C exercises

734 exercises graded by a sandboxed harness. Write code in the browser, submit, get feedback.

basics

Hello, World! — beginner
Sum two numbers — beginner
Max of three — beginner
Even or odd — beginner
Factorial — beginner
Nth Fibonacci — beginner
FizzBuzz to N — beginner
Is prime? — intermediate
Sum of array — beginner
Reverse array in place — beginner
Count characters — beginner
Reverse a string — beginner
Swap via pointers — beginner
Distance between points — intermediate
Implement strlen — intermediate
Absolute value — beginner
Greatest common divisor — beginner
Leap year check — beginner
Integer power (iterative) — beginner
Array mean — beginner
Strict atoi: parse an int with full error reporting — intermediate
Convert int to string in a fixed buffer — intermediate
Set a single bit — beginner
Clear a single bit — beginner
Toggle a single bit — beginner
Extract a byte from a 32-bit word — beginner
Swap two integers using XOR (no temporary) — beginner
Caesar cipher (educational toy) — beginner
Case-insensitive alphanumeric palindrome check — beginner
Count whitespace-separated words — beginner
Lowercase an ASCII string in place — beginner
Trim leading and trailing whitespace in place — beginner
Print sizeof for the canonical C types — beginner
Spot the signed/unsigned compare trap — beginner
Three-way classify using ternary — beginner
Day of week via Zeller's congruence — intermediate
Sum of decimal digits — beginner
Integer power by recursion — beginner
Parse a single `--key=value` flag — beginner
Implement my_strlen — beginner
Count a character in a string — beginner
Sum 1..n recursively — beginner
Integer power recursively — beginner
Isolate the lowest set bit — beginner
Is a number a power of two? — beginner
Sum an array — beginner
Maximum of an array — beginner
Add two integers — beginner
Square a number — beginner
Index a flattened matrix — beginner
Diagonal sum — beginner
Larger of two — beginner
Smaller of two — beginner
First character — beginner
Last character — beginner
Program status code — beginner
Clamp an exit code — beginner
Copy a string — beginner
String equality — beginner
Count format specifiers — beginner
Validate a format string — intermediate
Swap two integers — beginner
Sum of three — beginner
Bounded string copy — intermediate
Bounded string length — beginner
Fits in a byte? — beginner
Sign of a number — beginner
Strings equal — beginner
Prefix check — intermediate
Parse an integer — intermediate
Parse two integers — intermediate
Bounded concatenation — intermediate
Has a terminator in range? — beginner
Apply an operator — beginner
Even number test — beginner
Count flag arguments — intermediate
Find a flag's value — intermediate
Maximum of three — beginner
FizzBuzz word — beginner
Reverse an array in place — beginner
Palindrome check — beginner
Days in a month — beginner
Letter grade — beginner
Index of a substring — intermediate
Count occurrences — intermediate
Iterative factorial — beginner
Count vowels — beginner
Greatest common divisor — intermediate
Clamp a value — beginner
A counter with static state — intermediate
Running total — intermediate
Triple a value — beginner
Negate a value — beginner
Byte-swap a 32-bit value — intermediate
Read a big-endian uint32 — intermediate
Count words — intermediate
Validate a number string — intermediate
Sum a range — beginner
Product of a range — beginner
Fix the array sum (off-by-one) — intermediate
Fix the maximum (bad initial value) — intermediate

pointers-memory

Sum via pointer walking — intermediate
Implement strcpy — intermediate
Implement strstr — intermediate
Implement atoi — intermediate
Allocate sequence 1..N — intermediate
Deep-copy an int array — intermediate
Concat three strings (no leaks) — intermediate
Count set bits (Hamming weight) — intermediate
Reverse bits of a uint32 — intermediate
Swap two pointers via a function — beginner
Remove the n-th node from a linked list — intermediate
Allocate and free a dynamic 2-D array — intermediate
Grow a dynamic buffer with realloc — intermediate
Bump-pointer arena allocator — advanced
Detect a buffer overflow with a canary — intermediate
Safe overlapping copy (memcpy vs memmove) — intermediate
Grow a buffer and zero only the new region — intermediate
Destroy a linked list without use-after-free — intermediate
Set a value through a pointer — beginner
Read through a pointer — beginner
Nth element via pointer arithmetic — beginner
Distance between pointers — intermediate
Sum an array with a pointer — beginner
Fill an array — beginner
Find a character — beginner
String length by pointer — beginner
Dereference twice — intermediate
Swap two pointers — intermediate
Allocate a range — intermediate
Duplicate an array — intermediate
Zero-initialised allocation — intermediate
All zero? — beginner
Append with realloc — intermediate
Resize via a pointer-to-pointer — intermediate
Allocate, use, free — intermediate
Concatenate two arrays — intermediate
Are allocations balanced? — intermediate
Count leaked allocations — intermediate
Detect a use-after-free — intermediate
Validate a resource lifecycle — intermediate
Bounds-checked element pointer — intermediate
Is an index in bounds? — beginner
Capacity-checked copy — intermediate
Overflow-checked addition — intermediate

