New Adobe Zero-Day Shares Same Root Cause as Older Flaws
Earlier we talked about the out-of-band update for Flash Player that was released by Adobe (identified as APSB15-14) that was released to fix CVE-2015-3113. This update raised the Flash Player version to 188.8.131.52.
Our analysis of the current flaw reveals that the root cause of CVE-2015-3113 is similar to CVE-2015-3043. Both cause a buffer overflow within the Flash Player code. In fact, code targeting the previous exploit can also cause crashes in version 184.108.40.206 (the version immediately before this emergency update).
Both vulnerabilities can be used to run arbitrary code (i.e., malware) on user systems if they visit a site with a malicious Flash file. Users who visit a malicious or compromised site containing malicious Flash files that still use older, unpatched versions of Flash Player are at risk.
Both CVE-2015-3113 and CVE-2015-3043 are heap overflow vulnerabilities in the FLV audio parsing flow. They are both in how Flash Player processes audio with the Nellymoser codec; they can be triggered by modifying the FLV file’s audio tag. They both overflow a hardcoded length heap buffer with a length of 0x2000.
CVE-2015-3043 and CVE-2015-3113 both trigger this bug using sample_count * sample_size > 0x2000, and bypass the length check.
Old Patch for CVE-2015-3043
CVE-2015-3043 was originally patched in 220.127.116.11. This was done by limiting the sample count acquired from the FLV audio tag.
Figure 1. Original patch
We can see that the sample count is limited to 0x400. We can compute the biggest buffer size needed from this: FLV specifies a size of 4 as the biggest size per sample. The Nellymoser codec has a hardcoded multiple size of 2 (as seen in the code below). Therefore, the biggest buffer needed is 0x400 * 4 *2 = 0x2000.
Figure 2. Nellymoser doubling
New Patch in 18.104.22.168
However, the code underwent significant changes in 22.214.171.124. The code now looks like this:
Figure 3. New patch
The GetSampleCount function checks the final buffer size needed. If the final buffer size is larger than 0x2000, it will limit it to 0x2000. However, this ignores the Nellymoser decode function’s hardcoded double operation; this can be used to trigger a heap buffer overflow once again.
The analysis above shows that both the previous Flash zero-day and the current incident share the same underlying root cause. In fact, code targeting the previous zero-day will cause 126.96.36.199 to crash.
This incident highlights how important careful development of patches is, to prevent patched bugs from being re-exploited at a later time. Regression testing must also be a part of software development in order to check that old bugs do not threaten new versions of software.