diff --git a/Misc/NEWS.d/next/Library/2025-04-21-01-03-15.gh-issue-127081.WXRliX.rst b/Misc/NEWS.d/next/Library/2025-04-21-01-03-15.gh-issue-127081.WXRliX.rst
new file mode 100644
index 00000000000000..63fed60ced03c5
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-04-21-01-03-15.gh-issue-127081.WXRliX.rst
@@ -0,0 +1,2 @@
+Fix libc thread safety issues with :mod:`os` by replacing ``getlogin`` with
+``getlogin_r`` re-entrant version.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index b7300def8dc75f..58289d1d526fe7 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -9538,6 +9538,24 @@ os_getlogin_impl(PyObject *module)
     }
     else
         result = PyErr_SetFromWindowsErr(GetLastError());
+#elif defined (HAVE_GETLOGIN_R)
+# if defined (HAVE_MAXLOGNAME)
+    char name[MAXLOGNAME + 1];
+# elif defined (HAVE_UT_NAMESIZE)
+    char name[UT_NAMESIZE + 1];
+# else
+    char name[256];
+# endif
+    int err = getlogin_r(name, sizeof(name));
+    if (err) {
+        int old_errno = errno;
+        errno = -err;
+        posix_error();
+        errno = old_errno;
+    }
+    else {
+        result = PyUnicode_DecodeFSDefault(name);
+    }
 #else
     char *name;
     int old_errno = errno;
diff --git a/configure b/configure
index decb8f2449d162..6149296d89d84f 100755
--- a/configure
+++ b/configure
@@ -19240,6 +19240,12 @@ if test "x$ac_cv_func_getlogin" = xyes
 then :
   printf "%s\n" "#define HAVE_GETLOGIN 1" >>confdefs.h
 
+fi
+ac_fn_c_check_func "$LINENO" "getlogin_r" "ac_cv_func_getlogin_r"
+if test "x$ac_cv_func_getlogin_r" = xyes
+then :
+  printf "%s\n" "#define HAVE_GETLOGIN_R 1" >>confdefs.h
+
 fi
 ac_fn_c_check_func "$LINENO" "getpeername" "ac_cv_func_getpeername"
 if test "x$ac_cv_func_getpeername" = xyes
@@ -23263,6 +23269,33 @@ fi
 
 
 
+ac_fn_check_decl "$LINENO" "MAXLOGNAME" "ac_cv_have_decl_MAXLOGNAME" "#include <sys/params.h>
+" "$ac_c_undeclared_builtin_options" "CFLAGS"
+if test "x$ac_cv_have_decl_MAXLOGNAME" = xyes
+then :
+
+printf "%s\n" "#define HAVE_MAXLOGNAME 1" >>confdefs.h
+
+fi
+
+ac_fn_check_decl "$LINENO" "UT_NAMESIZE" "ac_cv_have_decl_UT_NAMESIZE" "#include <utmp.h>
+" "$ac_c_undeclared_builtin_options" "CFLAGS"
+if test "x$ac_cv_have_decl_UT_NAMESIZE" = xyes
+then :
+  ac_have_decl=1
+else case e in #(
+  e) ac_have_decl=0 ;;
+esac
+fi
+printf "%s\n" "#define HAVE_DECL_UT_NAMESIZE $ac_have_decl" >>confdefs.h
+if test $ac_have_decl = 1
+then :
+
+printf "%s\n" "#define HAVE_UT_NAMESIZE 1" >>confdefs.h
+
+fi
+
+
 # check for openpty, login_tty, and forkpty
 
 
diff --git a/configure.ac b/configure.ac
index 004797b5233c20..82df0b54c304c5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -5137,7 +5137,7 @@ AC_CHECK_FUNCS([ \
   faccessat fchmod fchmodat fchown fchownat fdopendir fdwalk fexecve \
   fork fork1 fpathconf fstatat ftime ftruncate futimens futimes futimesat \
   gai_strerror getegid geteuid getgid getgrent getgrgid getgrgid_r \
-  getgrnam_r getgrouplist gethostname getitimer getloadavg getlogin \
+  getgrnam_r getgrouplist gethostname getitimer getloadavg getlogin getlogin_r \
   getpeername getpgid getpid getppid getpriority _getpty \
   getpwent getpwnam_r getpwuid getpwuid_r getresgid getresuid getrusage getsid getspent \
   getspnam getuid getwd grantpt if_nameindex initgroups kill killpg lchown linkat \
@@ -5427,6 +5427,18 @@ PY_CHECK_FUNC([setgroups], [
 #endif
 ])
 
+AC_CHECK_DECL([MAXLOGNAME],
+              [AC_DEFINE([HAVE_MAXLOGNAME], [1],
+                         [Define if you have the 'MAXLOGNAME' constant.])],
+              [],
+              [@%:@include <sys/params.h>])
+
+AC_CHECK_DECLS([UT_NAMESIZE],
+              [AC_DEFINE([HAVE_UT_NAMESIZE], [1],
+                         [Define if you have the 'HAVE_UT_NAMESIZE' constant.])],
+              [],
+              [@%:@include <utmp.h>])
+
 # check for openpty, login_tty, and forkpty
 
 AC_CHECK_FUNCS([openpty], [],
diff --git a/pyconfig.h.in b/pyconfig.h.in
index aa086d49e90a5b..7b4cd544fdcae9 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -264,6 +264,10 @@
    */
 #undef HAVE_DECL_TZNAME
 
+/* Define to 1 if you have the declaration of 'UT_NAMESIZE', and to 0 if you
+   don't. */
+#undef HAVE_DECL_UT_NAMESIZE
+
 /* Define to 1 if you have the device macros. */
 #undef HAVE_DEVICE_MACROS
 
@@ -530,6 +534,9 @@
 /* Define to 1 if you have the 'getlogin' function. */
 #undef HAVE_GETLOGIN
 
+/* Define to 1 if you have the 'getlogin_r' function. */
+#undef HAVE_GETLOGIN_R
+
 /* Define to 1 if you have the 'getnameinfo' function. */
 #undef HAVE_GETNAMEINFO
 
@@ -795,6 +802,9 @@
 /* Define this if you have the makedev macro. */
 #undef HAVE_MAKEDEV
 
+/* Define if you have the 'MAXLOGNAME' constant. */
+#undef HAVE_MAXLOGNAME
+
 /* Define to 1 if you have the 'mbrtowc' function. */
 #undef HAVE_MBRTOWC
 
@@ -1563,6 +1573,9 @@
 /* Define to 1 if you have the <utmp.h> header file. */
 #undef HAVE_UTMP_H
 
+/* Define if you have the 'HAVE_UT_NAMESIZE' constant. */
+#undef HAVE_UT_NAMESIZE
+
 /* Define to 1 if you have the 'uuid_create' function. */
 #undef HAVE_UUID_CREATE