DVWA Command Injection Advanced Lab 2026 — Filter Bypass Complete Walkthrough | Hacking Lab16

This article is a detailed technical walkthrough of DVWA Lab 16, which teaches command injection filter bypass techniques across three security levels. It demonstrates how blacklist-based defenses fail systematically by showing specific gaps in Medium and High security filters, and contrasts these failures against proper allowlist validation. The article matters because it teaches both the offensive methodology (reading source code, identifying gaps, testing alternatives) and the defensive principle that will appear in professional penetration test reports.

• Blacklist gaps in Medium securityDVWA Medium security only blocks two strings: '&&' and ';'. The article demonstrates that pipe (|), URL-encoded newline (%0a), backtick, subshell $(), and logical OR (||) all pass through unfiltered. This single fact immediately enables five different bypass techniques, showing how incomplete blacklists create a false sense of protection.
• Reading source before testingThe article emphasizes viewing the PHP source code first to see the exact blacklist entries rather than attempting payloads blindly. Knowing Medium only strips two characters means the first bypass attempt succeeds immediately rather than wasting time testing blocked separators. This methodology — read, identify gaps, test alternatives — directly applies to real applications.
• IFS substitution bypasses space filtersThe Internal Field Separator (IFS) is a shell variable defaulting to whitespace. When the shell processes ${IFS}, it expands to a space without the payload containing a literal space character. The payload 'cat${IFS}/etc/passwd' passes through space filters, then the shell correctly expands it to 'cat /etc/passwd'. This demonstrates how shell variable substitution defeats character-level filtering.
• High security's space-position vulnerabilityHigh security blacklist includes '| ' (pipe followed by space) but not ' |' (space followed by pipe). The payload '127.0.0.1 |cat /etc/passwd' bypasses the filter because it contains ' |' not '| ' — these are different strings to the blacklist's string-matching function, yet the Linux shell executes them identically. This single-character-order distinction exemplifies why blacklist gaps are inevitable.
• Multiple command separator alternativesBeyond the basic && and semicolon, the article documents pipe (chains command output), URL-encoded newline (creates new command line), backtick (command substitution), subshell $() (alternative to backtick), and logical OR (||) as separators. Each performs command chaining without appearing in a typical two or three-entry blacklist.
• Allowlist validation defeats all bypassesThe article presents the correct fix: filter_var($target, FILTER_VALIDATE_IP) in PHP validates that input matches a valid IPv4 address format. No separator, encoding, IFS trick, or character arrangement makes '127.0.0.1|cat /etc/passwd' match a valid IP regex. This single check provides complete protection across all three security levels, replacing the entire blacklist arms race.
• Why blacklists fundamentally failLinux shell syntax is too rich and flexible for any complete blacklist. The article explains this as an arms race: developer adds two entries, attacker finds three bypasses; developer adds six more, attacker finds encoding bypasses. The correct defense is allowlist-based validation that defines what valid input should be rather than attempting to predict all invalid variations.
• Three-level comparison shows escalation patternLow security has no filter, Medium blocks only && and ;, High extends the blacklist substantially. Yet at each level, the fundamental problem repeats: the blacklist addition closes one gap while leaving others open. The space-position bypass works at High security specifically because the developer blocked one arrangement but not the equivalent arrangement with reversed character order.

Bottom line: Reading source code to identify specific blacklist gaps, then testing alternative separators (pipe, newline, backtick, IFS substitution) systematically, bypasses command injection filters at every level — but proper defense requires allowlist validation that accepts only the expected input format, making all separator bypasses impossible. This methodology and remediation approach directly transfers to professional penetration testing and real application assessments.