Incomplete URL substring sanitization¶
ID: rb/incomplete-url-substring-sanitization
Kind: problem
Security severity: 7.8
Severity: warning
Precision: high
Tags:
- correctness
- security
- external/cwe/cwe-020
Query suites:
- ruby-code-scanning.qls
- ruby-security-extended.qls
- ruby-security-and-quality.qls
Click to see the query in the CodeQL repository
Sanitizing untrusted URLs is an important technique for preventing attacks such as request forgeries and malicious redirections. Usually, this is done by checking that the host of a URL is in a set of allowed hosts.
However, treating the URL as a string and checking if one of the allowed hosts is a substring of the URL is very prone to errors. Malicious URLs can bypass such security checks by embedding one of the allowed hosts in an unexpected location.
Even if the substring check is not used in a security-critical context, the incomplete check may still cause undesirable behaviors when the check succeeds accidentally.
Recommendation¶
Parse a URL before performing a check on its host value, and ensure that the check handles arbitrary subdomain sequences correctly.
Example¶
The following example code checks that a URL redirection will reach the example.com domain, or one of its subdomains, and not some malicious site.
class AppController < ApplicationController
def index
url = params[:url]
# BAD: the host of `url` may be controlled by an attacker
if url.include?("example.com")
redirect_to url
end
end
end
The substring check is, however, easy to bypass. For example by embedding example.com in the path component: http://evil-example.net/example.com, or in the query string component: http://evil-example.net/?x=example.com. Address these shortcomings by checking the host of the parsed URL instead:
class AppController < ApplicationController
def index
url = params[:url]
host = URI(url).host
# BAD: the host of `url` may be controlled by an attacker
if host.include?("example.com")
redirect_to url
end
end
end
This is still not a sufficient check as the following URLs bypass it: http://evil-example.com http://example.com.evil-example.net. Instead, use an explicit whitelist of allowed hosts to make the redirect secure:
class AppController < ApplicationController
def index
url = params[:url]
host = URI(url).host
# GOOD: the host of `url` can not be controlled by an attacker
allowedHosts = [
'example.com',
'beta.example.com',
'www.example.com'
]
if allowedHosts.include?(host)
redirect_to url
end
end
end