Add os.path.basename as a sanitizer for py/path-injection

- Add test cases in path_injection.py demonstrating that os.path.basename
  prevents path traversal attacks (false positive scenarios)
- Add OsPathBasenameCall sanitizer class in PathInjectionCustomizations.qll
  that recognizes calls to os.path.basename (and posixpath/ntpath/genericpath
  variants) as barriers for the path-injection taint flow

os.path.basename strips all directory components from a path, returning only
the final filename. This makes it impossible for an attacker to inject path
traversal sequences like ../etc/passwd - the basename of such input would
just be 'passwd'.

Agent-Logs-Url: https://github.com/github/codeql/sessions/6603215b-21cd-4e05-8905-550434c7b9ff

Co-authored-by: hvitved <3667920+hvitved@users.noreply.github.com>

python/ql/lib/semmle/python/security/dataflow/PathInjectionCustomizations.qll

@@ -9,6 +9,7 @@ private import semmle.python.dataflow.new.DataFlow
 private import semmle.python.Concepts
 private import semmle.python.dataflow.new.RemoteFlowSources
 private import semmle.python.dataflow.new.BarrierGuards
+private import semmle.python.ApiGraphs
 
 /**
  * Provides default sources, and sinks for detecting
@@ -105,4 +106,25 @@ module PathInjection {
   class SanitizerFromModel extends Sanitizer {
     SanitizerFromModel() { ModelOutput::barrierNode(this, "path-injection") }
   }
+
+  /**
+   * A call to `os.path.basename`, considered as a sanitizer for path injection.
+   *
+   * `os.path.basename` returns the final component of a path, stripping any
+   * leading directory components. This prevents path traversal attacks since
+   * the result cannot contain directory separators or relative path components.
+   * See https://docs.python.org/3/library/os.path.html#os.path.basename
+   */
+  private class OsPathBasenameCall extends Sanitizer, DataFlow::CallCfgNode {
+    OsPathBasenameCall() {
+      exists(API::Node osPathModule |
+        (
+          osPathModule = API::moduleImport("os").getMember("path")
+          or
+          osPathModule = API::moduleImport(["posixpath", "ntpath", "genericpath"])
+        ) and
+        this = osPathModule.getMember("basename").getACall()
+      )
+    }
+  }
 }

python/ql/test/query-tests/Security/CWE-022-PathInjection/path_injection.py

@@ -150,3 +150,20 @@ def safe_set_of_files():
     if filename in SAFE_FILES:
         path = os.path.join(STATIC_DIR, filename)
         f = open(path) # $ SPURIOUS: Alert
+
+
+@app.route("/basename-sanitizer")
+def basename_sanitizer():
+    filename = request.args.get('filename', '')
+    # Secure mitigation pattern: os.path.basename strips all directory components,
+    # preventing path traversal attacks.
+    path = os.path.join(STATIC_DIR, os.path.basename(filename))
+    f = open(path)  # $ result=OK
+
+
+@app.route("/basename-no-join")
+def basename_no_join():
+    filename = request.args.get('filename', '')
+    # basename alone also prevents directory traversal
+    path = os.path.basename(filename)
+    f = open(path)  # $ result=OK