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HTTPS Hackable In 30 Seconds: DHS Alert

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Security experts are warning website operators to test whether their HTTPS traffic is vulnerable to a new crypto attack that can be used to grab sensitive information.

The so-called BREACH attack -- short for Browser Reconnaissance and Exfiltration via Adaptive Compression of Hypertext -- was detailed in a Department of Homeland Security (DHS) "BREACH vulnerability in compressed HTTPS" advisory, issued Friday, which warned that "a sophisticated attacker may be able to derive plaintext secrets from the ciphertext in an HTTPS stream." All versions of the transport layer security (TLS) and secure sockets layer (SSL) protocols are vulnerable.


But Prado said that numerous sites are at risk, and that crafting a related HTTPS fix would likely be "nontrivial." Still, the DHS advisory details mitigation strategies that businesses can employ, which include disabling HTTP compression such as gzip, as well as randomizing the secrets being transmitted in any particular request.

in informationweek

Por muito mais que que estude só aprendo uma coisa, que ainda tenho muita coisa para aprender.

A beleza de um código está em decompor problemas complexos em pequenos blocos simples.

"learn how to do it manually first, then use the wizzy tool to save time."

"Kill the baby, don't be afraid of starting all over again. Fail soon, learn fast."

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Mais info

Though the article is not full of details, we can infer a few things:
  • Attack uses compression with the same general principle as CRIME: the attacker can make a target system compress a sequence of characters which includes both a secret value (that the attacker tries to guess) and some characters that the attacker can choose. That's a chosen plaintext attack. The compressed length will depend on whether the attacker's string "looks like" the secret or not. The compressed length leaks through SSL encryption, because encryption hides contents, not length.
  • The article specifically speaks of "any secret that's [...] located in the body". So we are talking about HTTP-level compression, not SSL-level compression. HTTP compression applies on the request body only, not the header. So secrets in the header, in particular cookie values, are safe from that one.
  • Since there are "probe requests", then the attack requires some malicious code in the client browser; the attacker must also observe the encrypted bytes on the network, and coordinate both elements. This is the same setup as for CRIME and BEAST.
  • It is unclear (from the article alone, which is all I have right now to discuss on) whether the compressed body is one from the client or from the server. "Probe request" are certainly sent by the client (on behalf of the attacker) but responses from the server may include part of that which is sent in the request, so the "chosen plaintext attack" can work both ways.

In any case, "BREACH" looks like an attack methodology which needs to be adapted to the specific case of a target site. In that sense, it is not new at all; it was already "well-known" that compression leaks information and there was no reason to believe that HTTP-level compression was magically immune. Heck, it was discussed right here last year. It is a good thing, however, that some people go the extra mile to show working demonstrations because otherwise flaws would never be fixed. For instance, padding oracle attacks against CBC had been described and even prototyped in 2002, but it took an actual demo against ASP in 2010 to convince Microsoft that the danger was real. Similarly for BEAST in 2011 (the need for unpredictable IV for CBC mode was known since 2002 as well) and CRIME in 2012; BREACH is more "CRIME II": one more layer of pedagogy to strike down the unbelievers.

Unfortunately, a lot of people will get it wrong and believe it to be an attack against SSL, which it is not. It has nothing to do with SSL, really. It is an attack which forces an information leak through a low-bandwidth data channel, the data length, that SSL has never covered, and never claimed to cover.

The one-line executive summary is that thou shalt not compress.

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Ah! Um problema no HTTPS que não é no TLS...

No 1º post parecia um problema no TLS e estava a pensar que aquilo é tão perfeito que nem apercebia-me como é que tinha conseguido tal coisa :).

"[Os jovens da actual geração]não lêem porque não envolve um telecomando que dê para mirar e atirar, não falam porque a trapalhice é rainha e o calão é rei" autor: thoga31

Life is a genetically transmitted disease, induced by sex, with death rate of 100%.

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