data-structures

Linked list length — beginner
Insert at end — intermediate
Reverse a linked list — intermediate
Int stack (push/pop/peek) — intermediate
Circular int queue — intermediate
Int→int hash table (open addressing) — advanced
BST insert — intermediate
BST inorder to array — intermediate
Binary search — intermediate
Bubble sort — intermediate
Quicksort — advanced
Count distinct ints in a sorted array — beginner
Merge two sorted arrays — intermediate
Rotate array right by k — intermediate
Merge sort — intermediate
Heap sort — advanced
Counting sort (small range) — intermediate
3SUM: triples summing to zero — advanced
Reverse words of a sentence in place — intermediate
Longest substring without repeating characters — intermediate
Detect a cycle in a linked list — intermediate
LRU cache (int keys, fixed capacity) — advanced
Dijkstra shortest paths (small graph) — advanced
Trie: count words with a prefix — advanced
Final Project: todo-list-cli — add/remove/list — intermediate
Final Project: tiny key-value store — intermediate
Final Project: CSV row parser (no quoting) — intermediate
Final Project: tiny stack VM — advanced
Fixed-capacity circular queue — intermediate
Min-heap priority queue — advanced
Merge overlapping intervals — intermediate
LRU cache for string keys — advanced
Binary search (recursive implementation) — intermediate
Flatten a 2D array to 1D (row-major) — intermediate
Tower of Hanoi: count moves — intermediate
Are the brackets balanced? — beginner
A FIFO queue implemented with two stacks — intermediate
Find the middle node of a singly linked list — intermediate
Are two strings anagrams of each other? — beginner
Manhattan distance between points — beginner
Rectangle area — beginner
Compare fractions — intermediate
Dot product of 3-vectors — beginner
Name a colour — beginner
Turn right — beginner
Length of a linked list — beginner
Sum of a linked list — beginner
Balanced brackets — intermediate
Maximum nesting depth — beginner
Ring-buffer index — beginner
Queue length in a ring — intermediate
Find a free slot (linear probing) — intermediate
Lookup with linear probing — intermediate
Height of a binary tree — intermediate
Sum of a binary tree — intermediate
Bubble sort — intermediate
Is it sorted? — beginner
Linear search — beginner
Binary search — intermediate
djb2 string hash — intermediate
Map a hash to a bucket — beginner

file-handling

Write lines to file — intermediate
Read a whole file as text — intermediate
Count lines in a file — intermediate
Count lines from a string buffer — beginner
First line containing a substring — intermediate
Final Project: mini-grep — substring match in lines — intermediate
Final Project: mini-wc — line/word/byte counter — beginner
Final Project: mini-cat — concatenate buffers — beginner
Final Project: log-aggregator — count by level — intermediate
Read a file into a bounded buffer — beginner
Append a line to a file — beginner
Copy a file in chunks — intermediate
Count a byte's occurrences in a file — beginner
Get a file's size — beginner
Extract the Nth CSV field — intermediate
Read a value from INI text — intermediate
Take the last N lines — intermediate
Convert CRLF to LF — beginner
Detect a byte-order mark — beginner
Extract the final path component — beginner
Read a fixed-size record — intermediate
Is a file readable? — beginner
File size via fseek/ftell — intermediate
Read the first N bytes — intermediate
Read one binary int — intermediate
Write bytes to a file — intermediate
Write one binary int — intermediate
Count lines in a file — intermediate
Read the first line — intermediate
File size via stat — intermediate
Is it a regular file? — intermediate
Save a record — intermediate
Load a record — intermediate
Count CSV fields — beginner
Extract a CSV field — intermediate
Count lines at a level — intermediate
First error line number — intermediate

