MacOS Bypass Flaw Lets Attackers Sign Malicious Code as Apple

When is Apple-signed code not actually signed by Apple? When a hacker can manipulate the code-signing process to impersonate Apple and sign off on malicious code, bypassing common third-party security tools and tricking users into thinking illegitimate software is verified.

Such a bypass attack has been possible for years on macOS and older versions of OS X, thanks to a flaw in Apple code-signing APIs, explains Josh Pitts, staff engineer for research and exploitation at Okta. Pitts discovered the bypass flaw in third-party developers' interpretation of the APIs, which let unsigned malicious code appear as though it was verified by Apple and remain on a Mac until it was patched.

Operating systems use digital signatures to verify whether the code has been signed off by a trusted origin and hasn't been modified. Security tools use signed code for whitelisting, antivirus, incident response, and threat hunting. Detection and response teams use it to weed out untrusted code. The way many macOS security tools verify signatures has been fairly simple to bypass. As a result, popular security tools think Apple signed off on code when it hasn't.

"This really worried me a lot," says Pitts. "This allows an attacker to stay on a workstation … it's a nice way to hide in plain sight."

An attacker wouldn't need administrative access or memory corruption to bypass these code-signing checks, but several conditions need to be in place before an exploit would work.

Digging into the Details

On Windows, you can cryptographically sign "just about everything" from .NET binaries to PowerShell scripts, Pitts explains. On macOS and iOS, code signing focuses on the Mach-O binary and application bundles, which make sure only trusted code is executed in memory.

Here, the flaw exists in the difference between how the Mach-O loader loads signed code, and how improperly used code-signing APIs check the code. Attackers can exploit the vulnerability with a malformed Universal/Fat Binary, which is a binary format containing several Mach-O files. Each of these files targets a CPU architecture (i386, x86_64, PPC), found on Macs.

The following must be true for an exploit to work: Only the first Mach-O in the Fat/Universal file has to be signed by Apple. The malicious binary, or non-Apple code, must be adhoc signed and i386 compiled for an x86_64 bit target machine. The CPU_TYPE in the Fat header of the Apple binary must be set to an invalid type or CPU type not native to the host chipset.

Developers typically check the Mach-O binary or Fat/Universal binary with these APIs: SecStaticCodeCheckValidityWithErrors() or SecStaticCodeCheckValidity(). Flags -- kSecCSStrictValidate and KSecCSCheckAllArchitectures. The flags are supposed to ensure all code is cryptographically signed. However, Pitts says, the APIs "fall short by default."

Apple's code-signing APIs check the first binary for who signed the code, and then check the following binaries in the Fat/Universal file without verifying the CA root of trust. As a result, malicious code could slip past security software and get onto a target Mac. Third-party security, forensics, and incident response tools that use the official code-signing APIs are potentially affected.

An attacker could begin a successful bypass through a file-sharing or phishing attack. Once they're in, "whatever the user has access to, the malware can get access to," Pitts explains. This could include sensitive files containing personally identifiable information or financial data.

"To undermine a code signing implementation for a major OS would break a core security construct that many depend on for day to day security operations," writes Pitts in a blog post.

Affected Vendors and Available Patches

Security tools built into macOS are not exposed, and affected vendors and open-source projects have been alerted to the bypass, Okta reports. Developers are responsible for properly using the code-signing API, and POCs are released to help test their code.

Here are the affected vendors:

Mac users should apply all necessary fixes to protect against malicious software that tries to manipulate the code-signing process. "If enterprises are using illicit software, they need to update," Pitts emphasizes.

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