diff --git a/Lib/test/test_os/test_os.py b/Lib/test/test_os/test_os.py
index ddb8a63095bce5..67ab945101149d 100644
--- a/Lib/test/test_os/test_os.py
+++ b/Lib/test/test_os/test_os.py
@@ -2624,6 +2624,40 @@ def test_execve_invalid_env(self):
         with self.assertRaises(ValueError):
             os.execve(args[0], args, newenv)
 
+    # See https://github.com/python/cpython/issues/137934 and the other
+    # related issues for the reason why we cannot test this on Windows.
+    @unittest.skipIf(os.name == "nt", "POSIX-specific test")
+    def test_execve_env_concurrent_mutation_with_fspath_posix(self):
+        # Prevent crash when mutating environment during parsing.
+        # Regression test for https://github.com/python/cpython/issues/143309.
+
+        message = "hello from execve"
+        code = """
+        import os, sys
+
+        class MyPath:
+            def __fspath__(self):
+                mutated.clear()
+                return b"pwn"
+
+        mutated = KEYS = VALUES = [MyPath()]
+
+        class MyEnv:
+            def __getitem__(self): raise RuntimeError("must not be called")
+            def __len__(self): return 1
+            def keys(self): return KEYS
+            def values(self): return VALUES
+
+        args = [sys.executable, '-c', "print({message!r})"]
+        os.execve(args[0], args, MyEnv())
+        """.format(message=message)
+
+        # Use '__cleanenv' to signal to assert_python_ok() not
+        # to do a copy of os.environ on its own.
+        rc, out, _ = assert_python_ok('-c', code, __cleanenv=True)
+        self.assertEqual(rc, 0)
+        self.assertIn(bytes(message, "ascii"), out)
+
     @unittest.skipUnless(sys.platform == "win32", "Win32-specific test")
     def test_execve_with_empty_path(self):
         # bpo-32890: Check GetLastError() misuse
diff --git a/Misc/NEWS.d/next/Library/2025-12-31-20-43-02.gh-issue-143309.cdFxdH.rst b/Misc/NEWS.d/next/Library/2025-12-31-20-43-02.gh-issue-143309.cdFxdH.rst
new file mode 100644
index 00000000000000..5f30ed340bf0eb
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-12-31-20-43-02.gh-issue-143309.cdFxdH.rst
@@ -0,0 +1,3 @@
+Fix a crash in :func:`os.execve` on non-Windows platforms when
+given a custom environment mapping which is then mutated during
+parsing. Patch by Bénédikt Tran.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index e0276ce9e3906f..b26785abc3cd27 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -7291,8 +7291,8 @@ static EXECV_CHAR**
 parse_envlist(PyObject* env, Py_ssize_t *envc_ptr)
 {
     Py_ssize_t i, pos, envc;
-    PyObject *keys=NULL, *vals=NULL;
-    PyObject *key2, *val2, *keyval;
+    PyObject *keys = NULL, *vals = NULL;
+    PyObject *key = NULL, *val = NULL, *key2 = NULL, *val2 = NULL;
     EXECV_CHAR **envlist;
 
     i = PyMapping_Size(env);
@@ -7317,20 +7317,22 @@ parse_envlist(PyObject* env, Py_ssize_t *envc_ptr)
     }
 
     for (pos = 0; pos < i; pos++) {
-        PyObject *key = PyList_GetItem(keys, pos);  // Borrowed ref.
+        // The 'key' and 'val' must be strong references because of
+        // possible side-effects by PyUnicode_FS{Converter,Decoder}().
+        key = PyList_GetItemRef(keys, pos);
         if (key == NULL) {
             goto error;
         }
-        PyObject *val = PyList_GetItem(vals, pos);  // Borrowed ref.
+        val = PyList_GetItemRef(vals, pos);
         if (val == NULL) {
             goto error;
         }
 
 #if defined(HAVE_WEXECV) || defined(HAVE_WSPAWNV)
-        if (!PyUnicode_FSDecoder(key, &key2))
+        if (!PyUnicode_FSDecoder(key, &key2)) {
             goto error;
+        }
         if (!PyUnicode_FSDecoder(val, &val2)) {
-            Py_DECREF(key2);
             goto error;
         }
         /* Search from index 1 because on Windows starting '=' is allowed for
@@ -7339,39 +7341,38 @@ parse_envlist(PyObject* env, Py_ssize_t *envc_ptr)
             PyUnicode_FindChar(key2, '=', 1, PyUnicode_GET_LENGTH(key2), 1) != -1)
         {
             PyErr_SetString(PyExc_ValueError, "illegal environment variable name");
-            Py_DECREF(key2);
-            Py_DECREF(val2);
             goto error;
         }
-        keyval = PyUnicode_FromFormat("%U=%U", key2, val2);
+        PyObject *keyval = PyUnicode_FromFormat("%U=%U", key2, val2);
 #else
-        if (!PyUnicode_FSConverter(key, &key2))
+        if (!PyUnicode_FSConverter(key, &key2)) {
             goto error;
+        }
         if (!PyUnicode_FSConverter(val, &val2)) {
-            Py_DECREF(key2);
             goto error;
         }
         if (PyBytes_GET_SIZE(key2) == 0 ||
             strchr(PyBytes_AS_STRING(key2) + 1, '=') != NULL)
         {
             PyErr_SetString(PyExc_ValueError, "illegal environment variable name");
-            Py_DECREF(key2);
-            Py_DECREF(val2);
             goto error;
         }
-        keyval = PyBytes_FromFormat("%s=%s", PyBytes_AS_STRING(key2),
-                                             PyBytes_AS_STRING(val2));
+        PyObject *keyval = PyBytes_FromFormat("%s=%s", PyBytes_AS_STRING(key2),
+                                              PyBytes_AS_STRING(val2));
 #endif
-        Py_DECREF(key2);
-        Py_DECREF(val2);
-        if (!keyval)
+        if (!keyval) {
             goto error;
+        }
 
         if (!fsconvert_strdup(keyval, &envlist[envc++])) {
             Py_DECREF(keyval);
             goto error;
         }
 
+        Py_CLEAR(key);
+        Py_CLEAR(val);
+        Py_CLEAR(key2);
+        Py_CLEAR(val2);
         Py_DECREF(keyval);
     }
     Py_DECREF(vals);
@@ -7382,6 +7383,10 @@ parse_envlist(PyObject* env, Py_ssize_t *envc_ptr)
     return envlist;
 
 error:
+    Py_XDECREF(key);
+    Py_XDECREF(val);
+    Py_XDECREF(key2);
+    Py_XDECREF(val2);
     Py_XDECREF(keys);
     Py_XDECREF(vals);
     free_string_array(envlist, envc);