linux-sysprog

Echo argv with indices — intermediate
Env var with default — intermediate
Spawn child and collect exit code — intermediate
Fork+exec /bin/echo — advanced
Pipe roundtrip — advanced
Count SIGUSR1 deliveries — advanced
Copy bytes between fds — intermediate
Minimal cat: print file to fd — intermediate
Substitute $VAR with environment value — intermediate
Print a hex dump line — intermediate
Count env vars with prefix — intermediate
Final Project: mini-shell — argv tokenizer — intermediate
Order of operations to daemonize a process — advanced
Plan a pipe chain for N processes — advanced
Compute a sigprocmask blocking SIGINT/SIGTERM — intermediate
Map poll() events to per-fd readiness flags — intermediate
Write a buffer to two outputs — beginner
Return the last N bytes of a buffer (tail) — beginner
Read an integer environment variable with bounds — intermediate
Parse a `/etc/passwd`-style line — intermediate
Join two path components safely — intermediate
Atomic-write checklist — intermediate
Translate an errno value into a friendly string — beginner
Simulate an eventfd counter — beginner
One-shot timer: 'is now past the fire time?' — beginner
Classify a signal into an action — beginner
Refuse a Unix-socket peer unless their UID is on the allow-list — intermediate
Parse a /proc/self/maps line — beginner
Extract VmRSS from /proc/self/status — intermediate
Predict the result of a mutex-protected counter — beginner
One-shot init: which call performs the work? — beginner
Will this allocation fail under the rlimit? — beginner
Order the daemonisation steps — advanced
Compute a cgroup v2 cpu.max line — intermediate
Resolve a plugin's required symbols — intermediate
Check the mount-namespace flag — advanced
Catch SIGINT and set a flag — beginner
Time-out a blocking read with alarm() — intermediate
Parent process signals a child — intermediate
Translate legacy signal() into sigaction() — beginner
Graceful SIGTERM — set a flag, cleanup in main — intermediate
Create one thread that returns a value — beginner
Spawn N threads, join all — beginner
Pass an integer to each thread (by value) — intermediate
Sum an array using multiple threads — advanced
Demonstrate a race condition — intermediate
Fix the race with a pthread_mutex_t — intermediate
Build a tiny thread-safe counter type — intermediate
Decode a syslog priority value — beginner
Lowest free file descriptor — intermediate
Count open descriptors — beginner
Read with raw descriptors — intermediate
Write with raw descriptors — intermediate
errno after a failed open — intermediate
Set errno in your own function — intermediate
Interpret fork's return — beginner
Processes after N forks — beginner
Length of an argv vector — beginner
Is it an absolute path? — beginner
Decode an exit status — intermediate
Was the child signalled? — intermediate
Bytes left in a pipe — intermediate
Would the write block? — beginner
Does the default action terminate? — intermediate
Can the signal be caught? — beginner
getenv with a default — beginner
Boolean environment flag — beginner
Count command arguments — intermediate
Count pipeline stages — beginner
eventfd counter — beginner
timerfd expirations — beginner
Parallel array sum — advanced
Chunks needed for workers — beginner
Clamp to a soft limit — beginner
Over the hard limit? — beginner
Parse VmRSS from /proc status — intermediate
Parse thread count — intermediate
Same namespace? — beginner
cgroup memory remaining — beginner
Signal name to number — beginner
Signal number to name — beginner
Is it a termination signal? — beginner
Is it a stop signal? — beginner
Does the handler auto-restart syscalls? — intermediate
Does the handler get siginfo? — intermediate
Async-signal-safe function? — intermediate
Is the handler body safe? — intermediate
Raise a signal to yourself — intermediate
Count raised signals — intermediate
Signal yourself with kill — intermediate
Valid signal number? — beginner
Seconds remaining on a timer — beginner
Has the deadline passed? — beginner
Catch Ctrl-C — intermediate
Force-quit on second Ctrl-C — beginner
Correct flag type for a handler — beginner
Coalesced signals — beginner
Useful thread count — beginner
Ideal parallel runtime — beginner
Spawn and join N threads — advanced
Detached vs joinable — beginner
Double a value in a thread — advanced
Safe way to pass per-thread args — beginner
Return a value from a thread — advanced
Why not return a stack address? — beginner
Detect a lost update — intermediate
Is the operation atomic? — beginner
Are lock/unlock balanced? — intermediate
Critical section nesting — beginner
Opposite lock ordering — intermediate
Consistent lock order — intermediate
Does this need synchronisation? — beginner
Reentrant function variant — beginner
Bounded buffer occupancy — intermediate
Would the producer block? — beginner
Forgotten joins — beginner
Classify a threading bug — beginner

