Mathias Payer

The Matter of Heartbleed

The Heartbleed vulnerability took the Internet by surprise in April 2014. The vulnerability, one of the most consequential since the advent of the commercial Internet, allowed attackers to remotely read protected memory from an estimated 24--55% of popular HTTPS sites. In this work, we perform a comprehensive, measurement-based analysis of the vulnerabilitys impact, including (1) tracking the vulnerable population, (2) monitoring patching behavior over time, (3) assessing the impact on the HTTPS certificate ecosystem, and (4) exposing real attacks that attempted to exploit the bug. Furthermore, we conduct a large-scale vulnerability notification experiment involving 150,000 hosts and observe a nearly 50% increase in patching by notified hosts. Drawing upon these analyses, we discuss what went well and what went poorly, in an effort to understand how the technical community can respond more effectively to such events in the future.

SoK: Eternal War in Memory

Memory corruption bugs in software written in low-level languages like C or C++ are one of the oldest problems in computer security. The lack of safety in these languages allows attackers to alter the programs behavior or take full control over it by hijacking its control flow. This problem has existed for more than 30 years and a vast number of potential solutions have been proposed, yet memory corruption attacks continue to pose a serious threat. Real world exploits show that all currently deployed protections can be defeated. This paper sheds light on the primary reasons for this by describing attacks that succeed on todays systems. We systematize the current knowledge about various protection techniques by setting up a general model for memory corruption attacks. Using this model we show what policies can stop which attacks. The model identifies weaknesses of currently deployed techniques, as well as other proposed protections enforcing stricter policies. We analyze the reasons why protection mechanisms implementing stricter polices are not deployed. To achieve wide adoption, protection mechanisms must support a multitude of features and must satisfy a host of requirements. Especially important is performance, as experience shows that only solutions whose overhead is in reasonable bounds get deployed. A comparison of different enforceable policies helps designers of new protection mechanisms in finding the balance between effectiveness (security) and efficiency. We identify some open research problems, and provide suggestions on improving the adoption of newer techniques.

Code-pointer integrity

Systems code is often written in low-level languages like C/C++, which offer many benefits but also delegate memory management to programmers. This invites memory safety bugs that attackers can exploit to divert control flow and compromise the system. Deployed defense mechanisms (e.g., ASLR, DEP) are incomplete, and stronger defense mechanisms (e.g., CFI) often have high overhead and limited guarantees [19, 15, 9]. We introduce code-pointer integrity (CPI), a new design point that guarantees the integrity of all code pointers in a program (e.g., function pointers, saved return addresses) and thereby prevents all control-flow hijack attacks, including return-oriented programming. We also introduce code-pointer separation (CPS), a relaxation of CPI with better performance properties. CPI and CPS offer substantially better security-to-overhead ratios than the state of the art, they are practical (we protect a complete FreeBSD system and over 100 packages like apache and postgresql), effective (prevent all attacks in the RIPE benchmark), and efficient: on SPEC CPU2006, CPS averages 1.2% overhead for C and 1.9% for C/C++, while CPIs overhead is 2.9% for C and 8.4% for C/C++. A prototype implementation of CPI and CPS can be obtained from http://levee.epfl.ch.

Eternal War in Memory

Software written in low-level languages like C or C++ is prone to memory corruption bugs that allow attackers to access machines, extract information, and install malware. Real-world exploits show that all widely deployed protections can be defeated.

Fine-grained user-space security through virtualization

This paper presents an approach to the safe execution of applications based on software-based fault isolation and policy-based system call authorization. A running application is encapsulated in an additional layer of protection using dynamic binary translation in user-space. This virtualization layer dynamically recompiles the machine code and adds multiple dynamic security guards that verify the running code to protect and contain the application. The binary translation system redirects all system calls to a policy-based system call authorization framework. This interposition framework validates every system call based on the given arguments and the location of the system call. Depending on the user-loadable policy and an extensible handler mechanism the framework decides whether a system call is allowed, rejected, or redirect to a specific user-space handler in the virtualization layer. This paper offers an in-depth analysis of the different security guarantees and a performance analysis of libdetox, a prototype of the full protection platform. The combination of software-based fault isolation and policy-based system call authorization imposes only low overhead and is therefore an attractive option to encapsulate and sandbox applications to improve host security.

Fine-Grained Control-Flow Integrity Through Binary Hardening

Applications written in low-level languages without type or memory safety are prone to memory corruption. Attackers gain code execution capabilities through memory corruption despite all currently deployed defenses. Control-Flow Integrity CFI is a promising security property that restricts indirect control-flow transfers to a static set of well-known locations. We present Lockdown, a modular, fine-grained CFI policy that protects binary-only applications and libraries without requiring source-code. Lockdown adaptively discovers the control-flow graph of a running process based on the executed code. The sandbox component of Lockdown restricts interactions between different shared objects to imported and exported functions by enforcing fine-grained CFI checks using information from a trusted dynamic loader. A shadow stack enforces precise integrity for function returns. Our prototype implementation shows that Lockdown results in low performance overhead and a security analysis discusses any remaining gadgets.

