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.)
CWE-88 Base Draft
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 or 'neutralize' the intended arguments. This allows an…
Definition
What is CWE-88?
This vulnerability occurs when an application builds a command string for execution by another component, but fails to properly separate or 'neutralize' the intended arguments. This allows an attacker to inject additional command-line arguments, options, or switches by including argument-separating characters (like spaces or dashes) in untrusted input.
Developers often build command strings by inserting user input directly into a template, assuming only their specified arguments will run. They might even guard against obvious command injection by escaping shell metacharacters like semicolons or ampersands. However, if they don't also neutralize characters that separate arguments—such as spaces, commas, or hyphens—an attacker can sneak in entirely new flags or parameters. For example, inputting `file.txt --mode destructive` could be interpreted as two separate arguments, not a single filename.
The security impact depends on what the targeted command supports. Injected arguments can delete files, expose sensitive data, change system configurations, or bypass security checks. This flaw is subtle because the command itself isn't hijacked; instead, its intended behavior is altered by providing it with more—and potentially malicious—options than the developer planned for.
Real-world impact
Real-world CVEs caused by CWE-88
-
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.
-
Canonical Example - "-froot" argument is passed on to another program, where the "-f" causes execution as user "root"
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Web browser executes Telnet sessions using command line arguments that are specified by the web site, which could allow remote attackers to execute arbitrary commands.
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Web browser allows remote attackers to execute commands by spawning Telnet with a log file option on the command line and writing arbitrary code into an executable file which is later executed.
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Argument injection vulnerability in the mail function for PHP may allow attackers to bypass safe mode restrictions and modify command line arguments to the MTA (e.g. sendmail) possibly executing commands.
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Help and Support center in windows does not properly validate HCP URLs, which allows remote attackers to execute arbitrary code via quotation marks in an "hcp://" URL.
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Mail client does not sufficiently filter parameters of mailto: URLs when using them as arguments to mail executable, which allows remote attackers to execute arbitrary programs.
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Web browser doesn't filter "-" when invoking various commands, allowing command-line switches to be specified.
How attackers exploit it
Step-by-step attacker path
- 1
Consider the following program. It intends to perform an "ls -l" on an input filename. The validate_name() subroutine performs validation on the input to make sure that only alphanumeric and "-" characters are allowed, which avoids path traversal (CWE-22) and OS command injection (CWE-78) weaknesses. Only filenames like "abc" or "d-e-f" are intended to be allowed.
- 2
However, validate_name() allows filenames that begin with a "-". An adversary could supply a filename like "-aR", producing the "ls -l -aR" command (CWE-88), thereby getting a full recursive listing of the entire directory and all of its sub-directories. There are a couple possible mitigations for this weakness. One would be to refactor the code to avoid using system() altogether, instead relying on internal functions. Another option could be to add a "--" argument to the ls command, such as "ls -l --", so that any remaining arguments are treated as filenames, causing any leading "-" to be treated as part of a filename instead of another option. Another fix might be to change the regular expression used in validate_name to force the first character of the filename to be a letter or number, such as:
- 3
CVE-2016-10033 / [REF-1249] provides a useful real-world example of this weakness within PHPMailer.
- 4
The program calls PHP's mail() function to compose and send mail. The fifth argument to mail() is a set of parameters. The program intends to provide a "-fSENDER" parameter, where SENDER is expected to be a well-formed email address. The program has already validated the e-mail address before invoking mail(), but there is a lot of flexibility in what constitutes a well-formed email address, including whitespace. With some additional allowed characters to perform some escaping, the adversary can specify an additional "-o" argument (listing an output file) and a "-X" argument (giving a program to execute). Additional details for this kind of exploit are in [REF-1250].
Vulnerable code example
Vulnerable Perl
Consider the following program. It intends to perform an "ls -l" on an input filename. The validate_name() subroutine performs validation on the input to make sure that only alphanumeric and "-" characters are allowed, which avoids path traversal (CWE-22) and OS command injection (CWE-78) weaknesses. Only filenames like "abc" or "d-e-f" are intended to be allowed.
