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How the CVE Works:
The vulnerability resides in the `/3/ParseSetup` endpoint of H2O version 3.46.0.1. This endpoint processes user-supplied input by applying a regular expression (regex) to a user-controllable string. An attacker can exploit this by crafting a malicious regex pattern with excessive complexity, such as nested quantifiers or catastrophic backtracking. When the server attempts to evaluate this regex, it consumes significant computational resources, leading to a Denial of Service (DoS) condition. This renders the server unresponsive, disrupting services for legitimate users.
DailyCVE Form:
Platform: H2O
Version: 3.46.0.1
Vulnerability: Denial of Service (DoS)
Severity: High
Date: Mar 20, 2025
What Undercode Say:
Exploitation:
- Crafting Malicious Regex: Attackers can create regex patterns with exponential time complexity, such as `(a+)+` or
(a|a?)+, to trigger resource exhaustion. - Sending Payload: The malicious regex is sent to the `/3/ParseSetup` endpoint via a POST request.
- Observing Impact: The server becomes unresponsive due to CPU and memory exhaustion.
Protection:
- Input Validation: Sanitize and validate user-supplied regex patterns to prevent malicious inputs.
- Regex Timeout: Implement a timeout mechanism for regex evaluation to limit resource consumption.
- Rate Limiting: Restrict the number of requests to the vulnerable endpoint per user.
- Patch Application: Upgrade to a patched version of H2O if available.
Commands and Codes:
1. Exploit Example (Python):
import requests
url = "http://target-server:54321/3/ParseSetup"
malicious_regex = "(a+)+"
payload = {"input": "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "regex": malicious_regex}
response = requests.post(url, json=payload)
print(response.status_code)
2. Regex Timeout Implementation (Python):
import re
from functools import wraps
import signal
class TimeoutError(Exception):
pass
def timeout(seconds=10):
def decorator(func):
@wraps(func)
def wrapper(args, kwargs):
signal.signal(signal.SIGALRM, lambda signum, frame: (_ for _ in ()).throw(TimeoutError()))
signal.alarm(seconds)
try:
result = func(args, kwargs)
finally:
signal.alarm(0)
return result
return wrapper
return decorator
@timeout(5)
def safe_regex_eval(pattern, text):
return re.match(pattern, text)
try:
safe_regex_eval("(a+)+", "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
except TimeoutError:
print("Regex evaluation timed out.")
3. Rate Limiting with Nginx:
http {
limit_req_zone $binary_remote_addr zone=api_limit:10m rate=10r/s;
server {
location /3/ParseSetup {
limit_req zone=api_limit burst=20;
proxy_pass http://h2o_backend;
}
}
}
4. Monitoring Resource Usage:
top -b -n 1 | grep h2o
5. Upgrading H2O:
pip install --upgrade h2o
By following these steps, organizations can mitigate the risk of this vulnerability and protect their H2O deployments from potential DoS attacks.
References:
Reported By: https://github.com/advisories/GHSA-7qq7-pvm9-x8rf
Extra Source Hub:
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