Similar to https://bugs.python.org/issue25388 the following causes a crash on 3.5.1 and the latest 3.5 branch:

./python -c 'with open("vuln.py", "wb") as f: f.write(b"\x61\x73\x00\x0a\x79\x6e\x63\x5c\x0a\xef")'
./python vuln.py


Python 3.5.1+ (default, Jan  4 2016, 00:05:40) 
=================================================================
==24400==ERROR: AddressSanitizer: heap-use-after-free on address 0xf270f100 at pc 0x080ad09e bp 0xffef5ee8 sp 0xffef5ac0
READ of size 2 at 0xf270f100 thread T0
    #0 0x80ad09d in strncpy (/home/will/python/cpython/python+0x80ad09d)
    #1 0x8589b56 in parsetok /home/will/python/cpython/Parser/parsetok.c:235:13
    #2 0x858b301 in PyParser_ParseFileObject /home/will/python/cpython/Parser/parsetok.c:134:12
    #3 0x8439e0b in PyParser_ASTFromFileObject /home/will/python/cpython/Python/pythonrun.c:1150:15
    #4 0x843aa37 in PyRun_FileExFlags /home/will/python/cpython/Python/pythonrun.c:916:11
    #5 0x8438a98 in PyRun_SimpleFileExFlags /home/will/python/cpython/Python/pythonrun.c:396:13
    #6 0x84382a6 in PyRun_AnyFileExFlags /home/will/python/cpython/Python/pythonrun.c:80:16
    #7 0x813f194 in run_file /home/will/python/cpython/Modules/main.c:318:11
    #8 0x813f194 in Py_Main /home/will/python/cpython/Modules/main.c:768
    #9 0x8138070 in main /home/will/python/cpython/./Programs/python.c:69:11
    #10 0xf7558496 in __libc_start_main (/usr/lib32/libc.so.6+0x18496)
    #11 0x80715b7 in _start (/home/will/python/cpython/python+0x80715b7)

0xf270f100 is located 0 bytes inside of 8194-byte region [0xf270f100,0xf2711102)
freed by thread T0 here:
    #0 0x810c2a4 in __interceptor_cfree.localalias.1 (/home/will/python/cpython/python+0x810c2a4)
    #1 0x8139560 in _PyMem_RawFree /home/will/python/cpython/Objects/obmalloc.c:90:5
    #2 0x813852b in PyMem_Free /home/will/python/cpython/Objects/obmalloc.c:349:5
    #3 0x8596b05 in error_ret /home/will/python/cpython/Parser/tokenizer.c:198:9
    #4 0x8596b05 in decoding_fgets /home/will/python/cpython/Parser/tokenizer.c:636
    #5 0x8594df0 in tok_nextc /home/will/python/cpython/Parser/tokenizer.c:1016:21
    #6 0x858ebba in tok_get /home/will/python/cpython/Parser/tokenizer.c:1457:13
    #7 0x858fc79 in tok_get /home/will/python/cpython/Parser/tokenizer.c:1524:34
    #8 0x858e1da in PyTokenizer_Get /home/will/python/cpython/Parser/tokenizer.c:1804:18
    #9 0x85899a7 in parsetok /home/will/python/cpython/Parser/parsetok.c:208:16
    #10 0x858b301 in PyParser_ParseFileObject /home/will/python/cpython/Parser/parsetok.c:134:12
    #11 0x8439e0b in PyParser_ASTFromFileObject /home/will/python/cpython/Python/pythonrun.c:1150:15
    #12 0x843aa37 in PyRun_FileExFlags /home/will/python/cpython/Python/pythonrun.c:916:11
    #13 0x8438a98 in PyRun_SimpleFileExFlags /home/will/python/cpython/Python/pythonrun.c:396:13
    #14 0x84382a6 in PyRun_AnyFileExFlags /home/will/python/cpython/Python/pythonrun.c:80:16
    #15 0x813f194 in run_file /home/will/python/cpython/Modules/main.c:318:11
    #16 0x813f194 in Py_Main /home/will/python/cpython/Modules/main.c:768
    #17 0x8138070 in main /home/will/python/cpython/./Programs/python.c:69:11
    #18 0xf7558496 in __libc_start_main (/usr/lib32/libc.so.6+0x18496)

previously allocated by thread T0 here:
    #0 0x810c784 in realloc (/home/will/python/cpython/python+0x810c784)
    #1 0x8139541 in _PyMem_RawRealloc /home/will/python/cpython/Objects/obmalloc.c:84:12
    #2 0x8138506 in PyMem_Realloc /home/will/python/cpython/Objects/obmalloc.c:343:12
    #3 0x8594f1c in tok_nextc /home/will/python/cpython/Parser/tokenizer.c:1058:31
    #4 0x858e4c9 in tok_get /home/will/python/cpython/Parser/tokenizer.c:1354:17
    #5 0x858e1da in PyTokenizer_Get /home/will/python/cpython/Parser/tokenizer.c:1804:18
    #6 0x85899a7 in parsetok /home/will/python/cpython/Parser/parsetok.c:208:16
    #7 0x858b301 in PyParser_ParseFileObject /home/will/python/cpython/Parser/parsetok.c:134:12
    #8 0x8439e0b in PyParser_ASTFromFileObject /home/will/python/cpython/Python/pythonrun.c:1150:15
    #9 0x843aa37 in PyRun_FileExFlags /home/will/python/cpython/Python/pythonrun.c:916:11
    #10 0x8438a98 in PyRun_SimpleFileExFlags /home/will/python/cpython/Python/pythonrun.c:396:13
    #11 0x84382a6 in PyRun_AnyFileExFlags /home/will/python/cpython/Python/pythonrun.c:80:16
    #12 0x813f194 in run_file /home/will/python/cpython/Modules/main.c:318:11
    #13 0x813f194 in Py_Main /home/will/python/cpython/Modules/main.c:768
    #14 0x8138070 in main /home/will/python/cpython/./Programs/python.c:69:11
    #15 0xf7558496 in __libc_start_main (/usr/lib32/libc.so.6+0x18496)

SUMMARY: AddressSanitizer: heap-use-after-free (/home/will/python/cpython/python+0x80ad09d) in strncpy
Shadow bytes around the buggy address:
  0x3e4e1dd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x3e4e1de0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x3e4e1df0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x3e4e1e00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x3e4e1e10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x3e4e1e20:[fd]fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x3e4e1e30: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x3e4e1e40: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x3e4e1e50: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x3e4e1e60: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x3e4e1e70: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==24400==ABORTING