HTTP response smuggling vulnerability in Mozilla Firefox and Thunderbird before 1.5.0.4, when used with certain proxy servers, allows remote attackers to cause Firefox to interpret certain responses as if they were responses from two different sites via (1) invalid HTTP response headers with spaces between the header name and the colon, which might not be ignored in some cases, or (2) HTTP 1.1 headers through an HTTP 1.0 proxy, which are ignored by the proxy but processed by the client.