gh-140594: Fix an out of bounds read when feeding NUL byte to PyOS_StdioReadline

Lib/test/test_cmd_line.py

@@ -201,6 +201,25 @@ def test_run_module_bug1764407(self):
         self.assertTrue(data.find(b'1 loop') != -1)
         self.assertTrue(data.find(b'__main__.Timer') != -1)
 
+    @support.cpython_only
+    def test_null_byte_in_interactive_mode(self):
+        # gh-140594: heap-buffer-underflow in PyOS_StdioReadline when a NUL (\0) is present in interactive input
+        with os_helper.EnvironmentVarGuard() as env:
+            env.unset('PYTHONSTARTUP')
+            args = [sys.executable, '-I', '-S', '-q', '-i']
+            p = subprocess.Popen(
+                args,
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.STDOUT,
+                env=env.copy(),
+            )
+            out, _ = p.communicate(b'\x00', timeout=10)
+            self.assertEqual(
+                p.returncode, 0,
+                msg=f"Interpreter aborted on NUL input, output:\n{out[:500]!r}"
+            )
+
     def test_relativedir_bug46421(self):
         # Test `python -m unittest` with a relative directory beginning with ./
         # Note: We have to switch to the project's top module's directory, as per

Misc/NEWS.d/next/Security/2025-11-02-16-23-17.gh-issue-140594.YIWUpl.rst

@@ -0,0 +1,2 @@
+Fix a buffer overflow in :c:func:`!PyOS_StdioReadline` when a single NULL character is read from the standard input.
+Patch by Shamil Abdulaev.

Parser/myreadline.c

@@ -344,7 +344,7 @@ PyOS_StdioReadline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
             break;
         }
         n += strlen(p + n);
-    } while (p[n-1] != '\n');
+    } while (n > 0 && p[n-1] != '\n');
 
     pr = (char *)PyMem_RawRealloc(p, n+1);
     if (pr == NULL) {