Using {em Chandra} observations in the 2.15 deg$^{2}$ COSMOS legacy field, we present one of the most accurate measurements of the Cosmic X-ray Background (CXB) spectrum to date in the [0.3-7] keV energy band. The CXB has three distinct components: contributions from two Galactic collisional thermal plasmas at kT$sim$0.27 and 0.07 keV and an extragalactic power-law with photon spectral index $Gamma$=1.45$pm{0.02}$. The 1 keV normalization of the extragalactic component is 10.91$pm{0.16}$ keV cm$^{-2}$ s$^{-1}$ sr$^{-1}$ keV$^{-1}$. Removing all X-ray detected sources, the remaining unresolved CXB is best-fit by a power-law with normalization 4.18$pm{0.26}$ keV cm$^{-2}$ s$^{-1}$ sr$^{-1}$ keV$^{-1}$ and photon spectral index $Gamma$=1.57$pm{0.10}$. Removing faint galaxies down to i$_{AB}sim$27-28 leaves a hard spectrum with $Gammasim$1.25 and a 1 keV normalization of $sim$1.37 keV cm$^{-2}$ s$^{-1}$ sr$^{-1}$ keV$^{-1}$. This means that $sim$91% of the observed CXB is resolved into detected X-ray sources and undetected galaxies. Unresolved sources that contribute $sim 8-9%$ of the total CXB show a marginal evidence of being harder and possibly more obscured than resolved sources. Another $sim$1% of the CXB can be attributed to still undetected star forming galaxies and absorbed AGN. According to these limits, we investigate a scenario where early black holes totally account for non source CXB fraction and constrain some of their properties. In order to not exceed the remaining CXB and the $zsim$6 accreted mass density, such a population of black holes must grow in Compton-thick envelopes with N$_{H}>$1.6$times$10$^{25}$ cm$^{-2}$ and form in extremely low metallicity environments $(Z_odot)sim10^{-3}$.
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