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Taint

A PHP extension to detect XSS codes(tainted strings). It can also be used to spot SQL injection vulnerabilities, shell injection, etc.

The idea comes from https://wiki.php.net/rfc/taint, and I implemented it as a PHP extension, so no core patch is needed.

Please do not enable this extension in production environments, since it will slow down your app.

EXPERIMENTAL. Taint is a research-grade detection tool, not a security product. Detection coverage, warning behavior and INI settings may change between releases without any backward-compatibility guarantees.

Requirements

• PHP 8.0+ (master branch)
• PHP 7.x (php7 branch, taint 2.1.x releases)
• PHP 5.x (php5 branch, taint 1.x releases)

How it works

Strings received from user input are marked "tainted" at request startup, and the mark is tracked through string operations. When a tainted string reaches a dangerous sink (output, SQL query, shell command, file path, ...), taint raises a warning.

Taint sources are: $_GET, $_POST and $_COOKIE.

Design philosophy

• One bit, no context. The taint mark is a single bit stored on the string itself (zend_string), not on the variable. It carries no information about which context (HTML, SQL, shell, ...) the string will end up in, and taint never tries to analyze whether a value is "actually safe" for that context.
• Over-report rather than stay silent. Concatenation, interpolation and a whitelist of data-preserving string functions propagate the mark even when the result happens to be harmless. A warning means "user input reached this sink, go look at it", not "definitely exploitable". Noisy-but-auditable beats clever-but-silent.
• Everything else clears the mark. Any function taint does not explicitly understand — including escaping helpers like htmlspecialchars() — returns a fresh, unmarked string. Checks stay shallow on purpose: only top-level string arguments of sink calls are inspected.
• Zero core changes. The whole mechanism is implemented with user opcode handlers and internal-function handler swaps, so taint is a normal PECL extension that works on a stock PHP build. The price is that it conflicts with other extensions that hook the executor (see NOTE below).
• Report, don't block. Taint only raises warnings; it never alters or rejects data. It is a bug-finding instrument for developers, not a runtime defense.

When to use

Good fits:

• Auditing an existing codebase: run your test suite, or drive the app manually, with taint enabled, and the warnings will point out paths where raw user input reaches an output, SQL, shell, filesystem or code-execution sink.
• Regression checks in development/QA environments before a release — XSS, SQL injection, command injection, file-path injection.
• Understanding how user input flows through an unfamiliar application.

Not a good fit:

• Production. The instrumentation slows every request down, forces the OPcache JIT off and does not mix with xdebug (see NOTE below), and the warnings themselves may leak request data into logs.
• Runtime defense / WAF. Taint reports, it does not block, and its context-independent bit cannot decide whether a value is really safe.
• A substitute for escaping and parameterization. A clean run means "nothing taint could see", never "provably secure".

NOTE

Taint works by installing user opcode handlers and swapping internal function handlers. That makes it incompatible with the OPcache JIT — but not with OPcache itself:

• OPcache (the opcode cache) works fine together with taint.

• OPcache JIT cannot run at the same time: JIT-compiled code bypasses user opcode handlers, so the PHP engine refuses to start the JIT whenever an extension (like taint) has registered handlers. If opcache.jit is configured, PHP prints this warning at process startup and simply runs without the JIT:

  `

  Taint itself keeps working normally — you just get no JIT speedups while it is loaded. This is enforced by the engine; there is nothing to configure.

• Xdebug hooks the executor as well; do not enable taint and xdebug together (the test suite runs with opcache and xdebug disabled).

Taint is a detection tool for development use and is not designed for production deployment anyway.

Install

Install via PECL

Taint is a PECL extension, simply install it by:

`

Compile from source

`

Usage

When taint is enabled, if you pass a tainted string(which comes from $_GET, $_POST or $_COOKIE) to some dangerous functions, taint will warn you about that.

``

The above example will output something similar to: `

Detected sinks

Checks are shallow on purpose: only top-level string arguments are inspected, so dumping an array that contains tainted values does not warn.

Taint propagation

The taint mark survives string concatenation(., "{$var}" interpolation, .=) and these functions(both the plain call and the PHP 8.4+ frameless fast paths):

trim, rtrim, ltrim, substr, strstr, str_replace, str_ireplace, str_pad, sprintf, vsprintf, implode, join, explode, strtolower, strtoupper, strval, dirname, basename, pathinfo

For sprintf/vsprintf only %s specifiers carry the mark through — sprintf("%d", $t) produces a clean string, since numeric specifiers emit derived values, not the original bytes.

Any other function call produces a fresh, unmarked string — including escaping helpers such as htmlspecialchars(). Taint is a single, context-independent bit by design: it is meant to over-report during development rather than to understand which context a string is safe in.

API

Taint registers three global functions and no classes or constants.

``

Manually mark variables as tainted (by reference). Always returns true. Only non-empty strings are marked; other types and empty strings are silently ignored. When taint.enable is off, this is a no-op (still returns true). The mark lives on the string itself, so assignments just share it.

``

Clear the mark. Since the bit lives on the string itself, every copy/reference sharing the same string becomes clean at once. Always returns true.

``

Check whether a value carries the taint mark. Only strings can be tainted, other types return false. Always returns false when taint.enable is off.

``

INI settings

If you need to hide the errors for a particular script, you can: `

License

Taint is distributed under the PHP-3.01 license.