networking

TCP echo client (localhost) — advanced
One-shot TCP server — advanced
UDP send and receive — advanced
Parse an HTTP status line — intermediate
TCP connect with timeout — advanced
Find open ports on localhost — advanced
Parse a port number string — beginner
Compute network address from IP + prefix — intermediate
Does CIDR contain IP? — intermediate
Token-bucket rate limiter (single slot) — intermediate
Final Project: HTTP status-line parser — intermediate
Parse HTTP request line (method + path) — intermediate
Build an HTTP GET request — beginner
Parse an IPv4 dotted-quad to uint32 — intermediate
Does an IPv4 CIDR block contain an address? — intermediate
fd_set–style bit tracker for multiplexed I/O — advanced
Compute the IPv4/TCP one's-complement checksum — advanced
Parse the fixed fields of a mock IPv4 header — intermediate
Byte-swap a 32-bit integer — beginner
Compute CRC-8 over a byte stream (poly 0x07) — intermediate
Decode TCP flag byte into individual flags — beginner
Filter the fds reported as readable by epoll_wait — intermediate
Drain a non-blocking 'read' to EAGAIN — intermediate
Parse an IP literal into a discriminated v4/v6 union — beginner
Does an IPv4 address match a CIDR block? — beginner
Decode an HTTP/1.1 chunked body — intermediate
Parse a Cookie header into name/value pairs — intermediate
Choose the right keep-alive interval — intermediate
Decode IGMP membership-query bytes — advanced
Decode a DNS-wire label sequence (no pointer compression) — advanced
Parse the 5-byte TLS record header — advanced
Open a listening TCP socket on localhost — beginner
Connect from a TCP client to a listener — beginner
Send a message client → server (length-prefixed) — intermediate
One-shot echo server — intermediate
Multi-client server with threads (3 connections) — advanced
Localhost port checker (open/closed) — intermediate
Hand-build a minimal HTTP/1.0 GET request — intermediate
Decode a DNS question name — intermediate
Extract the SNI host from a TLS extension — intermediate
Classify an ARP frame — beginner
Parse a TLS record header — intermediate
Valid port number? — beginner
Well-known port? — beginner
Build a host:port string — beginner
Parse a host:port string — intermediate
Clamp the listen backlog — beginner
Allocate a connection id — beginner
Datagrams needed — beginner
Does the datagram fit? — beginner
Port from network bytes — beginner
Port to network bytes — beginner
Build an HTTP GET request — intermediate
Parse the status code — intermediate
Build an HTTP response — intermediate
Parse the request method — intermediate
Count ready descriptors — beginner
Any readable fd? — beginner
Milliseconds to timeval seconds — beginner
Milliseconds to timeval microseconds — beginner
Is it a loopback address? — beginner
Port within scan range? — beginner
Valid HTTP method? — beginner
HTTP status class — beginner
Internet checksum (16-bit ones-complement) — advanced
XOR checksum — beginner
Count epoll events of interest — beginner
Cap the maxevents argument — beginner
Would the call have blocked? — intermediate
Set the non-blocking flag — beginner
Valid hostname character? — beginner
Is the hostname well-formed? — intermediate
Does the path fit in sun_path? — beginner
Abstract socket address? — beginner
Count hextet groups — beginner
IPv6 loopback? — beginner
Parse a chunk size — intermediate
Is this the last chunk? — beginner
Is the TLS version acceptable? — beginner
Is it the HTTPS port? — beginner
Is it a stream socket? — beginner
Is it an internet family? — beginner
Reliable transport? — beginner
Connectionless transport? — beginner
Valid socket fd? — beginner
Did socket() fail? — beginner
Pack a dotted quad into a 32-bit address — intermediate
Unpack a 32-bit address — intermediate
htons via bytes — intermediate
ntohs via bytes — intermediate
Did bind succeed? — beginner
Does binding need privilege? — beginner
Valid backlog value? — beginner
Did accept return a connection? — beginner
Did connect succeed? — beginner
Non-blocking connect in progress? — intermediate
Did we receive everything? — beginner
Bytes left to send — beginner
Active connections — beginner
Peak connections — intermediate
Reads needed for a response — beginner
Did we receive the whole message? — beginner
Is this a localhost name? — beginner
Is it a safe test target? — beginner
Name a socket error — intermediate
Is the error retriable? — beginner
Parse the request line — intermediate
Find a header value — intermediate
SIP request method — intermediate
SIP status code — intermediate
Count IQ samples — beginner
Read a little-endian 16-bit sample — intermediate
DNS question count — intermediate
Is it a DNS response? — intermediate
TLS record content type — intermediate
TLS record length — intermediate

