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MS Edge CMarkup::Ensure­Delete­CFState use-after-free

(MS15-125, CVE-2015-6168)

Synopsis

A specially crafted web-page can trigger a memory corruption vulnerability in Microsoft Edge. I did not investigate this vulnerability thoroughly, so I cannot speculate on the potential impact or exploitability.

Known affected software and attack vectors

Repro

/<style>:first-letter{word-spacing:9

Variation

x<style>:first-letter{background-position:inherit

Description

At the time this issue was first discovered, Mem­GC was just introduced, and I had not yet fully appreciated what an impact it would have on mitigating use-after-free bugs. Despite Mem­GC being enabled in Microsoft Edge by default, this issue appeared to me to have been a use-after-free vulnerability. However, both Microsoft and ZDI (whom I sold the vulnerability to) describes it as a memory corruption vulnerability, so it's probably more complex than I assumed.

At the time, I did not consider this vulnerability to be of great interest, as there was no immediately obvious way of controlling the vulnerability in order to exploit it. So, I did not do any further investigation into the root cause and, if this was indeed a use-after-free, how come Mem­GC did not mitigate it? In hindsight, it would have been a good idea to investigate the root cause, as any use-after-free that is not mitigated by Mem­GC might provide hints on how to find more vulnerabilities that bypass it.

Time-line

Bug­Id report: AVR:Free microsoftedgecp.exe!EDGEHTML.dll!CMarkup::Ensure­Delete­CFState 7C0A0A This report was generated using Bug­Id, a Python script created to detect, analyze and id application bugs. Don't waste time manually analyzing issues and writing reports, but try Bug­Id out yourself today!
Id: AVR:Free microsoftedgecp.exe!EDGEHTML.dll!CMarkup::Ensure­Delete­CFState 7C0A0A
Description: Access violation while reading memory at 0x­F050FFC using a pointer to freed memory
Process binary: microsoftedgecp.exe
Location: EDGEHTML.dll!CMarkup::Ensure­Delete­CFState + 0x­D
Security impact: Potentially exploitable security issue
© Copyright 2016 by Sky­Lined.
Creative Commons License This work is licensed under a Creative Commons Attribution-Non‑Commercial 4.0 International License.

Last updated on 2016-11-21.
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