my $arg = GetArgument("filename");
do_listing($arg);
sub do_listing {
my($fname) = @_;
if (! validate_name($fname)) {
print "Error: name is not well-formed!\n";
return;
}
```
# build command*
my $cmd = "/bin/ls -l $fname";
system($cmd);
}
sub validate_name {
```
my($name) = @_;
if ($name =~ /^[\w\-]+$/) {
return(1);
}
else {
return(0);
}
} Secure code example
Secure Perl
However, validate_name() allows filenames that begin with a "-". An adversary could supply a filename like "-aR", producing the "ls -l -aR" command (CWE-88), thereby getting a full recursive listing of the entire directory and all of its sub-directories. There are a couple possible mitigations for this weakness. One would be to refactor the code to avoid using system() altogether, instead relying on internal functions. Another option could be to add a "--" argument to the ls command, such as "ls -l --", so that any remaining arguments are treated as filenames, causing any leading "-" to be treated as part of a filename instead of another option. Another fix might be to change the regular expression used in validate_name to force the first character of the filename to be a letter or number, such as:
if ($name =~ /^\w[\w\-]+$/) ... 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-88
- Implementation Where possible, avoid building a single string that contains the command and its arguments. Some languages or frameworks have functions that support specifying independent arguments, e.g. as an array, which is used to automatically perform the appropriate quoting or escaping while building the command. For example, in PHP, escapeshellarg() can be used to escape a single argument to system(), or exec() can be called with an array of arguments. In C, code can often be refactored from using system() - which accepts a single string - to using exec(), which requires separate function arguments for each parameter.
- Architecture and Design Understand all the potential areas where untrusted inputs can enter your product: parameters or arguments, cookies, anything read from the network, environment variables, request headers as well as content, URL components, e-mail, files, databases, and any external systems that provide data to the application. Perform input validation at well-defined interfaces.
- 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.
- Implementation Directly convert your input type into the expected data type, such as using a conversion function that translates a string into a number. After converting to the expected data type, ensure that the input's values fall within the expected range of allowable values and that multi-field consistencies are maintained.
- Implementation Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180, CWE-181). Make sure that your application does not inadvertently decode the same input twice (CWE-174). Such errors could be used to bypass allowlist schemes by introducing dangerous inputs after they have been checked. Use libraries such as the OWASP ESAPI Canonicalization control. Consider performing repeated canonicalization until your input does not change any more. This will avoid double-decoding and similar scenarios, but it might inadvertently modify inputs that are allowed to contain properly-encoded dangerous content.
- Implementation When exchanging data between components, ensure that both components are using the same character encoding. Ensure that the proper encoding is applied at each interface. Explicitly set the encoding you are using whenever the protocol allows you to do so.
- Implementation When your application combines data from multiple sources, perform the validation after the sources have been combined. The individual data elements may pass the validation step but violate the intended restrictions after they have been combined.
- Testing Use automated static analysis tools that target this type of weakness. Many modern techniques use data flow analysis to minimize the number of false positives. This is not a perfect solution, since 100% accuracy and coverage are not feasible.
Detection signals
How to detect CWE-88
Plexicus auto-detects CWE-88 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.
Frequently asked questions What is CWE-88?
How serious is CWE-88?
What languages or platforms are affected by CWE-88?
How can I prevent CWE-88?
How does Plexicus detect and fix CWE-88?
Where can I learn more about CWE-88?
Frequently asked questions
What is CWE-88?
This vulnerability occurs when an application builds a command string for execution by another component, but fails to properly separate or 'neutralize' the intended arguments. This allows an attacker to inject additional command-line arguments, options, or switches by including argument-separating characters (like spaces or dashes) in untrusted input.
How serious is CWE-88?
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-88?
MITRE lists the following affected platforms: PHP.
How can I prevent CWE-88?
Where possible, avoid building a single string that contains the command and its arguments. Some languages or frameworks have functions that support specifying independent arguments, e.g. as an array, which is used to automatically perform the appropriate quoting or escaping while building the command. For example, in PHP, escapeshellarg() can be used to escape a single argument to system(), or exec() can be called with an array of arguments. In C, code can often be refactored from using…
How does Plexicus detect and fix CWE-88?
Plexicus's SAST engine matches the data-flow signature for CWE-88 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-88?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/88.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-88
CWE-77 Parent
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-1427 Sibling
Improper Neutralization of Input Used for LLM Prompting
This vulnerability occurs when an application builds prompts for a Large Language Model (LLM) using external data, but does so in a way…
CWE-624 Sibling
Executable Regular Expression Error
This vulnerability occurs when an application uses a regular expression that can execute code, either because it directly contains…
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-917 Sibling
Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')
Expression Language Injection occurs when an application uses untrusted, external input to build an expression language statement—common…
Resources
Further reading
- MITRE — official CWE-88 https://cwe.mitre.org/data/definitions/88.html
- Argument injection issues https://seclists.org/bugtraq/2007/Feb/234ed
- Security issues with using PHP's escapeshellarg https://baesystemsai.blogspot.com/2013/11/security-issues-with-using-phps.html
- PHPMailer < 5.2.18 Remote Code Execution [CVE-2016-10033] https://legalhackers.com/advisories/PHPMailer-Exploit-Remote-Code-Exec-CVE-2016-10033-Vuln.html
- Pwning PHP mail() function For Fun And RCE https://exploitbox.io/paper/Pwning-PHP-Mail-Function-For-Fun-And-RCE.html
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