The malloc function in the GNU C Library (aka glibc or libc6) 2.26 could return a memory block that is too small if an attempt is made to allocate an object whose size is close to SIZE_MAX, potentially leading to a subsequent heap overflow. This occurs because the per-thread cache (aka tcache) feature enables a code path that lacks an integer overflow check.
The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.