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vulnerability note CVE-2011-3389 CVE-2012-1870

SSL, TLS: obtaining HTTPS Cookies, BEAST

Synthesis of the vulnerability

An attacker, who can control HTTPS connections of victim's web browser and which has a sufficient bandwidth, can use several SSL sessions in order to compute HTTP headers, such as cookies.
Impacted software: Asterisk Open Source, IPSO, SecurePlatform, CheckPoint Security Gateway, Debian, BIG-IP Hardware, TMOS, Fedora, HP-UX, Domino, Mandriva Linux, IIS, IE, Windows 2003, Windows 2008 R0, Windows 2008 R2, Windows 7, Windows Vista, Windows XP, Java OpenJDK, openSUSE, Opera, Oracle GlassFish Server, Oracle iPlanet Web Proxy Server, Oracle iPlanet Web Server, Java Oracle, Oracle Web Tier, SSL protocol, RHEL, SIMATIC, Sun AS, SUSE Linux Enterprise Desktop, SLES, Nessus.
Severity of this computer vulnerability: 1/4.
Consequences of a hack: data reading.
Attacker's origin: internet server.
Number of vulnerabilities in this bulletin: 3.
Creation date: 26/09/2011.
Références of this announce: 2588513, 2643584, 2655992, AST-2016-001, BID-49778, BID-54304, c03122753, CERTA-2012-AVI-381, CERTFR-2016-AVI-046, CERTFR-2019-AVI-311, CVE-2004-2770-REJECT, CVE-2011-3389, CVE-2012-1870, DSA-2368-1, DSA-2398-1, DSA-2398-2, FEDORA-2012-5916, FEDORA-2012-5924, FEDORA-2012-9135, FEDORA-2014-13764, FEDORA-2014-13777, HPSBUX02730, javacpuoct2011, MDVSA-2012:058, MDVSA-2012:096, MDVSA-2012:096-1, MDVSA-2012:097, MS12-006, MS12-049, openSUSE-SU-2012:0030-1, openSUSE-SU-2012:0063-1, openSUSE-SU-2012:0199-1, openSUSE-SU-2012:0229-1, openSUSE-SU-2012:0667-1, RHSA-2012:0034-01, RHSA-2013:1455-01, RHSA-2013:1456-01, sk74100, sk86440, SOL13400, SSA-556833, SSRT100710, SUSE-SU-2012:0114-1, SUSE-SU-2012:0114-2, SUSE-SU-2012:0122-1, SUSE-SU-2012:0122-2, swg21568229, VIGILANCE-VUL-11014, VU#864643.

Description of the vulnerability

The SSL/TLS protocol supports CBC (Cipher Block Chaining) encryption: a clear block is "XORed" (operation Exclusive OR) with the last encrypted block, and the result is encrypted. This dependence between a block and its previous block was the subject of several theoretical studies since 2002, and led to the definition of TLS 1.1 in 2006, which uses a different algorithm.

The HTTPS "protocol", used by web browsers, encapsulates an HTTP session in a SSL/TLS session. An HTTP query is like:
  GET /abcdefg HTTP/1.0
  Headers (cookies)
  ...
This query is fragmented in blocks of 8 bytes, which are encrypted by CBC. The first block is thus "GET /abc".

An attacker can setup a malicious web site, and invite the victim to connect. This web site can request the victim's web browser to load the page "/abcdefg" of a site secured by SSL/TLS.

The attacker controls the size of the requested url (via "/abcdefg"), so he can place the first byte of headers at the end of a block (the 7 other bytes are known: "P/1.1\r\n"). This blocks follows a block which is fully known ("defg HTT"). The attacker can then capture the encrypted SSL/TLS session, and memorize the last encrypted block. This block is used as an initialization vector to compute an XOR between "defg HTT" (block 2) encrypted, and a guessed character located at the end of "P/1.1\r\n" (block 3). The result is reinjected by the attacker at the end of the HTTP query in clear text. He captures the resulting encrypted block, and if it is the same as the third encrypted block, then the guessed character was correct. The attacker repeats these queries as many times as necessary.

An attacker, who can control HTTPS connections of victim's web browser and which has a sufficient bandwidth, can therefore use several SSL sessions in order to compute HTTP headers, such as cookies.
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