Server Side Request Forgery
Last updated
Last updated
Server-side request forgery (or SSRF) is a web security vulnerability that allows an attacker to induce the server-side application to make HTTP requests to an arbitrary domain of the attacker's choosing.
In typical SSRF examples, the attacker might cause the server to make a connection back to itself, or to other web-based services within the organization's infrastructure, or to external third-party systems.
Applications often block input containing non-whitelist hostnames, sensitive URLs, or IP addresses like loopback, IPv4 link-local, , etc. In this situation, it is sometimes possible to bypass the filter using various techniques.
You can try using a redirection to the desired URL to bypass the filter. To do this, return a response with the 3xx code and the desired URL in the Location header to the request from the vulnerable server, for example:
You can achieve redirection in the following ways:
bash, like nc -lvp 80 < response.txt
URL shortener services
Mock and webhook services, see
More flexible solutions such as a simple HTTP server on python
Also, if the application contains an open redirection vulnerability you can use it to bypass the URL filter, for example:
These bypass approaches work because the application only validates the provided URL, which triggers the redirect. It follows the redirect and makes a request to the internal URL of the attacker's choice.
You can try to use different URL schemes:
Node.js for Windows considers any single letter in a URL scheme as drive://filepath and set the protocol to file://.
References:
Java's URL will correctly handle the next URLs:
References:
You can try using a different IP address format to bypass the filter.
Dotted hexadecimal IP: 0x7f.0x0.0x0.0x1
Dotless hexadecimal IP: 0x7f001
Dotless hexadecimal IP with padding: 0x0a0b0c0d7f000001 (padding is 0a0b0c0d)
Dotless decimal IP: 2130706433
Dotted decimal IP with overflow (256): 383.256.256.257
Dotted octal IP: 0177.0.0.01
Dotless octal IP: 017700000001
Dotted octal IP with padding: 00177.000.0000.000001
Combined:
You can short-hand IP addresses by dropping the zeros:
IPv6 localhost:
IPv4-mapped IPv6 address: [::ffff:7f00:1]
IPv4-mapped IPv6 address: [::ffff:127.0.0.1]
Resolv::getaddresses is OS-dependent, therefore by playing around with different IP formats one can return blank values.
Proof of concept:
References:
Embedded credentials in a URL before the hostname, using the @ character: https://expected-host@evil-host
Indication a URL fragment using the # character: https://evil-host#expected-host
DNS naming hierarchy: https://expected-host.evil-host
URL-encode characters. This can help confuse URL-parsing code. This is particularly useful if the code that implements the filter handles URL-encoded characters differently than the code that performs the back-end HTTP request.
Combinations of these techniques together:
References:
If you want to get a A-record that resolves to an IP, use the following subdomain:
For example, domain resolves make-127-0-0-1-rr.1u.ms to 127.0.0.1:
Multiple records can be separated by -and-:
For example, domain resolves make-127-0-0-1-and-127-127-127-127-rr.1u.ms to 127.0.0.1 and 127.127.127.127:
Also, check sslip.io:
If the mechanisms in vulnerable application for checking and establishing a connection are independent and there is no caching of the DNS resolution response, you can bypass this by manipulating the DNS resolution response.
For example, if two requests go one after the other within 5 seconds, DNS resolution make-1-1-1-1-rebind-127-0-0-1-rr.1u.ms will return the address 1.1.1.1 by the first request, and the second - 127.0.0.1.
Also, check lock.cmpxchg8b.com:
Server-side processing of arbitrary HTML and JS data from a user can often be found when generating various documents, for example, to PDFs. If this functionality is vulnerable to HTML injection and/or XSS, you can use this to access internal resources:
Use HTTPLeaks to determine if any of the allowed HTML tags could be used to abuse the processing.
References:
References:
Many applications use in their flows IP addresses/domains, which they received directly from users in different HTTP headers, such as the X-Forwarded-For or Client-IP headers. Such application functionality can lead to a blind SSRF vulnerability if the header values are not properly validated.
Also notice the Referer header, which is used by server-side analytics software to track visitors. Such software often logs the Referer header from requests, since this allows to track incoming links.
The analytics software will actually visit any third-party URL that appears in the Referer header. This is typically done to analyze the contents of referring sites, including the anchor text that is used in the incoming links. As a result, the Referer header often represents fruitful attack surface for SSRF vulnerabilities.
@phithon_xg tweets and
Rare IP address formats, defined in :
IPv4-compatible IPv6 address (deprecated, q.v. : [::127.0.0.1]
IPv4-mapped IPv6 address with : [::ffff:7f00:1%25]
IPv4-mapped IPv6 address with : [::ffff:127.0.0.1%eth0]
Enclosed alphanumerics is a Unicode block of typographical symbols of an alphanumeric within a circle, a bracket or other not-closed enclosure, or ending in a full stop, q.v. .
The contains a number of features that are liable to be overlooked when implementing ad hoc parsing and validation of URLs:
If an application is running on a Windows server and exporting to a spreadsheet try to use function to gain a SSRF:
This is where the can be useful for searching the HTTP headers.
