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

What is CWE-77?

Step-by-step attacker path

1. 1

   Consider a "CWE Differentiator" application that uses an an LLM generative AI based "chatbot" to explain the difference between two weaknesses. As input, it accepts two CWE IDs, constructs a prompt string, sends the prompt to the chatbot, and prints the results. The prompt string effectively acts as a command to the chatbot component. Assume that invokeChatbot() calls the chatbot and returns the response as a string; the implementation details are not important here.

2. 2

   To avoid XSS risks, the code ensures that the response from the chatbot is properly encoded for HTML output. If the user provides CWE-77 and CWE-78, then the resulting prompt would look like:

3. 3

   However, the attacker could provide malformed CWE IDs containing malicious prompts such as:

4. 4

   This would produce a prompt like:

5. 5

   Instead of providing well-formed CWE IDs, the adversary has performed a "prompt injection" attack by adding an additional prompt that was not intended by the developer. The result from the maliciously modified prompt might be something like this:

How to prevent CWE-77

• Architecture and Design If at all possible, use library calls rather than external processes to recreate the desired functionality.
• Implementation If possible, ensure that all external commands called from the program are statically created.
• 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, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• Operation Run time: Run time policy enforcement may be used in an allowlist fashion to prevent use of any non-sanctioned commands.
• System Configuration Assign permissions that prevent the user from accessing/opening privileged files.

Frequently asked questions

Further reading

• MITRE — official CWE-77 https://cwe.mitre.org/data/definitions/77.html
• Seven Pernicious Kingdoms: A Taxonomy of Software Security Errors https://samate.nist.gov/SSATTM_Content/papers/Seven%20Pernicious%20Kingdoms%20-%20Taxonomy%20of%20Sw%20Security%20Errors%20-%20Tsipenyuk%20-%20Chess%20-%20McGraw.pdf
• Exploiting Software: How to Break Code https://www.amazon.com/Exploiting-Software-How-Break-Code/dp/0201786958
• Supplemental Details - 2022 CWE Top 25 https://cwe.mitre.org/top25/archive/2022/2022_cwe_top25_supplemental.html#problematicMappingDetails