cybersecurity

Strict integer parsing — intermediate
Fix the strcpy overflow — intermediate
Fix the sprintf overflow — intermediate
Detect weak passwords — intermediate
Parse an access log status code — intermediate
Constant-time byte compare — intermediate
Run a program without a shell — intermediate
Detect risky SQL chars — intermediate
Parse an http/https URL safely — intermediate
Detect world-writable files — intermediate
Parse an IPv4 header from bytes — advanced
Count suspicious IPs in a sample log — intermediate
Detect repeated failed logins — intermediate
Parse an HTTP request line — intermediate
Parse HTTP response headers — intermediate
Find dangerous C functions in toy source — intermediate
Replace unsafe strcpy with bounded copy — intermediate
Fix a toy buffer overflow — intermediate
Validate input length — intermediate
Detect command-injection characters in input — intermediate
Write a safe allowlist validator — intermediate
Check local file permissions — intermediate
Compare two files by content hash — intermediate
Build a 32-bit checksum tool — intermediate
Compare tokens in constant time — intermediate
Detect format string misuse in toy code — intermediate
Parse a sample Nmap-style report — intermediate
Localhost-only TCP connection checker — intermediate
Read a banner from a local service — intermediate
Password strength check (local) — intermediate
Redact secrets from a log line — intermediate
Why you don't scan public targets — beginner
HTML-escape user-provided text — intermediate
Detect weak crypto algorithm names — beginner
Validate a password against a policy — beginner
Prevent CSV injection by prefixing risky cells — intermediate
Pattern-match log file rotation candidates — beginner
Detect a malformed username (defensive check) — intermediate
Detect path-traversal patterns in a string — intermediate
Sanitize a user-supplied filename — intermediate
Extract the host from a URL safely — intermediate
Redact email addresses in a log line — intermediate
Redact credit-card-like number runs — intermediate
Bounded string concatenation (drop-in safer strncat) — intermediate
Validate a token against a fixed character allowlist — beginner
Detect format-string injection in user input — intermediate
Unfold an HTTP/1.1 continuation header — intermediate
Validate an HTTP method against an allowlist — beginner
Add two ints with overflow detection — intermediate
Multiply two ints with overflow detection — advanced
Validate a filename against an extension allowlist — beginner
Subtract two ints with overflow detection — intermediate
Refuse a DNS result that resolves to an internal range (SSRF defence) — intermediate
Bidirectional %XX URL encoding with an allow-list — beginner
Identify the file format from the first 8 bytes — beginner
Parse a mock ELF64 header — extract version, machine, entry — intermediate
Parse a /proc/net/tcp line and detect a public LISTEN — intermediate
Decide whether an fd-target string is suspicious — beginner
Strip dangerous environment variables before exec — intermediate
Detect a NUL byte hidden inside argv (within the allocated extent) — beginner
Flag a $PATH entry that is writable by anyone but root — intermediate
Validate that a mkstemp template ends with at least 6 X's — beginner
Reject when O_NOFOLLOW would have refused — intermediate
Identify file type from the first bytes — beginner
Reject a Host header with CR / LF / NUL / space — beginner
Is this syscall on the allow-list? — advanced
Pick the right entropy source for the use case — beginner
Score a setuid program against the 5-item checklist — intermediate
Compose the libFuzzer entry point — beginner
Build one line of an xxd-style hex dump — beginner
Count open ports in a saved nmap XML dump — intermediate
Detect brute-force IPs in auth.log — intermediate
Score a password against a policy struct — beginner
Flag SQL-injection metacharacters in input — intermediate
