Encountered a heap-buffer-overflow while itterating with pointers

Question

I have a program with the aim of finding the GCD of two strings s and t, i.e, return the largest x such that x|s and x|t. While testing this program, I encountered a heap-buffer-overflow. I am aware that this error implies a segmentation fault. However, I am not sure where in the program this error arises.

string gcdOfStrings(string str1, string str2) {

        const char *ptrI = str1.c_str(), *ptrJ = str2.c_str();
        char *str3 = new char[1000 + 1], *ptrK = str3;
        bool boolI = false, boolJ = false;

        while (*ptrI == *ptrJ) { *ptrK++ = *ptrI++; }

        ptrI = str1.c_str();

        while ( (*ptrI != '\0') || (*ptrJ != '\0') || (*ptrK != '\0') ) {

            if (*ptrK == '\0') { ptrK = str3; }
            
            if (*ptrI != *ptrK) { boolI = true; }
            if (*ptrI != '\0')  { ptrI++; }

            if (*ptrJ != *ptrK) { boolJ = true; }
            if (*ptrJ != '\0')  { ptrJ++; }

            if ( (boolI != true) && (boolJ != true) ) { *ptrK = '\0'; string t(str3); delete[] str3; return t; }

            ptrK++;
        }
        
        return "";
    }

Example 1:

Input: str1 = "ABCABC", str2 = "ABC" Output: "ABC"

Example 2:

Input: str1 = "ABABAB", str2 = "ABAB" Output: "AB"

Example 3:

Input: str1 = "ABABCB", str2 = "ABAB" Output: ""