February 6, 2019

Exploiting systemd-journald: Part 1

This is part one in a multipart series (read Part 2 here) on exploiting two vulnerabilities in systemd-journald, which were published by Qualys on January 9th. Specifically, the vulnerabilities were:

  • a user-influenced size passed to alloca(), allowing manipulation of the stack pointer (CVE-2018-16865)
  • a heap-based memory out-of-bounds read, yielding memory disclosure (CVE-2018-16866)

The affected program, systemd-journald, is a system service that collects and stores logging data. The vulnerabilities discovered in this service allow for user-generated log data to manipulate memory such that they can take over systemd-journald, which runs as root. Exploitation of these vulnerabilities thus allow for privilege escalation to root on the target system.

As Qualys did not provide exploit code, we developed a proof-of-concept exploit for our own testing and verification. There are some interesting aspects that were not covered by Qualys’ initial publication, such as how to communicate with the affected service to reach the vulnerable component, and how to control the computed hash value that is actually used to corrupt memory. We thought it was worth sharing the technical details for the community.

As the first in our series on this topic, the objective of this post is to provide the reader with the ability to write a proof-of-concept capable of exploiting the service with Address Space Layout Randomization (ASLR) disabled. In the interest of not posting an unreadably-long blog, and also not handing sharp objects to script-kiddies before the community has had chance to patch, we are saving some elements for discussion in future posts in this series, including details on how to control the key computed hash value.

Read more at Capsule8

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