COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record
CAPEC-664
Server Side Request Forgery
An adversary exploits improper input validation by submitting maliciously crafted input to a target application running on a server, with the goal of forcing the server to make a request either to itself, to web services running in the server’s internal network, or to external third parties. If successful, the adversary’s request will be made with the server’s privilege level, bypassing its authentication controls. This ultimately allows the adversary to access sensitive data, execute commands on the server’s network, and make external requests with the stolen identity of the server. Server Side Request Forgery attacks differ from Cross Site Request Forgery attacks in that they target the server itself, whereas CSRF attacks exploit an insecure user authentication mechanism to perform unauthorized actions on the user's behalf.
Abstraction
Standard
Typical severity
High
Likelihood of attack
High
Understand the attack
What is this attack pattern?
An adversary exploits improper input validation by submitting maliciously crafted input to a target application running on a server, with the goal of forcing the server to make a request either to itself, to web services running in the server’s internal network, or to external third parties. If successful, the adversary’s request will be made with the server’s privilege level, bypassing its authentication controls. This ultimately allows the adversary to access sensitive data, execute commands on the server’s network, and make external requests with the stolen identity of the server. Server Side Request Forgery attacks differ from Cross Site Request Forgery attacks in that they target the server itself, whereas CSRF attacks exploit an insecure user authentication mechanism to perform unauthorized actions on the user's behalf.
Execution flow
How does the attack proceed, step by step?
Explore
Find target application
Find target web application that accepts a user input and retrieves data from the server
Experiment
Examine existing application requests
Examine HTTP/GET requests to view the URL query format. Adversaries test to see if this type of attack is possible through weaknesses in an application's protection to Server Side Request Forgery
Techniques used
- Attempt manipulating the URL to retrieve an error response/code from the server to determine if URL/request validation is done.
- Use a list of XSS probe strings to specify as parameters to known URLs. If possible, use probe strings with unique identifiers.
- Create a GET request with a common server file path such as /etc/passwd as a parameter and examine output.
Exploit
Malicious request
Adversary crafts a malicious URL request that assumes the privilege level of the server to query internal or external network services and sends the request to the application
Requirements
What does the attack require?
Prerequisites
- Server must be running a web application that processes HTTP requests.
Skills required
- Medium: The adversary will have to detect the vulnerability through an intermediary service or specify maliciously crafted URLs and analyze the server response.
- High: The adversary will be required to access internal resources, extract information, or leverage the services running on the server to perform unauthorized actions such as traversing the local network or routing a reflected TCP DDoS through them.
Resources required
- [None] No specialized resources are required to execute this type of attack.
Impact
What does a successful attack lead to?
Modify Data
Affected scopes: Integrity, Confidentiality, Availability
Read Data
Affected scopes: Confidentiality
Resource Consumption
Affected scopes: Availability
Defence
How is it prevented and mitigated?
1
Handling incoming requests securely is the first line of action to mitigate this vulnerability. This can be done through URL validation.
2
Further down the process flow, examining the response and verifying that it is as expected before sending would be another way to secure the server.
3
Allowlist the DNS name or IP address of every service the web application is required to access is another effective security measure. This ensures the server cannot make external requests to arbitrary services.
4
Requiring authentication for local services adds another layer of security between the adversary and internal services running on the server. By enforcing local authentication, an adversary will not gain access to all internal services only with access to the server.
5
Enforce the usage of relevant URL schemas. By limiting requests be made only through HTTP or HTTPS, for example, attacks made through insecure schemas such as file://, ftp://, etc. can be prevented.
Real world
MITRE example instances
Example instance 1Detail
An e-commerce website allows a customer to filter results by specific categories. When the customer selects the category of choice, the web shop queries a back-end service to retrieve the requested products. The request may look something like:
POST /product/category HTTP/1.0
Content-Type: application/x-www-form-urlencoded
Content-Length: 200
vulnerableService=http://vulnerableshop.net:8080/product/category/check%3FcategoryName%3DsomeCategory
A malicious user can modify the request URL to look like this instead:
POST /product/category HTTP/1.0
Content-Type: application/x-www-form-urlencoded
Content-Length: 200
vulnerableService=http://localhost/server-status
or
vulnerableService = file:///etc/passwd
or
vulnerableService=dict://localhost:12345/info
If the exploit is successful, the server may return the data requested by the adversary
root:!:0:0::/:/usr/bin/ksh
daemon:!:1:1::/etc:
bin:!:2:2::/bin:
sys:!:3:3::/usr/sys:
adm:!:4:4::/var/adm:
uucp:!:5:5::/usr/lib/uucp:
guest:!:100:100::/home/guest:
nobody:!:4294967294:4294967294::/:
lpd:!:9:4294967294::/:
lp:*:11:11::/var/spool/lp:/bin/false
invscout:*:200:1::/var/adm/invscout:/usr/bin/ksh
nuucp:*:6:5:uucp login user:/var/spool/uucppublic:/usr/sbin/uucp/uucico
paul:!:201:1::/home/paul:/usr/bin/ksh
jdoe:*:202:1:My name:/home/myname:/usr/bin/ksh
Example instance 2Detail
The CallStranger attack is an observed example of SSRF. It specifically targets the UPnP (Universal Plug and Play) protocol used by various network devices and gaming consoles. To execute the attack, an adversary performs a scan of the LAN to discover UPnP enabled devices, and subsequently a list of UPnP services they use. Once the UPnP service endpoints are listed, a vulnerability in the UPnP protocol is used to send these endpoints as encrypted to a verification server via the UPnP Callback method. Because the encryption is done on the client side, the server returns an encrypted list of services which is decrypted on the client side. The adversary then has a list of services running the vulnerable UPnP protocol, which the adversary can leverage to make spoofed requests. [ REF-646 ]