Skip to content
Switch branches/tags
Go to file
Cannot retrieve contributors at this time


Some bounds checks are elided by Apple's compiler and possibly others, leading to a possible attack especially in 32-bit builds.

Although triggered by a compiler optimization, this is a bug in Cap'n Proto, not the compiler.

Discovered by

Kenton Varda <> <>






  • Remotely segfault a 32-bit application by sending it a malicious message.
  • Exfiltration of memory from such applications might be possible, but our current analysis indicates that other checks would cause any such attempt to fail (see below).
  • At present I've only reproduced the problem on Mac OS using Apple's compiler. Other compilers and platforms do not seem to apply the problematic optimization.

Fixed in


The following text contains speculation about the exploitability of this bug. This is provided for informational purposes, but as such speculation is often shown to be wrong, you should not rely on the accuracy of this section for the safety of your service. Please update your library.

During regular testing in preparation for a release, it was discovered that when Cap'n Proto is built using the current version of Apple's Clang compiler in 32-bit mode with optimizations enabled, it is vulnerable to attack via specially-crafted malicious input, causing the application to read from an invalid memory location and crash.

Specifically, a malicious message could contain a far pointer pointing outside of the message. Cap'n Proto messages are broken into segments of continuous memory. A far pointer points from one segment into another, indicating both the segment number and an offset within that segment. If a malicious far pointer contained an offset large enough to overflow the pointer arithmetic (wrapping around to the beginning of memory), then it would escape bounds checking, in part because the compiler would elide half of the bounds check as an optimization.

That is, the code looked like (simplification):

word* target = segmentStart + farPointer.offset;
if (target < segmentStart || target >= segmentEnd) {

To most observers, this code would appear to be correct. However, as it turns out, pointer arithmetic that overflows is undefined behavior under the C standard. As a result, the compiler is allowed to assume that the addition on the first line never overflows. Since farPointer.offset is an unsigned number, the compiler is able to conclude that target < segmentStart always evaluates false. Thus, the compiler removes this part of the check. Unfortunately, in the case of overflow, this is exactly the part of the check that we need.

Based on both fuzz testing and logical analysis, I believe that the far pointer bounds check is the only check affected by this optimization. If true, then it is not possible to exfiltrate memory through this vulnerability: the target of a far pointer is always expected to be another pointer, which in turn points to the final object. That second pointer will go through its own bounds checking, which will fail (unless it somehow happens to point back into the message bounds, in which case no harm is done).

I believe this bug does not affect any common 64-bit platform because the maximum offset expressible by a far pointer is 2^32 bytes. In order to trigger the bug in a 64-bit address space, the message location would have to begin with 0xFFFFFFFF (allocated in the uppermost 4GB of address space) and the target would have to begin with 0x00000000 (allocated in the lowermost 4GB of address space). Typically, on 64-bit architectures, the upper half of the address space is reserved for the OS kernel, thus a message could not possibly be located there.

I was not able to reproduce this bug on other platforms, perhaps because the compiler optimization is not applied by other compilers. On Linux, I tested GCC 4.9, 5.4, and 6.3, as well as Clang 3.6, 3.8, and 4.0. None were affected. Nevertheless, the compiler behavior is technically allowed, thus it should be assumed that it can happen on other platforms as well.

The specific compiler version which was observed to be affected is:

$ clang++ --version
Apple LLVM version 8.1.0 (clang-802.0.41)
Target: x86_64-apple-darwin16.5.0
Thread model: posix
InstalledDir: /Applications/

(Note: Despite being Clang-based, Apple's compiler version numbers have no apparent relationship to Clang version numbers.)

Preventative measures

The problem was caught by running Cap'n Proto's standard fuzz tests in this configuration. These tests are part of the Cap'n Proto test suite which runs when you invoke make check, which Cap'n Proto's installation instructions suggest to all users.

However, these fuzz tests were introduced after the 0.5.x release branch, hence are not currently present in release versions of Cap'n Proto, only in git. A 0.6 release will come soon, fixing this.

The bugfix commit forces the compiler to perform all checks by casting the relevant pointers to uintptr_t. According to the standard, unsigned integers implement modulo arithmetic, rather than leaving overflow undefined, thus the compiler cannot make the assumptions that lead to eliding the check. This change has been shown to fix the problem in practice. However, this quick fix does not technically avoid undefined behavior, as the code still computes pointers that point to invalid locations before they are checked. A technically-correct solution has been implemented in the next commit, 2ca8e41140ebc618b8fb314b393b0a507568cf21. However, as this required more extensive refactoring, it is not appropriate for cherry-picking, and will only land in versions 0.6 and up.