CWE-77 Improper Neutralization of Special Elements used in a Command

The product constructs all or part of a command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended command when it is sent to a downstream component.

Many protocols and products have their own custom command language. While OS or shell command strings are frequently discovered and targeted, developers may not realize that these other command languages might also be vulnerable to attacks.

• Integrity, Confidentiality, Availability: Execute Unauthorized Code or Commands — If a malicious user injects a character (such as a semi-colon) that delimits the end of one command and the beginning of another, it may be possible to then insert an entirely new and unrelated command that was not intended to be executed. This gives an attacker a privilege or capability that they would not otherwise have.

• [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.

• Automated Static Analysis: 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.)

• CVE-2026-27806: Device management product allows injection of Tcl/expect commands • CVE-2022-1509: injection of sed script syntax ("sed injection") • CVE-2024-5184: ML-based email analysis product uses an API service that allows a malicious user to inject a direct prompt and take over the service logic, forcing it to leak the standard hard-coded system prompts and/or execute unwanted prompts to leak sensitive data. • CVE-2020-11698: anti-spam product allows injection of SNMP commands into configuration file • CVE-2019-12921: image program allows injection of commands in "Magick Vector Graphics (MVG)" language. • CVE-2022-36069: Python-based dependency management tool avoids OS command injection when generating Git commands but allows injection of optional arguments with input beginning with a dash (CWE-88), potentially allowing for code execution. • CVE-1999-0067: Canonical example of OS command injection. CGI program does not neutralize "|" metacharacter when invoking a phonebook program. • CVE-2020-9054: Chain: improper input validation (CWE-20) in username parameter, leading to OS command injection (CWE-78), as exploited in the wild per CISA KEV. • CVE-2021-41282: injection of sed script syntax ("sed injection") • CVE-2019-13398: injection of sed script syntax ("sed injection")