Lack of error checking on calls to UTF8toUTF16, rule filter bypass

Summary

Event filtering in Sysmon For Linux incorrectly assumes event data, such as executable image paths, will be valid UTF-8 and that conversion to UTF-16 will always succeed. This can result in incorrect filtering results and event logging bypass.

Details

Event filtering on Linux makes two calls to UTF8toUTF16 before comparing event data against filter rules:

#if defined __linux__
        // on Linux, convert data to UTF16, on heap
        size_t fieldValueLen = UTF8toUTF16( NULL, (PCHAR)fieldValue, 0 );
        WCHAR *fieldValueUTF16 = (WCHAR *)malloc(fieldValueLen * sizeof(WCHAR));
        if (fieldValueUTF16 == NULL) {
            printf("Out of memory\n");
            return Failed;
        }
        UTF8toUTF16( fieldValueUTF16, (PCHAR)fieldValue, fieldValueLen );
        fieldValue = fieldValueUTF16;
#endif

https://github.com/Sysinternals/SysmonCommon/blob/1ca3832963dfce9f0e4a3d08fdcbd6de1df0cf94/rules.c#L1805-L1815

The first call to it is done in such a way that it just computes the length of the would-be converted string, the value of which is then used in a call to malloc, where the second call performs actual conversion on the newly allocated heap space.

The issue is that Linux paths are not required to be UTF-8 and calls to UTF8toUTF16 can fail, which is not accounted for:

The first call to UTF8toUTF16 results in error, with returns zero. https://github.com/Sysinternals/SysmonForLinux/blob/f5d6219ec099acf61c1d3b3eecdabaed69faefe4/linuxWideChar.c#L38
malloc is then called with argument 0 (fieldValueLen * sizeof(WCHAR)), which can return a valid pointer on the heap.
UTF8toUTF16 is called again to perform the conversion on this newly allocated heap space, but no data will be written because the argument to len is zero https://github.com/Sysinternals/SysmonForLinux/blob/f5d6219ec099acf61c1d3b3eecdabaed69faefe4/linuxWideChar.c#L58
The code continues to call MatchFilterOnSpecificRule with our pointer on the heap, which will result in either comparing against invalid heap data or a NULL pointer dereference.

This could be used to bypass filter rules where a match should be found for an event. For example, an executable at path /tmp/� (where the “�” is hex FF) with the following filter rule:

...
    <ProcessCreate onmatch="include">
        <Image condition="begin with">/tmp</Image>
    </ProcessCreate>
...

On Fixing

The invalid heap access could be resolved by checking for errors from UTF8toUTF16. For example: https://github.com/inickles/SysmonCommon/commit/01a772320d385146cd91a71b186e9eaa7c912963

However, this does not fully mitigate the issue of being a potential event filter bypass. FilterEventRules will default to excluding events if no matches were found and there are multiple rules defined: https://github.com/Sysinternals/SysmonCommon/blob/73ae2ac398dcba2ae01c2e40664f662c9fc270c8/rules.c#L1968

It seems the conversion to UTF-16 is done to be able to code shared with the Windows version, but in do so Sysmon For Linux apparently assumes these conversions will always succeed, which won’t always be the case.

Other issues in assuming UTF8toUTF16 will succeed can be found in Sysmon For Linux config parsing in https://github.com/Sysinternals/SysmonCommon/blob/73ae2ac398dcba2ae01c2e40664f662c9fc270c8/xml.cpp, though these are seemingly less serious, where a filter value might be terminated earlier than expected.

Sysinternals / SysmonForLinux