Many speakers of American English have long mispronounced the phrase social security by assimilating the medial hushing sound /ʃ/of the first word to the initial hissing sound /s/of both. This change––for it is a change––can straightforwardly be reckoned a case of (so-called) assimilation at a distance, but this would be a unique instance of /ʃ/ > /s/ in any context, let alone a non-contiguous one, in English, hence suspect as an assimilation. Typologically, as is true of /s/ before /i/ in the phrase at issue, the directionality is rather from /s/ to /ʃ/ and not the reverse, i. e., a garden-variety case of palatalization, observable in the histories of many languages, where a dental (here the hiss-sibilant) becomes a palatal (here the hush-sibilant) before a front vowel (here /i/).
The replacement of /ʃ/ by /s/ in non-normative speech is to be explained otherwise, specifically as an UNMARKING. The palatal /ʃ/ is marked for compactness, whereas the dental /s/ is unmarked for this feature. Additionally, it is important to keep firmly in mind that the unique change at issue occurs only in this fixed phrase, where the context is a compound (consisting of an adjective plus a substantive). Now, it is a fact that the process of composition (as, for that matter, derivational morphology generally) is often accompanied by an unmarking of the individual constituents that go to make up the compositum. What this means is that some marked aspect of an individual constituent is replaced by its unmarked counterpart when that constituent enters into a compound.
Taking the same process in a non-Indo-European language like Japanese for comparison, one sees that compounding regularly involves the replacement of a phonetically voiceless (actually, a phonemically tense) obstruent at the beginning of the second constituent of the compound by its phonetically voiced (resp. phonemically lax) counterpart, e.g., fuufu ‘husband and wife’ + kenka ‘quarrel’ > fuufugenka ‘marital strife’—and never the other way around. Tenseness being marked and laxness unmarked for obstruents in languages with phonemic protensity (like English or Japanese or Serbo-Croatian or French), the replacement of the initial /k/ of the second constituent kenka by /g/ is clearly an unmarking, completely parallel to the replacement in the phrase social security of the medial /ʃ/ by /s/. This phrase, moreover, has a superordinate meaning that is not the simple product of social + security. Thus the replacement of the hushing by the hissing sibilant is completely consistent with the nature of composition, namely the subordination of individual constituents to the resultant compound both formally and semantically. The normative pronunciation of the first constituent does not, of course, undermine the status of the phrase as a compound. But compared to the non-normative pronunciation, it has simply not exploited the semiotic potential attendant on compounding that the latter has.