fix(llm-security): AST-taint clears taint on reassignment (#29) + sink test coverage (#28)

py-ast-taint.py Assign handler never removed a name from the tainted set on reassignment, so a source-then-constant/sanitizer rebind (g=os.getenv(); g='safe'; os.system(g), or x=shlex.quote(x)) kept stale taint and fired false AST-CMD-EXEC findings. Assign now clears taint for target names when the RHS is not a source. No cross-expression propagation added — the f-string/concat/alias recall gap (#27) stays deferred to v8.

#28: added fixtures+assertions locking the tainted subprocess/os.system AST-CMD-EXEC sink and the open(...,'w') AST-FILE-WRITE sink. Suite 1931/0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Claude-Session: https://claude.ai/code/session_01TcQyMTQfyrsAapaCMPxTtQ

tests/fixtures/ast-scan/reassign.py

@@ -0,0 +1,17 @@
+import os
+import shlex
+
+# Taint must CLEAR when a name is rebound to a non-source value (#29).
+# Neither function below may produce a finding.
+
+
+def reassigned_constant():
+    g = os.getenv("G")      # tainted source
+    g = "safe-constant"     # rebound to a literal -> taint must clear
+    os.system(g)            # must NOT be flagged
+
+
+def sanitized_reassignment():
+    x = input("path> ")     # tainted source
+    x = shlex.quote(x)      # rebound to a non-source call -> taint must clear
+    os.system(x)            # must NOT be flagged

tests/fixtures/ast-scan/sinks.py

@@ -0,0 +1,20 @@
+import os
+import subprocess
+
+# Locks the subprocess/os.system command sinks and the file-write sink (#28).
+
+
+def run_user_command():
+    cmd = input("cmd> ")             # tainted source
+    subprocess.run(cmd, shell=True)  # sink: subprocess.* -> AST-CMD-EXEC
+
+
+def shell_from_env():
+    target = os.getenv("TARGET")     # tainted source
+    os.system(target)                # sink: os.system -> AST-CMD-EXEC
+
+
+def leak_to_file():
+    payload = os.environ["DATA"]     # tainted source
+    log = open("out.txt", "w")       # write handle
+    log.write(payload)               # sink: file.write -> AST-FILE-WRITE (high)