open_basedir confines PHP file operations to a configured path tree. Like disable_functions, it is enforced inside the interpreter's file API, so anything that reaches the filesystem outside that API, or that confuses the path check, escapes it. This matters when a foothold needs to read config/secrets or drop a payload beyond the allowed directory.

In-interpreter tricks#

Some bypasses stay within PHP by abusing how the check resolves paths:

• chdir() plus relative traversal: walking in and out of permitted directories with sequences of chdir() and .. has historically desynchronized the resolved base from the checked base on some versions, allowing access outside the tree.
• Symlink races (TOCTOU), not plain symlinks: PHP resolves a symlink to its real target before applying open_basedir, so a static link under an allowed directory that points outside it is rejected like any outside path. The version-specific vector is a race: swap the path from a legitimate in-base target to a symlink pointing outside between PHP's check and its open. This needs a concrete resolution or race bug on the target's PHP build; do not expect a plain symlink to work.
• glob:// and wrapper quirks: enumeration wrappers sometimes list entries the direct check would deny.

These are version-sensitive; test against the exact PHP build.

Going outside the file API#

The robust escapes leave the interpreter's file layer entirely:

• Subprocess: if any command-execution path is available (see disable_functions bypass), run cat/cp as a child process; open_basedir does not apply to the spawned process.
• FFI: call libc fopen/open/read directly through FFI::cdef, bypassing PHP's checked file functions.
• Loadable native code: an LD_PRELOAD library or loaded extension reads and writes the real filesystem.

Exploitation notes#

• Check the current confinement and your position first:

  php

  echo ini_get('open_basedir');
  echo getcwd();
  

• open_basedir and disable_functions are independent; a host may set one and not the other. If command execution is available, the subprocess route solves both at once.

• The in-interpreter traversal tricks are the only option when no execution and no FFI exist, so keep them in reserve for tightly locked PHP-only footholds.

Tools#

• Version-matched open_basedir bypass snippets; FFI one-liners.

References#

• PHP manual: open_basedir, FFI
• Public open_basedir bypass writeups (version-specific)