Render a finding struct as Markdown — beginner
Parse an HTTP request line into method/path/version — intermediate
Read the 24-byte pcap global header — intermediate
Extract the method from a SIP request — intermediate
Classify an 802.11 frame as mgmt/ctrl/data — intermediate
Extract the BLE Complete Local Name from an advertisement — intermediate
Count global symbols in an ELF .symtab buffer — intermediate
Detect a firmware blob's type from its magic bytes — beginner
Read a filename from a mock MFT record — intermediate
Recover an 8.3 filename from a FAT directory entry — intermediate
Parse the 20-byte header of an IQ-samples capture — intermediate
Count unique non-empty subdomains in a wordlist — beginner
Count HIGH-severity entries in a mock CVE JSON feed — intermediate
Score a URL for phishing structural smells — intermediate
Format a contactless-card UID as colon-hex — beginner
Detect a glibc-style stack canary pattern — intermediate
A libFuzzer-shape target that never crashes — intermediate
Constant-time byte equality — intermediate
Write a USTAR tar header for one file — intermediate
Bounded base64 decoder — intermediate
Classify a TCP scan from its flag byte — intermediate
Detect port scanners in a connection log — intermediate
Validate JWT structure — beginner
Flag a Log4Shell JNDI marker — beginner
Compute a CRC-32 checksum — intermediate
Audit a file mode for risky bits — beginner
Read the machine type from a PE header — intermediate
Base64 encode — intermediate
Dictionary attack against a hash — beginner
Validate PKCS#7 padding — intermediate
Would this write overflow? — beginner
Safe write length — beginner
Index in bounds? — beginner
Is a sub-range valid? — intermediate
strlcpy-style copy — intermediate
strlcat-style append — intermediate
Validate a username — intermediate
Validate an integer in range — intermediate
Overflow-checked multiply — intermediate
Overflow-checked size addition — intermediate
Detect format specifiers in input — intermediate
Count percent signs — beginner
Detect shell metacharacters — intermediate
Allowlist a safe argument — intermediate
Check a return code — beginner
Did every step succeed? — beginner
Ensure NUL termination — intermediate
Safe append position — beginner
Parse a bounded unsigned int — intermediate
Parse a hex byte — intermediate
Is the file-access approach TOCTOU-safe? — intermediate
Does it create atomically? — intermediate
Does it refuse to follow symlinks? — intermediate
Does the path contain ..? — intermediate
Is the password strong enough? — intermediate
Does it contain a special character? — beginner
Detect SQL metacharacters — intermediate
Escape single quotes — intermediate
Bounds-safe byte read for a fuzzer — intermediate
Enough input to parse? — beginner
Constant-time equality — intermediate
Which compare is timing-safe? — beginner
Base64 encoded length — beginner
Valid base64 character? — beginner
Are you authorized to test? — beginner
Is the action ethical? — beginner
Is the target in scope? — beginner
Should you stop? — beginner
Is it a private (RFC1918) address? — intermediate
Is it a safe lab target? — beginner
Extract the request path — intermediate
Suspicious request path? — intermediate
Count 404 responses — intermediate
Count distinct client IPs — advanced
Count failed logins — intermediate
Brute-force threshold reached? — beginner
Does the code call strcpy? — beginner
Count unsafe calls — intermediate
Does the code call system()? — beginner
Safer alternative to system()? — beginner
Does the query use a placeholder? — beginner
Is the query built by concatenation? — beginner
Is the scan target allowed? — beginner
Number of ports to scan — beginner
Is it an ELF file? — intermediate
Is the ELF 64-bit? — intermediate
Parse a labelled /proc field — intermediate
Parent PID from status — beginner
Is the setuid bit set? — intermediate
Is the file world-writable? — intermediate
Is the syscall on the allowlist? — intermediate
