Behind the Masq: Yet more DNS, and DHCP, vulnerabilities

Posted by Fermin J. Serna, Staff Software Engineer, Matt Linton, Senior Security Engineer and Kevin Stadmeyer, Technical Program Manager

Our team has previously posted about DNS vulnerabilities and exploits. Lately, we’ve been busy reviewing the security of another DNS software package: Dnsmasq. We are writing this to disclose the issues we found and to publicize the patches in an effort to increase their uptake.

Dnsmasq provides functionality for serving DNS, DHCP, router advertisements and network boot. This software is commonly installed in systems as varied as desktop Linux distributions (like Ubuntu), home routers, and IoT devices. Dnsmasq is widely used both on the open internet and internally in private networks.

We discovered seven distinct issues (listed below) over the course of our regular internal security assessments. Once we determined the severity of these issues, we worked to investigate their impact and exploitability and then produced internal proofs of concept for each of them. We also worked with the maintainer of Dnsmasq, Simon Kelley, to produce appropriate patches and mitigate the issue.

These patches have been upstreamed and are now committed to the project’s git repository. In addition to these patches we have also submitted another patch which will run Dnsmasq under seccomp-bpf to allow for additional sandboxing. This patch has been submitted to the DNSmasq project for review and we have also made it available here for those who wish to integrate it into an existing install (after testing, of course!). We believe the adoption of this patch will increase the security of DNSMasq installations.

We would like to thank Simon Kelley for his help in patching these bugs in the core Dnsmasq codebase. Users who have deployed the latest version of Dnsmasq (2.78) will be protected from the attacks discovered here. Android partners have received this patch as well and it will be included in Android's monthly security update for October. Kubernetes versions 1.5.8, 1.6.11, 1.7.7, and 1.8.0 have been released with a patched DNS pod. Other affected Google services have been updated.

During our review, the team found three potential remote code executions, one information leak, and three denial of service vulnerabilities affecting the latest version at the project git server as of September 5th 2017.

CVE	Impact	Vector	Notes	PoC
CVE-2017-14491	RCE	DNS	Heap based overflow (2 bytes). Before 2.76 and this commit overflow was unrestricted.	PoC, instructions and ASAN report
CVE-2017-14492	RCE	DHCP	Heap based overflow.	PoC, instructions and ASAN report
CVE-2017-14493	RCE	DHCP	Stack Based overflow.	PoC, instructions and ASAN report
CVE-2017-14494	Information Leak	DHCP	Can help bypass ASLR.	PoC and Instructions
CVE-2017-14495	OOM/DoS	DNS	Lack of free() here.	PoC and  instructions
CVE-2017-14496	DoS	DNS	Invalid boundary checks here. Integer underflow leading to a huge memcpy.	PoC, instructions and ASAN report
CVE-2017-13704	DoS	DNS	Bug collision with CVE-2017-13704

It is worth expanding on some of these:

• CVE-2017-14491 is a DNS-based vulnerability that affects both directly exposed and internal network setups. Although the latest git version only allows a 2-byte overflow, this could be exploited based on previous research. Before version 2.76 and this commit the overflow is unrestricted. 

==1159==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x62200001dd0b at pc 0x0000005105e7 bp 0x7fff6165b9b0 sp0x7fff6165b9a8

WRITE of size 1 at 0x62200001dd0b thread T0

  #0 0x5105e6 in add_resource_record

/test/dnsmasq/src/rfc1035.c:1141:7

  #1 0x5127c8 in answer_request /test/dnsmasq/src/rfc1035.c:1428:11

  #2 0x534578 in receive_query /test/dnsmasq/src/forward.c:1439:11

  #3 0x548486 in check_dns_listeners 

/test/dnsmasq/src/dnsmasq.c:1565:2

  #4 0x5448b6 in main /test/dnsmasq/src/dnsmasq.c:1044:7

  #5 0x7fdf4b3972b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0)

  #6 0x41cbe9 in _start (/test/dnsmasq/src/dnsmasq+0x41cbe9)

• CVE-2017-14493 is a trivial-to-exploit DHCP-based, stack-based buffer overflow vulnerability. In combination with CVE-2017-14494 acting as an info leak, an attacker could bypass ASLR and gain remote code execution.

dnsmasq[15714]: segfault at 1337deadbeef ip 00001337deadbeef sp 00007fff1b66fd10 error 14 in libnss_files-2.24.so[7f7cfbacb000+a000]

• Android is affected by CVE-2017-14496 when the attacker is local or tethered directly to the device—the service itself is sandboxed so the risk is reduced. Android partners received patches on 5 September 2017 and devices with a 2017-10-01 security patch level or later address this issue.

Proofs of concept are provided so you can check if you are affected by these issues, and verify any mitigations you may deploy.

We would like to thank the following people for discovering, investigating impact/exploitability and developing PoCs: Felix Wilhelm, Fermin J. Serna, Gabriel Campana, Kevin Hamacher, Ron Bowes and Gynvael Coldwind of the Google Security Team.