twice.
A crawler fetches billions of pages without hammering any site. We design the URL frontier, deduplication and politeness rules.
Two people must never buy the same seat. We design seat holds, locking and payment timeouts for a booking system.
Drive-style services store files and keep every device in sync. We design chunked storage, a metadata service and conflict handling.
As you type, the search box suggests completions in milliseconds. We design the data structure, ranking and update pipeline.
Notifications look simple until you send millions across push, SMS and email. We design a reliable, respectful pipeline.
Money systems must never lose or double-count a taka. We design a wallet transfer with a ledger, idempotency and reconciliation.
Ride sharing needs live location, fast nearby-driver search and reliable trip state. We design the matching core.
Video is heavy. We design upload, transcoding into many qualities, and streaming that adapts to each viewer's network.
A feed shows posts from people you follow, fast. The key choice is when to build each user's feed: at write time or at read time.
Chat needs low latency, reliable delivery and millions of open connections. We walk through the design step by step.
Pastebin looks like a URL shortener with bigger values. The difference is where you store the content, and how you handle expiry and abuse.
The classic first case study. We design a service like bit.ly, covering ID generation, fast redirects, storage and scaling.
Placing an order touches payment, stock and shipping in different services. A saga keeps them consistent without a distributed transaction.
Instead of saving only the latest balance, event sourcing saves every deposit and withdrawal. That gives a full history, and some new problems.
CQRS splits the model that changes data from the model that reads it. It can solve scale and complexity problems, and it can add many.
When servers start, stop and move all the time, hard-coded addresses break. Service discovery keeps a live map of who is where.
A distributed lock keeps two servers from doing the same job at once. Learn the Redis recipe, its dangers, and when to avoid locks entirely.
Many systems need exactly one node in charge. Learn how elections work, why majorities matter and how to stop two leaders at once.
A Bloom filter can tell you an item is definitely not in a huge set, using a fraction of the memory. Learn the trick and where it is used.
REST is simple and universal. gRPC is fast and strongly typed. Learn the trade-offs and where each belongs in a system.
Chat, live scores and notifications need the server to push data. Compare the three main ways and see how to scale connections.
Images, videos and backups belong in object storage, not in a database. Learn how it works and the patterns that make uploads safe and cheap.
Searching text with LIKE is slow. Elasticsearch builds an inverted index for fast, ranked search. Learn how it works and where it fits.
RabbitMQ is a flexible message broker built around exchanges and queues. Learn how routing works and how to avoid losing or duplicating work.
Kafka stores streams of events in a distributed log that many readers can replay. Here are the core ideas and the trade-offs.
Redis is more than a cache. Learn why it is so fast, its core data structures, how persistence works and the limits you must plan for.
Before you design, estimate. Learn the numbers worth remembering and walk through a worked example for requests, storage and bandwidth.
If a payment request times out, did it go through? Idempotency makes retrying safe. Learn idempotent methods and how idempotency keys work.
An API gateway is a single entry point in front of many services. Learn what belongs in it, what does not, and how it can become a bottleneck.
A message queue lets one part of your system hand work to another without waiting. Learn the guarantees, the failure cases and the main tools.
Logic flaws hide in the rules of the app, not in the code patterns scanners look for. Here is how to map a workflow, where the money-moving mistakes usually sit, and how to write a report a triager accepts.
Rate limiting protects your service from abuse, bugs and traffic spikes. Learn the four main algorithms and how to run them across many servers.
Plain hash-mod-N remaps almost every key when you add a server. Consistent hashing moves only a small share. Here is how the ring works.
Replication keeps copies of your database on other servers. It adds read capacity and safety, but it brings replication lag and failover decisions.
When one database server can no longer hold your data or writes, you shard. Learn the strategies, the pain points and why sharding should be a last resort.
SQL and NoSQL are tools for different jobs. Learn what each is good at, the four NoSQL families, and why PostgreSQL is a strong default.
A content delivery network puts copies of your files close to users. Here is how it works, how to control it with headers, and what it cannot cache.
Caching is the cheapest way to make a system faster. Learn the main strategies, when each fits, and the hard parts: invalidation and stampedes.
A load balancer sits in front of your servers and shares traffic between them. Learn L4 vs L7, the main algorithms and how health checks keep users away from dead servers.
CAP is the most quoted and most misunderstood idea in distributed systems. Here is what it actually says, and what it means when you pick a database.
Latency and throughput are not the same, and improving one can hurt the other. Learn p50, p95 and p99, and why the average hides real problems.
A feature that fetches a URL for the user can be turned around to fetch your internal network instead. Here is how server-side request forgery works, why cloud metadata is the usual target, and the defences that hold up.
