  Let $F$ be a finite extension of $\mathbb{Q}_p$. Let $W(k)$ denote the Witt
vectors of an algebraically closed field $k$ of characteristic $\ell$ different
from $p$ and $2$, and let $\mathcal{Z}$ be the spherical Hecke algebra for
$GL_n(F)$ over $W(k)$. Given a Hecke character $\lambda:\mathcal{Z}\to R$,
where $R$ is an arbitrary $W(k)$-algebra, we introduce the universal unramified
module $\mathcal{M}_{\lambda,R}$. We show $\mathcal{M}_{\lambda,R}$ embeds in
its Whittaker space and is flat over $R$, resolving a conjecture of Lazarus. It
follows that $\mathcal{M}_{\lambda,k}$ has the same semisimplification as any
unramified principle series with Hecke character $\lambda$.
  In the setting of mod-$\ell$ automorphic forms, Clozel, Harris, and Taylor
formulate a conjectural analogue of Ihara's lemma. It predicts that every
irreducible submodule of a certain cyclic module $V$ of mod-$\ell$ automorphic
forms is generic. Our result on the Whittaker model of
$\mathcal{M}_{\lambda,k}$ reduces the Ihara conjecture to the statement that
$V$ is generic.
