The data's authenticated, but the authentication is accidentally broken. SYN(seq_num) and SYN(seq_num+3) can both generate the same cookie using a different (b

The data's authenticated, but the authentication is accidentally broken. SYN(seq_num) and SYN(seq_num+3) can both generate the same cookie using a different (but still valid) maximum segment size. To lump this into an existing category of bugs, syn cookies are a kind of HMAC, only the implementation is custom and nonstandard. It isn't a surprise that a bespoke HMAC leaks, but to the credit of kernel developers syn cookies the initial 1996 specification pre-dates the common understanding. (But to its demerit, it looks like the DJB spec (http://cr.yp.to/syncookies.html http://cr.yp.to/syncookies.html) would have not had this issue, since the MSS was encoded in the top bits of the cookie and not the bottom bits.)