diff --git a/Makefile.pre.in b/Makefile.pre.in
index f86d7363e0900f..75a892e94b0965 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -1679,6 +1679,8 @@ Programs/_bootstrap_python.o: Programs/_bootstrap_python.c $(BOOTSTRAP_HEADERS)
 _bootstrap_python: $(LIBRARY_OBJS_OMIT_FROZEN) Programs/_bootstrap_python.o Modules/getpath.o Modules/Setup.local
 	$(LINKCC) $(PY_LDFLAGS_NOLTO) -o $@ $(LIBRARY_OBJS_OMIT_FROZEN) \
 		Programs/_bootstrap_python.o Modules/getpath.o $(LIBS) $(MODLIBS) $(SYSLIBS)
+	# Dummy pybuilddir.txt  is needed for _bootstrap_python to be runnable
+	@echo "none" > ./pybuilddir.txt
 
 
 ############################################################################
diff --git a/Misc/NEWS.d/next/Security/2026-06-16-14-58-02.gh-issue-151544._bexVy.rst b/Misc/NEWS.d/next/Security/2026-06-16-14-58-02.gh-issue-151544._bexVy.rst
new file mode 100644
index 00000000000000..418e3b4b967794
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2026-06-16-14-58-02.gh-issue-151544._bexVy.rst
@@ -0,0 +1,4 @@
+:file:`Modules/Setup.local` is no longer used as a landmark to discover
+whether Python is running in a source tree, as it could potentially affect
+actual installs. The :file:`pybuilddir.txt` file is now the sole indicator
+of running in a source tree.
diff --git a/Modules/getpath.py b/Modules/getpath.py
index b89d7427e3febd..0e4f1e87e7342a 100644
--- a/Modules/getpath.py
+++ b/Modules/getpath.py
@@ -129,8 +129,7 @@
 # checked by looking for the BUILDDIR_TXT file, which contains the
 # relative path to the platlib dir. The executable_dir value is
 # derived from joining the VPATH preprocessor variable to the
-# directory containing pybuilddir.txt. If it is not found, the
-# BUILD_LANDMARK file is found, which is part of the source tree.
+# directory containing pybuilddir.txt.
 # prefix is then found by searching up for a file that should only
 # exist in the source tree, and the stdlib dir is set to prefix/Lib.
 
@@ -177,7 +176,6 @@
 
 if os_name == 'posix' or os_name == 'darwin':
     BUILDDIR_TXT = 'pybuilddir.txt'
-    BUILD_LANDMARK = 'Modules/Setup.local'
     DEFAULT_PROGRAM_NAME = f'python{VERSION_MAJOR}'
     STDLIB_SUBDIR = f'{platlibdir}/python{VERSION_MAJOR}.{VERSION_MINOR}{ABI_THREAD}'
     STDLIB_LANDMARKS = [f'{STDLIB_SUBDIR}/os.py', f'{STDLIB_SUBDIR}/os.pyc']
@@ -190,7 +188,6 @@
 
 elif os_name == 'nt':
     BUILDDIR_TXT = 'pybuilddir.txt'
-    BUILD_LANDMARK = f'{VPATH}\\Modules\\Setup.local'
     DEFAULT_PROGRAM_NAME = f'python'
     STDLIB_SUBDIR = 'Lib'
     STDLIB_LANDMARKS = [f'{STDLIB_SUBDIR}\\os.py', f'{STDLIB_SUBDIR}\\os.pyc']
@@ -512,13 +509,9 @@ def search_up(prefix, *landmarks, test=isfile):
         platstdlib_dir = real_executable_dir
         build_prefix = joinpath(real_executable_dir, VPATH)
     except (FileNotFoundError, PermissionError):
-        if isfile(joinpath(real_executable_dir, BUILD_LANDMARK)):
-            build_prefix = joinpath(real_executable_dir, VPATH)
-            if os_name == 'nt':
-                # QUIRK: Windows builds need platstdlib_dir to be the executable
-                # dir. Normally the builddir marker handles this, but in this
-                # case we need to correct manually.
-                platstdlib_dir = real_executable_dir
+        # We used to check for an alternate landmark here, but now we require
+        # BUILDDIR_TXT to exist. (gh-151544; CVE-2026-12003)
+        pass
 
     if build_prefix:
         if os_name == 'nt':
diff --git a/Tools/wasm/wasi/__main__.py b/Tools/wasm/wasi/__main__.py
index b57bcaca924380..abe30fe0b4c77f 100644
--- a/Tools/wasm/wasi/__main__.py
+++ b/Tools/wasm/wasi/__main__.py
@@ -368,6 +368,10 @@ def configure_wasi_python(context, working_dir):
         file.write(f'#!/bin/sh\nexec {host_runner} {python_wasm} "$@"\n')
     exec_script.chmod(0o755)
     log("🏃", f"Created {exec_script} (--host-runner)... ")
+    pybuilddir_txt = working_dir / "pybuilddir.txt"
+    if not pybuilddir_txt.exists():
+        os.symlink(CHECKOUT / "pybuilddir.txt", pybuilddir_txt)
+        log("📝", f"Symlinked {pybuilddir_txt} to normal location")
     sys.stdout.flush()