Description Brandon Sterne (:bsterne) 2011-09-23 14:39:04 PDT

Fabian Yamaguchi of Recurity Labs on behalf of BSI (German Federal Office for Information Security) reported the following to security@mozilla.org:

Method: nsLocalFile::EnsureShortPath

Component:
The bug is contained in the nsLocalFile Implementation for Windows.

Summary:
The method nsLocalFile::EnsureShortPath contains a call to GetShortPathName, which potentially overflows a stack­based buffer.

Overview:
The method nsLocalFile::EnsureShortPath reads a path name into a local stack­buffer by using the Win32 API function GetShortPathNameW. This function expects the number of wide­characters that the destination buffer can hold as its last argument. In EnsureShortPath, this third parameter has mistakenly been chosen to contain the number of bytes the buffer consists of, which is twice as much as the number of wide­characters it can hold. Therefore, it may be possible to trigger a stack­ based buffer­overflow.

Actual Results:
A local stack­buffer can potentially be overflowed.

Expected Results:
The buffer cannot be overflowed.

Additional Information:
The vulnerable code is the following.
void nsLocalFile::EnsureShortPath()
{
[...]
    WCHAR thisshort[MAX_PATH];
    DWORD thisr = ::GetShortPathNameW(mWorkingPath.get(), thisshort,
                                      sizeof(thisshort));
[...]
}