Default-deny action — beginner
Minimum-size guard — beginner
Read stays within input? — intermediate
Count open ports in nmap XML — intermediate
Count scanned hosts — beginner
Count accepted logins — beginner
Count invalid-user attempts — beginner
Build a finding block — intermediate
Severity rank — beginner
pcap captured length — intermediate
Read a little-endian uint32 — intermediate
802.11 frame type — intermediate
802.11 frame subtype — intermediate
BLE AD structure length — beginner
BLE AD type — beginner
ELF symbol binding — intermediate
ELF symbol type — intermediate
Is it gzip-compressed? — beginner
Identify firmware container — intermediate
Is it an MFT record? — beginner
MFT first-attribute offset — intermediate
Is the directory entry deleted? — beginner
Recover the first character — beginner
Count subdomains — intermediate
Count subdomains of a domain — intermediate
Valid CVE id? — intermediate
Extract the CVE year — intermediate
URL contains an @ ? — beginner
Phishiness score — intermediate
UID bytes to hex — intermediate
Single-size UID? — beginner
Is the canary intact? — beginner
Is it a terminator canary? — intermediate
Total evidence size — beginner
Does the bundle fit? — beginner
Is it a known Kali tool? — beginner
Tool category — beginner
Distinct ports touched — intermediate
Is this a port scan? — beginner
Owner permission bits — intermediate
World-writable file? — intermediate
Object-level authorization: stop IDOR / BOLA — intermediate
Bounds-Safe HTTP Request-Line Parser — intermediate
Flag Sensitive API Endpoints — beginner
BOLA Guard: Allowed-Object Set Check — intermediate
BFLA: Rank-Based Function Authorization — intermediate
BFLA: Role-Action Allowlist Check — intermediate
Mass Assignment: Field Allowlist Guard — beginner
Mass Assignment: Allowlist the Fields a Client May Set — intermediate
JWT Structure Splitter — intermediate
Validate a JWT Segment Charset (base64url) — beginner
Fixed-Window Rate Limiter — intermediate
Token-Bucket Rate Limiter — advanced
Count Unique API Endpoint Paths — intermediate
Normalize an API Request Path Safely — intermediate
Constant-Time CSRF Token Comparison — intermediate
Validate CSRF Token Format — beginner
Detect Missing Security Response Headers — intermediate
Bounds-Safe HTTP Header Line Parser — intermediate
Path Traversal Detector — intermediate
Path Traversal: Stay Within Base — advanced
Audit File Modes for Privilege-Escalation Bits — beginner
Flag Over-Privileged Container Capabilities — intermediate
Zip Slip: Reject Unsafe Archive Entries — intermediate
Zip-Slip Safe Path Join — intermediate
Parse a Dotted-Quad IPv4 Address Safely — intermediate
SSRF Defence: Reject Private IPv4 Ranges — intermediate
CIDR Membership Check: ip_in_cidr — intermediate
Usable Hosts in a CIDR Subnet — beginner
Parse a MAC Address Safely — intermediate
Detect the Broadcast MAC Address — beginner
Classify a TCP/UDP Port Number — beginner
Flag High-Risk Exposed Ports — intermediate
Validate a DNS Hostname Safely — intermediate
Count DNS Labels Safely — beginner
Detect Plaintext Password Storage — intermediate
Validate a bcrypt Hash Structure — intermediate
Defensive Password Strength Meter — intermediate
Banned-Password Denylist Check — beginner
Validate a Canonical SHA-256 Hex Digest — beginner
Constant-Time Digest Comparison — intermediate
Spot the ECB Fingerprint — advanced
Flag Weak Cryptographic Algorithms — beginner
Detect Parameterized vs Concatenated SQL — intermediate
Count SQL Parameter Placeholders — intermediate
Set-Cookie Security-Attribute Auditor — intermediate
Audit a Hardened Session Cookie — beginner
Allowlist Validation for SQL Identifiers — beginner
ORDER BY Column Allowlist — beginner
SSRF Guard: Block Internal IPv4 Ranges — intermediate
SSRF Defense: URL Scheme Allow-List — beginner
Server-Side Access Control: Stop the IDOR — intermediate
Count Owned Objects (IDOR-Safe Access Check) — intermediate