Why are elliptic curves over a field of characteristic 2 or 3 insecure? The following is a quotation from my cryptography course:


  Recent results on the discrete logarithm raise big concerns on the security of elliptic curves over a binary field. 


What are these results? Also, is characteristic three safe?
There is no known subexponential-cost algorithm for computing discrete logs in elliptic curves over fields of small characteristic—barring standard generic algorithms on groups of smooth order, transfers to $\operatorname{GF}(2^n)$, etc.—but there seems to be exploitable structure that just hasn't been worked out yet.  The most recent survey seems to be from 2015:


  Stephen D. Galbraith and Pierrick Gaudry, ‘Recent progress on the elliptic curve discrete logarithm problem’, IACR Cryptology ePrint Archive: Report 2015/1022, 2015-10-22.


See in particular §10.2, ‘A subexponential algorithm for elliptic curves over $\mathbb F_{2^n}$?’, p. 18.