CWE-99 Class Draft High likelihood

Improper Control of Resource Identifiers ('Resource Injection')

This vulnerability occurs when an application accepts user input as a resource identifier (like a file path or port number) without proper validation, allowing an attacker to access or manipulate…

Definition

What is CWE-99?

This vulnerability occurs when an application accepts user input as a resource identifier (like a file path or port number) without proper validation, allowing an attacker to access or manipulate resources outside the intended scope.
A resource injection flaw arises when two conditions are met. First, an attacker can control the identifier used to access a system resource, such as part of a filename, a database key, a network port, or a configuration setting. Second, by controlling this identifier, the attacker gains unauthorized capabilities—like reading sensitive files, overwriting protected data, redirecting network traffic, or altering application behavior—that would normally be restricted. In practice, this happens because the application treats unvalidated user input as a direct reference to a resource. For example, an attacker might supply a path like '../../etc/passwd' to traverse directories, or specify a remote server address to exfiltrate data. The core failure is a lack of input validation and authorization checks before the resource is accessed, effectively letting users dictate which system resources the application uses.
Real-world impact

Real-world CVEs caused by CWE-99

  • CVE-2013-4787

    chain: mobile OS verifies cryptographic signature of file in an archive, but then installs a different file with the same name that is also listed in the archive.

How attackers exploit it

Step-by-step attacker path

  1. 1

    The following Java code uses input from an HTTP request to create a file name. The programmer has not considered the possibility that an attacker could provide a file name such as "../../tomcat/conf/server.xml", which causes the application to delete one of its own configuration files.

  2. 2

    The following code uses input from the command line to determine which file to open and echo back to the user. If the program runs with privileges and malicious users can create soft links to the file, they can use the program to read the first part of any file on the system.

  3. 3

    The kind of resource the data affects indicates the kind of content that may be dangerous. For example, data containing special characters like period, slash, and backslash, are risky when used in methods that interact with the file system. (Resource injection, when it is related to file system resources, sometimes goes by the name "path manipulation.") Similarly, data that contains URLs and URIs is risky for functions that create remote connections.

Vulnerable code example

Vulnerable Java

The following Java code uses input from an HTTP request to create a file name. The programmer has not considered the possibility that an attacker could provide a file name such as "../../tomcat/conf/server.xml", which causes the application to delete one of its own configuration files.

Vulnerable Java 
String rName = request.getParameter("reportName");
  File rFile = new File("/usr/local/apfr/reports/" + rName);
  ...
  rFile.delete();
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-99

  • Implementation Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, it can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
Detection signals

How to detect CWE-99

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.)

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Frequently asked questions

Frequently asked questions

What is CWE-99?

This vulnerability occurs when an application accepts user input as a resource identifier (like a file path or port number) without proper validation, allowing an attacker to access or manipulate resources outside the intended scope.

How serious is CWE-99?

MITRE rates the likelihood of exploit as High — this weakness is actively exploited in the wild and should be prioritized for remediation.

What languages or platforms are affected by CWE-99?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-99?

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and…

How does Plexicus detect and fix CWE-99?

Plexicus's SAST engine matches the data-flow signature for CWE-99 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-99?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/99.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-99

CWE-74 Parent

Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')

This vulnerability occurs when an application uses untrusted external input to build a command, query, or data structure for another…

CWE-1236 Sibling

Improper Neutralization of Formula Elements in a CSV File

This vulnerability occurs when an application writes user-supplied data into a CSV file without properly sanitizing special characters.…

CWE-75 Sibling

Failure to Sanitize Special Elements into a Different Plane (Special Element Injection)

This vulnerability occurs when an application fails to properly filter or encode user-supplied data containing special characters or…

CWE-77 Sibling

Improper Neutralization of Special Elements used in a Command ('Command Injection')

This vulnerability occurs when an application builds a system command using untrusted user input without properly sanitizing it. An…

CWE-78 Sibling

Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

OS Command Injection occurs when an application builds a system command using untrusted, external input without properly sanitizing it.…

CWE-79 Sibling

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

This vulnerability occurs when a web application fails to properly sanitize or encode user-supplied input before displaying it on a…

CWE-88 Sibling

Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')

This vulnerability occurs when an application builds a command string for execution by another component, but fails to properly separate…

CWE-89 Sibling

Improper Neutralization of Special Elements used in an SQL Command ('SQL Injection')

SQL Injection occurs when an application builds a database query using untrusted user input without properly sanitizing it. This allows an…

CWE-91 Sibling

XML Injection (aka Blind XPath Injection)

XML Injection occurs when an application fails to properly validate or escape user-controlled input before including it in XML documents…

Resources

Further reading

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