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Shellshock，又称Bashdoor，是Bash shell的一系列安全漏洞。最早在2014年9月24日公开。许多互联网守护进程，如网页服务器，使用bash来处理某些命令，从而允许攻击者在易受攻击的Bash版本上执行任意代码。这可使攻击者在未授权的情况下访问计算机系统。

A simple Shellshock logo, similar to the Heartbleed bug logo

Shellshock, also known as Bashdoor, is a family of security bugs in the widely used Unix Bash shell, the first of which was disclosed on 24 September 2014. Many Internet-facing services, such as some web server deployments, use Bash to process certain requests, allowing an attacker to cause vulnerable versions of Bash to execute arbitrary commands. This can allow an attacker to gain unauthorized access to a computer system.

Stéphane Chazelas contacted Bash's maintainer, Chet Ramey, on 12 September 2014 telling Ramey about his discovery of the original bug, which he called "Bashdoor". Working together with security experts, he soon had a patch as well. The bug was assigned the CVE identifier CVE-2014-6271. It was announced to the public on 24 September 2014 when Bash updates with the fix were ready for distribution.

The first bug causes Bash to unintentionally execute commands when the commands are concatenated to the end of function definitions stored in the values of environment variables. Within days of the publication of this, intense scrutiny of the underlying design flaws discovered a variety of related vulnerabilities (CVE-2014-6277, CVE-2014-6278, CVE-2014-7169, CVE-2014-7186, and CVE-2014-7187); which Ramey addressed with a series of further patches.

Attackers exploited Shellshock within hours of the initial disclosure by creating botnets of compromised computers to perform distributed denial-of-service attacks and vulnerability scanning. Security companies recorded millions of attacks and probes related to the bug in the days following the disclosure.

Shellshock could potentially compromise millions of unpatched servers and other systems. Accordingly, it has been compared to the Heartbleed bug in its severity.

概述

取得等角视角的旋转步骤

等角格纸

各座标轴之间角度相等(120°)的视角，就是等角的观点。以正方体为例，先从一个面看过去，然后以垂直方向为转轴旋转视角±45°，接著以水平方向为转轴旋转视角约±35.264° (准确来说是 arcsin 1⁄√3)。值得注意的是，平面上正方体(右图左上)的边界恰好是一个正六角形：所有黑线的长度相同，而且正方体每一面的面积相同。等角格纸可以在不需要计算的情况下帮忙达到目标。

等角的观点也可以想成是：从一个正方体空间的顶点，看向对面的顶点。x轴向右下延伸、y轴向左下延伸、z轴向上延伸。沿个轴的线彼此成120°。 　 　

数学

有8种可能的视角符合等角，取决于观察者的卦限。以第一挂限为例，三维上的点a_x,y,z等角投影到二维上成点 b_x,y，数学上可以写成旋转矩阵：

${\begin{bmatrix}\mathbf {c} _{x}\\\mathbf {c} _{y}\\\mathbf {c} _{z}\\\end{bmatrix}}={\begin{bmatrix}1&0&0\\0&{\cos \alpha }&{\sin \alpha }\\0&{-\sin \alpha }&{\cos \alpha }\\\end{bmatrix}}{\begin{bmatrix}{\cos \beta }&0&{-\sin \beta }\\0&1&0\\{\sin \beta }&0&{\cos \beta }\\\end{bmatrix}}{\begin{bmatrix}\mathbf {a} _{x}\\\mathbf {a} _{y}\\\mathbf {a} _{z}\\\end{bmatrix}}={\frac {1}{\sqrt {6}}}{\begin{bmatrix}{\sqrt {3}}&0&-{\sqrt {3}}\\1&2&1\\{\sqrt {2}}&-{\sqrt {2}}&{\sqrt {2}}\\\end{bmatrix}}{\begin{bmatrix}\mathbf {a} _{x}\\\mathbf {a} _{y}\\\mathbf {a} _{z}\\\end{bmatrix}}$

其中 α = arcsin(1⁄√3) ≈ 35.264° 且 β = 45°。

接著正投影到xy平面:

${\begin{bmatrix}\mathbf {b} _{x}\\\mathbf {b} _{y}\\0\\\end{bmatrix}}={\begin{bmatrix}1&0&0\\0&1&0\\0&0&0\\\end{bmatrix}}{\begin{bmatrix}\mathbf {c} _{x}\\\mathbf {c} _{y}\\\mathbf {c} _{z}\\\end{bmatrix}}$

另外7种符合等角的视角，可以借由旋转反方向或镜像来达成。

历史与限制

研磨机 (1822), 等角投影

三国演义中的等角投影

等角的概念存在了数个世纪，最早由William Farish(1759–1837)在1822年形式化。 十九世纪中期，等角投影成为工程师重要的工具。不久之后等角投影和轴测投影被加入欧美国家的建筑学课程中。 然而，轴测投影发源于中国。在它对中国艺术的功能，就如同透视投影之于西洋艺术。轴测投影和相关的绘图法则对电脑图学 有所帮助。

难以辨识高度的情况，无法从局部看出两球的高度差

潘洛斯阶梯 画著一座循环的阶梯

如同各种平行投影的方法，等角投影所绘制的物体不会因为远近而改变大小。虽然建筑制图等时候，需要直接测量长度，但却会造成视觉上失真。不像透视投影呈现眼睛和照相机的结果。等角投影有时会造成高度难以辨识(如右左下图)。这被用来创造不可能出现的图形，如潘洛斯阶梯。

在电玩和像素画的用途

等角投影提供有限的3D效果，在1980~90年代，因为当时的微电脑能够负荷，而被用在早期的电玩中。

这种风格也被用在精灵和像素画，用来呈现复古游戏的风格。

参见

• 图形投影
• 轴测投影

外部链接

• Isometric Projection