Python 3.5 scan_eol() Buffer Over-Read

2015.11.03
Credit: John Leitch
Risk: High
Local: Yes
Remote: No
CVE: N/A
CWE: CWE-119

Title: Python 3.5 scan_eol() Buffer Over-read Credit: John Leitch (john@autosectools.com), Bryce Darling (darlingbryce@gmail.com) Url1: http://autosectools.com/Page/Python-scan_eol-Buffer-Over-read Url2: http://bugs.python.org/issue24989 Resolution: Fixed Python 3.5 suffers from a vulnerability caused by the behavior of the scan_eol() function. When called, the function gets a line from the buffer of a BytesIO object by searching for a newline character starting at the position in the buffer. However, if the position is set to a value that is larger than the buffer, this logic will result in a call to memchr that reads off the end of the buffer: /* Move to the end of the line, up to the end of the string, s. */ start = PyBytes_AS_STRING(self->buf) + self->pos; maxlen = self->string_size - self->pos; if (len < 0 || len > maxlen) len = maxlen; if (len) { n = memchr(start, '\n', len); In some applications, it may be possible to exploit this behavior to disclose the contents of adjacent memory. The buffer over-read can be observed by running the following script: import tempfile a = tempfile.SpooledTemporaryFile() a.seek(0b1) a.readlines() Which, depending on the arrangement of memory, may produce an exception such as this: 0:000> g (698.188): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling. This exception may be expected and handled. eax=fff8a14c ebx=0a0a0a0a ecx=00000000 edx=05bb1000 esi=061211b0 edi=89090909 eip=61c6caf2 esp=010af8dc ebp=010af914 iopl=0 nv up ei ng nz ac po nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010292 python35!memchr+0x62: 61c6caf2 8b0a mov ecx,dword ptr [edx] ds:002b:05bb1000=???????? 0:000> k1 ChildEBP RetAddr 010af8e0 61b640f1 python35!memchr+0x62 [f:\dd\vctools\crt_bld\SELF_X86\crt\src\INTEL\memchr.asm @ 125] To fix this issue, it is recommended that scan_eol() be updated to check that the position is not greater than or equal to the size of the buffer. A proposed patch is attached.

References:

http://bugs.python.org/issue24989


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