Din 6271

[Emmaline]

Thesemitigations involve making changes that could lead to unintended consequences if they are not performed correctly. It is recommended that only experienced system administrators who fully understand the implications of these changes apply the mitigations documented in this article. These mitigations have only been subjected to limited testing by Red Hat.

Important: Given that package updates are available to correct the flaws these mitigations were provided for, these mitigations should only be used in exceptional circumstances and not as a replacement for installing the updated packages. Red Hat recommends all users of Bash to upgrade to the updated packages as swiftly as possible.

The following mod_security rules can be used to reject HTTP requests containing data that may be interpreted by Bash as a function definition if set in its environment. They can be used to block attacks against web services, such as attacks against CGI applications.

Request Header values:

Note that this is a weak workaround, as an attacker could easily send one or two characters per packet, which would avoid matching this signature check. It may, in conjunction with logging, provide an overview of automated attempts at exploiting this vulnerability.

The shellshock vulnerability arises from the fact that you can create environment variables with specially-crafted values before calling the Bash shell. These variables can contain code, which gets executed as soon as the shell is invoked. The initial patch for CVE-2014-6271 was found to be incomplete. The patch for CVE-2014-7169 addresses this, but it does not entirely remove the vulnerable functionality. If you entirely remove the vulnerable functionality from Bash, all possible attack variants will be mitigated.

LD_PRELOAD is an environment variable used by the runtime linking process by searching for shared libraries at alternate locations and by forcibly loading and linking libraries that would be used. This forces some symbols/functions to be loaded and prioritized over built in functions a program. This code creates a new function "strip_env" with the gcc "constructor" attribute set. Functions with this attribute set will execute before entering main(). See -Attributes.html for more details.

Note that this is potentially very dangerous. It is recommend that you just apply this workaround to specific services that may be exploitable on your system. This can be achieved by adding bash_ld_preload.so to the LD_PRELOAD environment variable in the script that will initialize the service. However, this will not work for services that use SUID binaries in some capacity (e.g. httpd with SUEXEC), as SUID binaries will not honour the LD_PRELOAD command.

The controller is based on a programmable fixed-frequency, peak current-mode control architecture. The DC-DC converter can be configured in a myriad of different switching configurations including boost, buck-boost, buck (ground-referenced switch), and SEPIC.

The A6271-1 provides a cost-effective solution using an external logic-level MOSFET and minimum additional external components. The maximum LED current is set with a single external sense resistor and can be accurately modulated using a current reference input (analog control). External PWM dimming is possible via the PWMIN input, which also provides a shutdown mode. As an alternative, an internal PWM dimming circuit can be used by programming the PWMIN and DR pins. Either PWM scheme controls the PWMOUT output which drives an external p-channel MOSFET connected in series with the LED string. This MOSFET is also used to isolate the load during certain fault conditions, including output shorts to ground

The A6271-1 has been carefully designed to minimize electromagnetic emissions through distributed decoupling and an externally programmable frequency dither circuit configured for the EMI specification CISPR 25. It is also possible to program the fundamental switching frequency below 150 kHz where most EMI standards begin.

The A6271-1 has a comprehensive set of integrated protection features to protect the IC, the LED driver system, and the LED string against faults. Fixed-output overvoltage protection ensures no maximum voltage rating violations, even under a single point failure of the programmable-output overvoltage protection circuit. Other protection features include: LED overload (boost), output under voltage (buck or buck-boost), input supply (VIN) under voltage, 5 V regulator (VREG) output under voltage, high-side supply (PWM PMOS) under voltage, and thermal protection.

ISO 6271:2015 specifies a spectrophotometric method for estimating the colour of clear liquids in terms of platinum-cobalt units (Pt-Co units). It is applicable to clear liquids having a colour characteristic similar to those of the platinum-cobalt colour scale specified in Annex A. For products with colours more intense than the Pt-Co stock solution the method specified in ISO 4630 applies.

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Recently, there have been news going around regarding "CVE-2014-6271" (See USN-2362-1), which is a vulnerability in Bash. How do I know if I am affected by this, how can I fix it, and why should I care?

Bash is the default interactive shell in Ubuntu. When you are interfacing with the terminal (either through the terminal emulator, over a tty, or ssh), you are generally typing commands that bash will read, and execute. Even if you do not use the terminal at all, you still have Bash.

Bash and the OS keep track of a set of environment variables that describe the current logged-on user, where to look for programs on the hard disk, and other such functions. By crafting an environment variable with a specific structure, an attacker might be able to execute code next time Bash starts.

Once they set this variable, the next time bash opens for any reason, your attacker's code will be run. This is especially fearsome with sudo -s, as it spawns bash as the super-user (an administrative user rule that has full control over your computer's data and programs). Even if you only start bash as a standard user, that user's files can be deleted.

It is important to note that even if you do not use bash yourself, many programs will spawn bash by themselves as part of their operation. Even in this case, you are vulnerable. However, Ubuntu's /bin/sh is not bash, so only programs that explicitly invoke bash and not the default scripting shell are affected.

vectors involving the ForceCommand feature in OpenSSH sshd, the mod_cgi and mod_cgid modules in the Apache HTTP Server, scripts executed by unspecified DHCP clients, and other situations in which setting the environment occurs across a privilege boundary from Bash execution.

The USN Bulletin states that new versions have been released for Ubuntu 14.04 Trusty Tahr, 12.04 Precise Pangolin, and 10.04 Lucid Lynx. If you are not on one of these LTS versions, but are on a reasonably-recent version, you'll most likely be able to find a patched package.

I'm using Natty 11.04, which is EOL (and I have updated /etc/apt/sources.list to use old-releases.ubuntu.com), so I have to build from source. I wanted to build a .deb, so at least the package manage is "aware" the bash version is not the default one. I am not 100% succesful - however, the package is registered as "newer" and the bash binary ends up fixed, so here is what I did:

Then you can try building with dpkg-buildpackage -us -uc or debuild command. Note - either of these will re-unpack the source from the zip - thus overriding any patches you may have had! Still, run one of these once so the source is unpacked and built (note debuild may still fail in the end due to texlive, but it should unpack and build the source).

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