Security vulnerabilities are often the result of software bugs. It is important to keep software up-to-date, as malicious parties often can exploit bugs in outdated software. Package managers were created to automate the process of package update and installation, however, if the package manager is not secure, it may represent another avenue of attack!
Package managers are normally run with unrestricted access in order to allow them to modify critical system software. The package manager's actions, therefore, affect the entire system and make the security of package managers vital.
Given their critical role, the expectation would be for package managers to be extremely secure. We examined ten popular package managers (APT, YUM, YaST, etc.) for Linux and BSD systems and found vulnerabilities in all of them. The attacks discussed here can give attackers the ability to read or erase any file on your computer, capture passwords, set up a backdoor into your system, etc. without needing to compromise any keys or passwords.
Packages are essentially trusted forever once they or repository metadata describing them has been signed. This means that even after a package has a vulnerability discovered in it, clients will continue to be willing to install that insecure package. An attacker can replay the correctly signed packages or metadata from a previous release and your computer will install software with flaws that the attacker can exploit.
The default behavior of package managers is to install the most recent version of a requested package that is available from the distribution. Keeping a client safe would seem to be just a matter of retrieving the latest repository metadata provided by the distribution on a regular basis.
Unfortunately, replay attacks on package managers are still very possible even if a client frequently requests updated repository metadata. The problem is that most distributions use a network of publicly provided servers to host their mirrors. In most cases, the people and organizations running these mirrors are only superficially examined before they are listed as an official public mirror. Once listed as a public mirror, a distribution will begin directing requests to the mirror.
To give an example of how easy it is for a malicious party to obtain a mirror, we ran an experiment where we created a fake administrator and company name and leased a server from a hosting provider. We were able to get our mirror listed on every distribution we tried (Ubuntu, Fedora, OpenSuSE, CentOS, and Debian) and our mirrors were contacted by thousands of clients, even including military and government computers!
Since there is so little verification of who owns and runs a mirror, there is no guarantee that the mirror is benevolent. They may, in fact, be talking to an attacker who has setup a mirror and become listed as an official mirror for the distribution.
Some distributions do simple checking that the mirrors respond with updated and valid content. However, such checks only serve to discover benevolent mirrors that have fallen behind. They are unable to detect a maliciously operated mirror. Attackers can respond to the distribution's queries with the expected answer while at the same time targeting the public at large or individual clients with different responses. These targeted responses can include old signed files or manipulated unsigned files.
When a user installs a package, their package manager retrieves from a repository mirror the list of available packages. A malicious mirror can respond to the client with an outdated signed list of packages or an arbitrary list of outdated packages, depending on characteristics of the package manager (described in the FAQ). The client only knows that files it receives are properly signed by their distribution. They do not know that what they have received is an old list of packages.
Due to the way existing package managers are currently implemented, it is even possible to provide a client an older list of files than a list they have already seen. That is, if a client already obtained an updated, signed list of packages yesterday but they are talking to a malicious mirror today, the malicious mirror can provide the client a signed list of packages from the year before. The malicious mirror needs only to have a copy of signed files that were made available by the distribution. They do not need to have compromised the signing key.
A malicious mirror doesn't have to provide a client outdated packages with vulnerabilities known to the attacker in order to leave the client vulnerable. All that an attacker needs to do is to continuously serve the client the same repository metadata. Over time, even though vulnerabilities become known in various packages and new versions become available, the client will never see them when updating.
For example, it is known that an earlier version of OpenSSL for Debian has a security flaw. The list of files from the repository that previously included this package is still correctly signed. Using this old signed file list, a malicious mirror can keep a client on the insecure version of OpenSSL by responding to the client's package manager with the old list of files.
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We would like to thank Ryan Giobbi and the CERT Coordination Center for contacting many of the major package managers and distributions. We appreciate James Hamilton of Offshoot Inc. creating the logo at the top of this page. We also appreciate the comments given to us by Ryan Giobbi, Igor Crk, Michael Piatek, Angelos Stavrou, Chris Gniady, Yoshi Kohno, and Wenjun Hu.