If active neutrinos undergo non-standard (`secret) interactions (NS$ u$I) the cosmological evolution of the neutrino fluid might be altered, leaving an imprint in cosmological observables. We use the latest publicly available CMB data from Planck to constrain NS$ u$I inducing $ u- u$ scattering, under the assumption that the mediator $phi$ of the secret interaction is very light. We find that the effective coupling constant of the interaction, $g_mathrm{eff}^4 equiv langle sigma vrangle T_ u^2$, is constrained at $< 2.35times10^{-27}$ (95% credible interval), which stregthens to $g_mathrm{eff}^4 < 1.64times10^{-27}$ when Planck non-baseline small-scale polarization is considered. Our findings imply that after decoupling at $Tsimeq 1$ MeV, cosmic neutrinos are free streaming at redshifts $z>3800$, or $z>2300$ if small-scale polarization is included. These bounds are only marginally improved when data from geometrical expansion probes are included in the analysis to complement Planck. We also find that the tensions between CMB and low-redshift measurements of the expansion rate $H_0$ and the amplitude of matter fluctuations $sigma_8$ are not significantly reduced. Our results are independent on the underlying particle physics model as long as $phi$ is very light. Considering a model with Majorana neutrinos and a pseudoscalar mediator we find that the coupling constant $g$ of the secret interaction is constrained at $lesssim 7times 10^{-7}$. By further assuming that the pseudoscalar interaction comes from a dynamical realization of the see-saw mechanism, as in Majoron models, we can bound the scale of lepton number breaking $v_sigma$ as $gtrsim (1.4times 10^{6})m_ u$.
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