Headline

Microsoft Windows UTF-8 Buffer Overruns

When Microsoft released UTF-8 support for the -A interfaces of the Windows API, it appears to have introduced buffer overrun conditions.

1 year ago

Packet Storm

Open in Source

#windows #microsoft

Hi @ll,

almost 4 years ago, with Windows 10 1903, after more than a year
beta-testing in insider previews, Microsoft finally released UTF-8
support for the -A interfaces of the Windows API.

https://docs.microsoft.com/en-us/windows/uwp/design/globalizing/use-utf8-code-page#activeCodePage

| If the ANSI code page is configured for UTF-8, -A APIs typically
| operate in UTF-8. This model has the benefit of supporting
| existing code built with -A APIs without any code changes.

The last claim is but a bloody DANGEROUS lie!
As shown hereafter, it must read instead:
“This model has the malefit of causing buffer overruns in existing
code!”
For 30 years, the documentation of the -A interfaces for file and
directory management of the Win32 API states:
“The maximum path name length is 260 characters.”

See https://msdn.microsoft.com/en-us/library/aa363855.aspx
“CreateDirectoryA function” for example:

| For the ANSI version of this function, there is a default string
| size limit for paths of 248 characters (MAX_PATH - enough room
| for a 8.3 filename). …
…
| The 255 character limit per path segment still applies.

This constitutes a contractual GUARANTEE for the product behaviour!
The documentation for the file systems supported by Windows says
too “The maximum length of a file name segment is 255 characters.”
See https://msdn.microsoft.com/en-us/library/ee681827.aspx
“File System Functionality Comparison”
With these 2 contractually GUARANTEED preconditions, the following
code is safe, i.e. not susceptible to a buffer overrun:
CreateDirectoryA() fails as soon as szPath exceeds the documented
limit which is less than the buffer size of 260 characters.

CHAR szANSI[] = "€"; // or one of the following other 122 characters
// from ANSI code page 1252:
// ‚ƒ„…†‡ˆ‰Š‹ŒŽ‘’“”•–—˜™š›œžŸ ¡¢£¤¥¦§¨©ª«¬®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂ
// ÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ
CHAR szPath[MAX_PATH] = "";
do
strcat(szPath, szANSI);
while (CreateDirectoryA(szPath));
With UTF-8 support enabled, the same code now suffers from a
buffer overrun:

CHAR szUTF8[] = u"€"; // or “\xE2\x82\xAC”
CHAR szPath[MAX_PATH] = "";
do
strcat(szPath, szUTF8);
while (CreateDirectoryA(szPath), NULL);

STRIKE 1!
Given the 2 guarantees from 1) and 2), the following code is
also safe and not susceptible to a buffer overrun: see
https://msdn.microsoft.com/en-us/library/aa365740.aspx
“WIN32_FIND_DATAA structure” and “FindFirstFile function”
https://msdn.microsoft.com/en-us/library/aa364418.aspx

wfd.cFileName can NEVER receive a file/directory name longer
than 255 characters, so the concatenation of C: or .\ (as
well as C:\ and …\ too) and wfd.cFileName NEVER overruns a
buffer of MAX_PATH!

#define PATTERN "C:" // or "C:\" or ".\" or "…\"

WIN32_FIND_DATAA wfd;
CHAR szPath[MAX_PATH] = PATTERN;
HANDLE hFind = FindFirstFileA(szPath, &wfd);
if (hFind != INVALID_HANDLE_VALUE) {
do {
strcat(szPath + strlen(PATTERN) - 1, wfd.cFileName);
GetFileAttributesA(szPath); // do something with the
… // found file system objects
} while (FindNextFile(hFind, &wfd));
FindClose(hFind);
}
With UTF-8 support enabled and a file or directory named
€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€
(or other 86 characters from the above 123) present in the
CWD of drive C:, FindFirstFileA() (and FindNextFileA() too)
return a string of 86 * 3 = 258 characters in wfd.cFileName
which causes a buffer overrun in previously safe code!

STRIKE 2!
The following code enumerates ALL file system objects in a
(root) directory or network share:

WIN32_FIND_DATAA wfd;
HANDLE hFind = FindFirstFileA("\\host\share\*", &wfd);
if (hFind != INVALID_HANDLE_VALUE) {
do { // code to process wfd.cFileName omitted
} while (FindNextFile(hFind, &wfd));
FindClose(hFind);
}
With UTF-8 support enabled and a directory or file named
€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€
(i.e. 87 or more of the 123 characters from above) present,
both FindFirstFile() and FindNextFile() FAIL with the
previously impossible, NEVER encountered Win32 error code
234 alias ERROR_MORE_DATA: wfd.cFileName is to short for
UTF-8 encoded file/directory segment names!

STRIKE 3!