Scalability is the ability to handle more load without a redesign. Here is the difference between scaling up and scaling out, and where systems usually break first.
On 21 October a compromised release pipeline pushed a credential-stealing LiteLLM 1.107.0 to PyPI and Docker for about two hours. What the malware does, how to tell if you are affected, and the order to clean up and rotate.
On 23 October CISA added two critical flaws to its Known Exploited Vulnerabilities catalog: a command injection in Collabora Online's built-in server (Nextcloud) and an SQL injection in Arista VeloCloud Orchestrator. What is reported, and what to do first.
Recursive released Flint on 21 October: a free, Apache 2.0 mixture-of-experts coding model built to run locally. The speed and benchmark numbers come from the company itself, so here is what is reported, what was tested by others, and what to check before relying on it.
A language model cannot reliably tell your instructions from text it reads along the way. Here is how prompt injection works, why tools and private data make it serious, and the design choices that limit the damage.
Railway disclosed on 21 October that an unauthorised party may have read encrypted environment variables, and decrypted some. Here is what the company has said, what is still unknown, and a practical order for rotating API keys, database passwords and tokens.
Reuters and researcher Snorre Fagerland report a Chinese-speaking attacker pointed open-source AI agents at online shops, breaching more than 100 companies for an estimated $5,000. What is confirmed, what is not, and what site owners should do.
The 21 October Node.js security release fixes a high-severity Brotli decoder bug (CVE-2026-111497) that can allow remote code execution, plus three TLS and DNS flaws. Who is exposed, which versions fix it, and how to check your servers.
Between 16 and 20 October 2026 CISA added eight vulnerabilities to its Known Exploited Vulnerabilities catalog. Here is what was added, what CISA says about the BeyondTrust bug, and what to patch first.
The PHP project fixed two unauthenticated remote code execution bugs on 21 October 2026, CVE-2026-109618 and CVE-2026-109619. Every PHP install serving web requests is affected. Here is who is at risk, what is confirmed, and what to do today.
Deleting a leaked key in a new commit does not remove it. Here is how to find every secret in a repository's history, what to do in the first hour, and how to stop the next one before it is pushed.
How dangling DNS records let attackers take over your subdomains, how to find vulnerable ones safely, and the inventory and verification habits that prevent it.
A practical JWT security checklist covering signature verification, algorithm pinning, key strength, claim validation, token lifetime, revocation and safe storage.
The CORS mistakes that let other websites read your users' data, how to check your own API in minutes, and secure configurations for Express, Next.js and nginx.
Choose the right rate limiting algorithm, key limits on the right identity, respond with 429 correctly, and implement layered limits with Redis, Express and nginx.
The eight HTTP security headers every website should send, the exact values to use, copy-paste configs for nginx, Next.js and Express, and the mistakes that break them.
What each cookie security attribute does, the recommended settings for session cookies, and copy-paste examples for Express, Next.js and PHP.
A walkthrough of an IDOR from the first request to the final report, and how to write it up so triage accepts it the first time.
Tokens only protect you if every state-changing route checks them, and checks them correctly. Here are the gaps we see most often.
Before you send a single request, an hour spent reading the program carefully saves days of wasted testing and rejected reports.
AI assistants now write whole pull requests. Review habits built for human authors need adjusting, especially around security.
You do not need a security team to be secure. A handful of weekly habits covers most of what small teams get breached over.
Triage analysts read dozens of reports a day. Here is what makes them accept an IDOR quickly, and what makes them close it as informative.
What to run, what to skip, and when to stop collecting and start testing one endpoint properly.
React escapes output by default, which makes the few places it does not even more important to find.
A good SECURITY.md tells reporters exactly where to go and what to expect, and saves maintainers from public zero-day issues.
Roll out CSP in report-only mode, fix what it finds, then enforce. A nonce-based policy stops most XSS without breaking your pages.
Starting from one domain name, you can find most of what an organisation exposes to the internet, and fix it before someone else finds it.
AI tools are changing what junior security work looks like. Hiring managers are adjusting what they test for, and candidates should adjust too.
Loose redirect URI validation remains one of the most common ways OAuth logins leak authorisation codes and tokens.
Old vulnerabilities are a free curriculum. Read them the right way and patterns start appearing in the code you test today.
Public storage, over-broad access keys and exposed databases cause a large share of cloud data leaks. All three are fixable this week.
Engagement problems in engineering and security teams are usually fixable by managers, with clarity, ownership and regular feedback.
An intercepting proxy is the core tool of web testing. A few minutes of setup makes every testing session faster and cleaner.
Automation speeds up testing, but programs limit request rates for good reasons. Here is how to script responsibly.
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