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Microsoft Windows UTF-8 Buffer Overruns ≈ Mobile Hacker For Hire

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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.

0) <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!”

1) 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!

2) 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”

3) 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));

4) 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!

5) 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);
}

6) 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!

7) 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);
}

8) 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!

stay tuned, and far away from UTF-8 in Windows
Stefan Kanthak

PS: for the full story of Microsoft’s EPIC failures with UTF-8
in Windows, see
<https://skanthak.homepage.t-online.de/quirks.html#quirk33>
<https://skanthak.homepage.t-online.de/quirks.html#quirk32>
<https://skanthak.homepage.t-online.de/quirks.html#quirk31>

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