Analysis of the CVE-2013-6435 Flaw in RPM
The RPM Package Manager (RPM) is a powerful command-line driven package management system capable of installing, uninstalling, verifying, querying, and updating software packages. RPM was originally written in 1997 by Erik Troan and Marc Ewing. Since then RPM has been successfully used in all versions of Red Hat Linux and currently in Red Hat Enterprise Linux.
RPM offers considerable advantages over traditional open-source software install methodology of building from source via tar balls, especially when it comes to software distribution and management. This has led to other Linux distributions to accept RPM as either the default package management system or offer it as an alternative to the ones which are default in those distributions.
Like any big, widely used software, over time several features are added to it and also several security flaws are found. On several occasions Red Hat has found and fixed security issues with RPM.
Florian Weimer of Red Hat Product Security discovered an interesting flaw in RPM, which was assigned CVE-2013-6435. Firstly, let's take a brief look at the structure of an RPM file. It consists of two main parts: the RPM header and the payload. The payload is a compressed CPIO archive of binary files that are installed by the RPM utility. The RPM header, among other things, contains a cryptographic checksum of all the installed files in the CPIO archive. The header also contains a provision for a cryptographic signature. The signature works by performing a mathematical function on the header and archive section of the file. The mathematical function can be an encryption process, such as PGP (Pretty Good Privacy), or a message digest in the MD5 format.
If the RPM is signed, one can use the corresponding public key to verify the integrity and even the authenticity of the package. However, RPM only checked the header and not the payload during the installation.
When an RPM is installed, it writes the contents of the package to its target directory and then verifies its checksum against the value in the header. If the checksum does not match, that means something is wrong with the package (possibly someone has tampered with it) and the file is removed. At this point RPM refuses to install that particular package.
Though this may seem like the correct way to handle things, it has a bad consequence. Let's assume RPM installs a file in the /etc/cron.d directory and then verifies its checksum. This offers a small race-window, in which crond can run before the checksum is found to be incorrect and the file is removed. There are several ways to prolong this window as well. So in the end we achieve arbitrary code execution as root, even though the system administrator assumes that the RPM package was never installed.
The approach Red Hat used to solve the problem is:
- Require the size in the header to match with the size of the file in the payload. This prevents anyone from tampering with the payload, because the header is cryptographically verified. (This fix is already present in the upstream version of RPM)
- Set restrictive permissions while a file is being unpacked from an RPM package. This will only allow root to access those file. Also, several programs, including cron, perform a check for permission sanity before running those files.
Another approach to mitigate this issue is the use of the O_TMPFILE flag. Linux kernel 3.11 and above introduced this flag, which can be passed to open(2), to simplify the creation of secure temporary files. Files opened with the O_TMPFILE flag are created, but they are not visible in the file system. As soon as they are closed, they are deleted. There are two uses for these files: race-free temporary files and creation of initially unreachable files. These unreachable files can be written to or changed same as regular files. RPM could use this approach to create a temporary, unreachable file, run a checksum on it, and either delete it or atomically link it to set the file up, without being vulnerable to the attack described above. However, as mentioned above, this feature is only available in Linux kernel 3.11 and above, was added to glibc 2.19, and is slowly making its way into GNU/Linux distributions.
The risk mentioned above is greatly reduced if the following precautions are followed:
- Always check signatures of RPM packages before installing them. Red Hat RPMs are signed with cryptographic keys provided at https://access.redhat.com/security/team/key. When installing RPMs from Red Hat or Fedora repositories, Yum will automatically validate RPM packages via the respective public keys, unless explicitly told not to (via the “nogpgcheck” option and configuration directive).
- Package downloads via Red Hat software repositories are protected via TLS/SSL so it is extremely difficult to tamper with them in transit. Fedora uses a whole-file hash chain rooted in a hash downloaded over TLS/SSL from a Fedora-run central server.
The above issue (CVE-2013-6435) has been fixed along with another issue (CVE-2014-8118), which is a potentially exploitable crash in the CPIO parser.
Red Hat customers should update to the latest versions of RPM via the following security advisories: