Listen to this Post
BuildKit is a toolkit for converting source code to build artifacts in an efficient, expressive, and repeatable manner. Prior to version 0.31.1, a critical vulnerability existed in how BuildKit handled `SecurityMode` values provided by custom frontends. BuildKit allows users to specify custom frontends using the `syntax` directive or the `–build-arg BUILDKIT_SYNTAX` flag. A custom frontend is essentially a container image that implements the BuildKit frontend API and can control the build process.
The vulnerability arises in the `executor/oci/spec_linux.go` component, which is responsible for constructing the OCI specification for the build container. This specification defines the security context, including Seccomp and AppArmor profiles, Linux capabilities, and other isolation mechanisms. Normally, if a user wants to disable these security protections, they must explicitly grant the `security.insecure` enment to the frontend. This enment acts as a safeguard, ensuring that only trusted frontends can run without sandboxing.
However, a malicious custom frontend could place an invalid or unsupported `SecurityMode` value in a crafted build request. Due to improper input validation (CWE-20), the `spec_linux.go` logic treated this unsupported value as a non-sandbox mode. Critically, it did so without checking for the `security.insecure` enment. As a result, the build container would be created with Seccomp and AppArmor protections completely disabled. Other security measures, such as Linux capabilities, were still applied to these containers, but the loss of Seccomp and AppArmor significantly weakened the container’s security posture.
This bypass could allow an attacker to execute system calls or perform actions that would normally be blocked by the default security profiles. The vulnerability is particularly dangerous because it does not require the user to explicitly opt-in to insecure behavior. Simply using a malicious or compromised custom frontend is sufficient to trigger the bypass. The issue was addressed in BuildKit version 0.31.1, which properly validates the `SecurityMode` input and enforces the `security.insecure` enment check.
DailyCVE Form:
Platform: BuildKit (Moby)
Version: < 0.31.1
Vulnerability: Seccomp/AppArmor bypass
Severity: Medium (CVSS 5.3)
date: 2026-08-19
Prediction: 2026-06-24 (already patched)
What Undercode Say:
Check BuildKit version buildkitd --version Verify if running a vulnerable version (< v0.31.1) Example vulnerable request (crafted by malicious frontend) The frontend sends an invalid SecurityMode value spec_linux.go treats it as non-sandbox mode without security.insecure check Exploit proof-of-concept (simplified) A custom frontend can set SecurityMode to an unsupported string e.g., "invalid-mode" which bypasses the enment check
Exploit: (Educational Purposes!)
A malicious custom frontend can be created and specified via `syntax=malicious/frontend` in a Dockerfile or --build-arg BUILDKIT_SYNTAX=malicious/frontend. The frontend sends a build request with an invalid `SecurityMode` value (e.g., "unsupported"). The BuildKit executor, in versions prior to 0.31.1, processes this value without validating it against the allowed modes and without requiring the `security.insecure` enment. Consequently, the build container is launched with `seccomp` and `AppArmor` disabled, allowing the attacker to execute restricted system calls and bypass mandatory access controls.
Protection:
- Upgrade to BuildKit version v0.31.1 or later.
- Only use BuildKit frontends from trusted providers.
- Avoid using untrusted `syntax` directives or `BUILDKIT_SYNTAX` build arguments.
- Implement network-level restrictions to prevent BuildKit from pulling untrusted frontend images.
- Regularly audit and monitor BuildKit logs for suspicious frontend activity.
Impact:
- Confidentiality: Low – potential information disclosure via unrestricted system calls.
- Integrity: Low – possible modification of build artifacts or host system.
- Availability: Low – risk of denial of service through unrestricted resource usage.
- Overall: An attacker with control over a custom frontend can bypass critical sandboxing mechanisms, potentially leading to container escape, privilege escalation, or unauthorized access to host resources. The vulnerability is mitigated by the fact that Linux capabilities remain restricted, limiting the full impact.
🎯Let’s Practice Exploiting & Learn Patching For Free:
🎓 Live Courses & Certifications:
Join Undercode Academy for Verified Certifications
🚀 Request a Custom Project:
Secure, high-velocity infrastructure and disruptive technological engineering. Contact our engineering team for high-tier development and proprietary systems:
[email protected]
💎 Smart Architecture | 🛡️ Secure by Design | ⭐ Trusted by Thousands
Sources:
Reported By: github.com
Extra Source Hub:
Undercode
