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Server-Side Request Forgery (SSRF)

Description

SSRF flaws occur whenever a web application is fetching a remote resource without validating the user-supplied URL. It allows an attacker to coerce the application to send a crafted request to an unexpected destination, even when protected by a firewall, VPN, or another type of network access control list (ACL).

As modern web applications provide end-users with convenient features, fetching a URL becomes a common scenario. As a result, the incidence of SSRF is increasing. Also, the severity of SSRF is becoming higher due to cloud services and the complexity of architectures.

How to Prevent

Developers can prevent SSRF by implementing some or all the following defense in depth controls:

From Network layer:

• Segment remote resource access functionality in separate networks to reduce the impact of SSRF
• Enforce “deny by default” firewall policies or network access control rules to block all but essential intranet traffic. Hints: ~ Establish an ownership and a lifecycle for firewall rules based on applications. ~ Log all accepted and blocked network flows on firewalls (see A09:2021-Security Logging and Monitoring Failures).

From Application layer:

• Sanitize and validate all client-supplied input data
• Enforce the URL schema, port, and destination with a positive allow list
• Do not send raw responses to clients
• Disable HTTP redirections
• Be aware of the URL consistency to avoid attacks such as DNS rebinding and “time of check, time of use” (TOCTOU) race conditions

Do not mitigate SSRF via the use of a deny list or regular expression. Attackers have payload lists, tools, and skills to bypass deny lists.

Additional Measures to consider:

• Don't deploy other security relevant services on front systems (e.g. OpenID). Control local traffic on these systems (e.g. localhost)
• For frontends with dedicated and manageable user groups use network encryption (e.g. VPNs) on independent systems to consider very high protection needs

Source: OWASP TOP 10 Server-Side Request Forgery (SSRF)

Cheatsheets

Server-Side Request Forgery (SSRF) Cheatcheats

Cards

Data-validation-&-encoding

• Data-validation-&-encoding 2
• Data-validation-&-encoding 7
• Data-validation-&-encoding Q

Session-management

• Session-management 4
• Session-management 10

Authorization

• Authorization 2
• Authorization 8
• Authorization 9

Cryptography

• Cryptography 7
• Cryptography J

Cornucopia

• Cornucopia 3
• Cornucopia 5
• Cornucopia Q

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