Online optimizations driven by hardware performance monitoring

Hardware performance monitors provide detailed direct feedback about application behavior and are an additional source of infor-mation that a compiler may use for optimization. A JIT compiler is in a good position to make use of such information because it is running on the same platform as the user applications. As hardware platforms become more and more complex, it becomes more and more difficult to model their behavior. Profile information that captures general program properties (like execution frequency of methods or basic blocks) may be useful, but does not capture sufficient information about the execution platform. Machine-level performance data obtained from a hardware performance monitor can not only direct the compiler to those parts of the program that deserve its attention but also determine if an optimization step actually improved the performance of the application. This paper presents an infrastructure based on a dynamic compiler+runtime environment for Java that incorporates machine-level information as an additional kind of feedback for the compiler and runtime environment. The low-overhead monitoring system provides fine-grained performance data that can be tracked back to individual Java bytecode instructions. As an example, the paper presents results for object co-allocation in a generational garbage collector that optimizes spatial locality of objects on-line using measurements about cache misses. In the best case, the execution time is reduced by 14% and L1 cache misses by 28%.

Safe Loading - A Foundation for Secure Execution of Untrusted Programs

The standard loader (ld.so) is a common target of attacks. The loader is a trusted component of the application, and faults in the loader are problematic, e.g., they may lead to local privilege escalation for SUID binaries. Software-based fault isolation (SFI) provides a framework to execute arbitrary code while protecting the host system. A problem of current approaches to SFI is that fault isolation is decoupled from the dynamic loader, which is treated as a black box. The sandbox has no information about the (expected) execution behavior of the application and the connections between different shared objects. As a consequence, SFI is limited in its ability to identify devious application behavior. This paper presents a new approach to run untrusted code in a user-space sandbox. The approach replaces the standard loader with a security-aware trusted loader. The secure loader and the sandbox together cooperate to allow controlled execution of untrusted programs. A secure loader makes security a first class concept and ensures that the SFI system does not allow any unchecked code to be executed. The user-space sandbox builds on the secure loader and subsequently dynamically checks for malicious code and ensures that all control flow instructions of the application adhere to an execution model. The combination of the secure loader and the user-space sandbox enables the safe execution of untrusted code in user-space. Code injection attacks are stopped before any unintended code is executed. Furthermore, additional information provided by the loader can be used to support additional security properties, e.g., in lining of Procedure Linkage Table calls reduces the number of indirect control flow transfers and therefore limits jump-oriented attacks. This approach implements a secure platform for privileged applications and applications reachable over the network that anticipates and confines security threats from the beginning.

Control-Flow Integrity: Precision, Security, and Performance

Memory corruption errors in C/C++ programs remain the most common source of security vulnerabilities in today’s systems. Control-flow hijacking attacks exploit memory corruption vulnerabilities to divert program execution away from the intended control flow. Researchers have spent more than a decade studying and refining defenses based on Control-Flow Integrity (CFI); this technique is now integrated into several production compilers. However, so far, no study has systematically compared the various proposed CFI mechanisms nor is there any protocol on how to compare such mechanisms. We compare a broad range of CFI mechanisms using a unified nomenclature based on (i) a qualitative discussion of the conceptual security guarantees, (ii) a quantitative security evaluation, and (iii) an empirical evaluation of their performance in the same test environment. For each mechanism, we evaluate (i) protected types of control-flow transfers and (ii) precision of the protection for forward and backward edges. For open-source, compiler-based implementations, we also evaluate (iii) generated equivalence classes and target sets and (iv) runtime performance.

HI-CFG: Construction by Binary Analysis and Application to Attack Polymorphism

Security analysis often requires understanding both the control and data-flow structure of a binary. We introduce a new program representation, a hybrid information- and control-flow graph (HI-CFG), and give algorithms to infer it from an instruction-level trace. As an application, we consider the task of generalizing an attack against a program whose inputs undergo complex transformations before reaching a vulnerability. We apply the HI-CFG to find the parts of the program that implement each transformation, and then generate new attack inputs under a user-specified combination of transformations. Structural knowledge allows our approach to scale to applications that are infeasible with monolithic symbolic execution. Such attack polymorphism shows the insufficiency of any filter that does not support all the same transformations as the vulnerable application. In case studies, we show this attack capability against a PDF viewer and a word processor.