Tag: openssh

Google 寫了一篇關於 CVE-2015-7547 的安全性問題：「CVE-2015-7547: glibc getaddrinfo stack-based buffer overflow」。

Google 的工程師在找 OpenSSH 連到某台特定主機就會 segfault 的通靈過程中，發現問題不在 OpenSSH，而是在更底層的 glibc 導致 segfault：

Recently a Google engineer noticed that their SSH client segfaulted every time they tried to connect to a specific host. That engineer filed a ticket to investigate the behavior and after an intense investigation we discovered the issue lay in glibc and not in SSH as we were expecting.

由於等級到了 glibc 這種每台 Linux 都有裝的情況，在不經意的情況下發生 segfault，表示在刻意攻擊的情況下可能會很糟糕，所以 Google 投入了人力研究，想知道這個漏洞到底可以做到什麼程度：

Thanks to this engineer’s keen observation, we were able determine that the issue could result in remote code execution. We immediately began an in-depth analysis of the issue to determine whether it could be exploited, and possible fixes. We saw this as a challenge, and after some intense hacking sessions, we were able to craft a full working exploit!

在研究過程中 Google 發現 Red Hat 的人也在研究同樣的問題：「(CVE-2015-7547) - In send_dg, the recvfrom function is NOT always using the buffer size of a newly created buffer (CVE-2015-7547)」：

In the course of our investigation, and to our surprise, we learned that the glibc maintainers had previously been alerted of the issue via their bug tracker in July, 2015. (bug). We couldn't immediately tell whether the bug fix was underway, so we worked hard to make sure we understood the issue and then reached out to the glibc maintainers. To our delight, Florian Weimer and Carlos O’Donell of Red Hat had also been studying the bug’s impact, albeit completely independently! Due to the sensitive nature of the issue, the investigation, patch creation, and regression tests performed primarily by Florian and Carlos had continued “off-bug.”

攻擊本身需要繞過反制機制 (像是 ASLR)，但仍然是可行的，Google 的人已經成功寫出 exploit code：

Remote code execution is possible, but not straightforward. It requires bypassing the security mitigations present on the system, such as ASLR. We will not release our exploit code, but a non-weaponized Proof of Concept has been made available simultaneously with this blog post.

技術細節在 Google 的文章裡也有提到，buffer 大小固定為 2048 bytes，但取得時有可能超過 2048 bytes，於是造成 buffer overflow：

glibc reserves 2048 bytes in the stack through alloca() for the DNS answer at _nss_dns_gethostbyname4_r() for hosting responses to a DNS query.

Later on, at send_dg() and send_vc(), if the response is larger than 2048 bytes, a new buffer is allocated from the heap and all the information (buffer pointer, new buffer size and response size) is updated.

另外 glibc 官方的 mailing list 上也有說明：「[PATCH] CVE-2015-7547 --- glibc getaddrinfo() stack-based buffer overflow」。