CWE-918 Base Incomplete

Server-Side Request Forgery (SSRF)

Server-Side Request Forgery (SSRF) occurs when a web application fetches a remote resource based on user-controlled input, but fails to properly validate or restrict where those requests are sent.…

Definition

What is CWE-918?

Server-Side Request Forgery (SSRF) occurs when a web application fetches a remote resource based on user-controlled input, but fails to properly validate or restrict where those requests are sent. This allows an attacker to trick the server into making unauthorized connections to internal systems or external domains.
SSRF flaws are dangerous because they let attackers bypass firewalls and access sensitive internal infrastructure that should be unreachable from the outside. A common scenario involves an application that takes a URL parameter for fetching an image or importing data, but an attacker manipulates it to target internal admin panels, cloud metadata services, or other backend systems. This can lead to information disclosure, internal service enumeration, or even remote code execution if the server processes the malicious response. Preventing SSRF requires a defense-in-depth approach: implement allowlists for permitted domains and protocols, validate and sanitize all user input used in network requests, and avoid using raw URLs to fetch resources. Network-level controls like segregating internal services and enforcing outbound firewall rules are also critical. While SAST and DAST tools can detect the vulnerable pattern, Plexicus uses AI to analyze the specific context and suggest the precise code fix—such as implementing a secure URL validator—saving significant manual remediation time across your application portfolio.
Vulnerability Diagram CWE-918
Server-Side Request Forgery (SSRF) Attacker url=http://169.254.… Web app fetch(url) no host allowlist runs in VPC Cloud metadata 169.254.169.254/iam Internal services redis:6379, admin:8080 Server fetches attacker URL → reaches resources only the server can.
Real-world impact

Real-world CVEs caused by CWE-918

  • CVE-2024-3095

    SSRF in LLM application development framework because the URL retriever allows connections to local addresses using a crafted Location header

  • CVE-2023-32786

    Chain: LLM integration framework has prompt injection (CWE-1427) that allows an attacker to force the service to retrieve data from an arbitrary URL, essentially providing SSRF (CWE-918) and potentially injecting content into downstream tasks.

  • CVE-2021-26855

    Server Side Request Forgery (SSRF) in mail server, as exploited in the wild per CISA KEV.

  • CVE-2021-21973

    Server Side Request Forgery in cloud platform, as exploited in the wild per CISA KEV.

  • CVE-2016-4029

    Chain: incorrect validation of intended decimal-based IP address format (CWE-1286) enables parsing of octal or hexadecimal formats (CWE-1389), allowing bypass of an SSRF protection mechanism (CWE-918).

  • CVE-2002-1484

    Web server allows attackers to request a URL from another server, including other ports, which allows proxied scanning.

  • CVE-2004-2061

    CGI script accepts and retrieves incoming URLs.

  • CVE-2010-1637

    Web-based mail program allows internal network scanning using a modified POP3 port number.

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 4

    Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable pseudo

MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.

Vulnerable pseudo 
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
  // Untrusted input flows directly into the sensitive sink.
  return executeUnsafe(input);
}
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-918

  • Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
  • Implementation Validate input at trust boundaries; use allowlists, not denylists.
  • Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
  • Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
  • Operation Monitor logs for the runtime signals listed in the next section.
Detection signals

How to detect CWE-918

Automated Static Analysis High

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Plexicus auto-fix

Plexicus auto-detects CWE-918 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

Get a demo Try Plexicus free
Frequently asked questions

Frequently asked questions

What is CWE-918?

Server-Side Request Forgery (SSRF) occurs when a web application fetches a remote resource based on user-controlled input, but fails to properly validate or restrict where those requests are sent. This allows an attacker to trick the server into making unauthorized connections to internal systems or external domains.

How serious is CWE-918?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-918?

MITRE lists the following affected platforms: AI/ML, Web Server.

How can I prevent CWE-918?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

How does Plexicus detect and fix CWE-918?

Plexicus's SAST engine matches the data-flow signature for CWE-918 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-918?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/918.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-918

CWE-441 Parent

Unintended Proxy or Intermediary ('Confused Deputy')

A confused deputy vulnerability occurs when a system receives a request from a client and forwards it to an external destination without…

CWE-1021 Sibling

Improper Restriction of Rendered UI Layers or Frames

This vulnerability occurs when a web application fails to properly control whether its pages can be embedded within frames or UI layers…

Resources

Further reading

Ready when you are

Don't Let Security
Weigh You Down.

Stop choosing between AI velocity and security debt. Plexicus is the only platform that runs Vibe Coding Security and ASPM in parallel — one workflow, every codebase.

Get started free Book a demo