Return Flow Guard

[DannyWei, lywang, FlowerCode] of Tencent Xuanwu Lab

Here is a preliminary documentation of the RFG implementation. We will update it once we have new findings and corrections.

We analyzed the Return Flow Guard introduced in Windows 10 Redstone 2 14942, released on October 7, 2016.


Microsoft introduced Control Flow Guard in Windows 8.1 to protect against malicious modification of indirect call function pointers. CFG checks the target function pointer before each indirect call. However, CFG cannot detect modification of the return address on stack, or Return Oriented Programming.
The newly added RFG effectively stops these kind of attacks by saving the return address to fs:[rsp] at the entry of each function, and compare it with the return address on stack before returning.
Enabling RFG require both compiler and operating system support. During compilation, the compiler instruments the file by reserving a certain number of instruction spaces in the form of nop instructions.
When the target executable runs on a supported operating system, the reserved spaces are dynamically replaced with RFG instructions to check function return addresses. Otherwise, these nop instructions will not interfere with normal execution flow of the program.
The difference between RFG and GS (Buffer Security Check) is that the stack cookie can be obtained by using information leak or brute forcing, the RFG return address is written to the Thread Control Stack out of reach of attackers. This significantly increased the difficulty of the attack.
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BadTunnel – A New Hope

This article purposes a new attack model to hijack TCP/IP broadcast protocol across different network segment, named “BadTunnel”.

With this method, NetBIOS Name Service Spoofing can be achieved, regardless of the attacker and the victim is on the same or different network, the firewalls and NAT devices in between. All it need is the victim navigate to a malicious web page with IE or Edge, or open a specially crafted document, and the attacker can hijack the victim’s NetBIOS name query to spoof as print server or file server in the local network.

By hijacking the WAPD name, the attacker can hijack all network communications, including but not limited to usual web accesses, Windows Update service and Microsoft Crypto API Certificate revocation list updates. Once the hijack is successful, it is easy to achieve arbitrary execution of program on the target system by using Evilgrade [1].

This method is effective on all Windows versions before the June 2016 patch, and can be exploited through all Internet Explorer, Microsoft Edge, and Microsoft Office versions, and can also be exploited through third-party applications. In fact, BadTunnel attack can be conducted on anywhere that a file URI scheme or UNC path can be embedded. For example, if a shortcut’s icon path is pointed to the malicious file URI scheme or UNC path, the BadTunnel attack can be triggered at the moment the user sees it in the Windows Explorer, which means BadTunnel can also be exploited through web pages, emails, USB flash drives and many other ways. It can even impact Web servers and SQL servers [2].

(This article does not include all contents covered by the BadTunnel research, the remaining part will be released in my presentation “BadTunnel: How do I get Big Brother power” on BlackHat US 2016.)
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Exceptions in Exceptions – Abusing Special Cases in System Exception Handling to Achieve Unbelievable Vulnerability Exploitation

Memory Read / Write / Execute attributes are one of the most important part of system security. Usually it is mandatory to have writable attribute set before overwriting a block of memory, and executable attribute set before executing code in a block of memory, otherwise an exception is generated. However, there are some special cases in the Windows exception handling procedure that we can take advantage of. By abusing such exceptions, we could write to the unwritable, and execute the unexecutable.
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Use Chakra engine again to bypass CFG

This post is initially inspired by a talk with @TK, during which I learned the process and detail on how to successfully bypass CFG (reference: use Chakra JIT to bypass DEP and CFG). Due to my interest in its technology, I spent some time reading related materials and found another position to bypass CFG. I would like to thanks @TK for enlightening me on the ideas and techniques mentioned in this post.

There are plenty of articles that focus on the analysis of CFG, if you are interested, you may refer to my previous speech on HitCon 2015(《spartan 0day & exploit》). To be clear, this post is the part that is not revealed in my speech. At this point, the method to implement arbitrary code execution on edge through a write to memory is completely revealed.

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Drag & Drop Security Policy of IE Sandbox

There is a kind of vulnerability that uses the flaw of whitelist applications in ElevationPolicy settings to accomplish sandbox bypass. A DragDrop policy setting similar to ElevationPolicy in the IE registry attracts our attention. In this post, the writer will try every possible means to break IE sandbox from the perspective of an attacker by analyzing all obstacles ahead to detail the drag drop security policy of IE sandbox.

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Bypass DEP and CFG using JIT compiler in Chakra engine

JIT Spray is a popular exploitation technique first appeared in 2010. It embeds shellcode as immediate value into the executable code the JIT compiler generates. Currently, all major JIT engine, including Chakra, already have many mitigations in place against this technique, such as random NOP instruction insertion, constant blinding, etc.

This article points out two weaknesses in Chakra’s JIT Spray mitigation (in Windows 8.1 and older operating systems, and Windows 10, respectively), allowing attackers to use JIT Spray to execute shellcode, bypassing DEP. I will also discuss a method to bypass CFG using Chakra’s JIT compiler.

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A “WormHole” vulnerability on PC

The WormHole vulnerability that recently has raised security concerns actually comes from a series of inappropriate developing habits. Similar problems are very common on PC, but most of them are mitigated by Microsoft’s default firewall. I wish this post with a lot of discussion on WormHole could more or less enhance the security awareness of some developers.

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Poking a Hole in the Patch–Escaping from IE Sandbox with a Poorly Patched Vulnerability

James Forshaw reported a vulnerability to Microsoft regarding Windows Audio Service in November 2014. In our analysis, we discovered that the patch Microsoft release later did not completely solve the problem. With a combination of techniques, we successfully bypassed the patch and can exploit the vulnerability on patched system.
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Research report on using JIT to trigger RowHammer

RowHammer is a problem with some DDR3 in which repeatedly accessing a row of memory can cause bit flips in adjacent rows. However it need to run the customized asm code on target machine to trigger RowHammer, so RowHammer is not easy to be used to attack. We have an idea that try to use script language to trigger RowHammer. If it works, RowHammer will be more dangerous. In order to verify our idea, we analyzed the Java Hotspot, Chrome V8, .NET CoreCLR and Firefox SpiderMonfey.
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