CVE-2026-93318 in BuildKit
Summary
by MITRE • 10/05/2026
A malicious image can advertise DiffIDs from another image while containing different layer contents. In affected versions, BuildKit could use the advertised DiffIDs to derive cache and snapshot identity without validating that they matched the actual layer contents.
If a BuildKit daemon with shared or persistent cache first processes such a malicious image, a later build using the victim image may mount the attacker-controlled layer contents as the base image. This can allow code from the malicious image to run in the victim build, for example by replacing a commonly executed path such as /bin/sh. The attacker-controlled code may read build secrets mounted into the build, access other build resources, alter output artifacts, or hang the build.
The issue affects both regular snapshotters and lazy-pulling snapshotters such as stargz.
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Analysis
by VulDB Data Team • 10/05/2026
This vulnerability represents a critical integrity failure within Docker BuildKit's caching mechanism, specifically targeting how layer identities are validated during image construction. The core technical flaw lies in the daemon's reliance on advertised DiffIDs from an incoming image manifest to derive cache keys and snapshot identities without performing cryptographic verification against the actual content of the layers. In container build systems like BuildKit, each filesystem layer is assigned a unique identifier known as a DiffID, which serves as a fingerprint for that specific set of files. When building images, the system checks if it already possesses a cached version of these layers to optimize performance by reusing existing snapshots rather than recomputing them. However, in affected versions, this process lacks a critical validation step where the advertised identifier is compared against the computed hash of the downloaded layer data. This omission allows an attacker to craft a malicious image that claims to have the same DiffID as a legitimate, widely used base image while containing entirely different file contents.
The operational impact of this flaw is severe because it enables cache poisoning attacks in environments where BuildKit daemons utilize shared or persistent caches. If such a daemon processes a malicious image with spoofed identifiers before processing a victim's build request for the same nominal base image, the system will mistakenly believe that the cached snapshot corresponds to the legitimate layer content. Consequently, when the subsequent build begins, it mounts the attacker-controlled filesystem as if it were the trusted base image. This substitution allows arbitrary code execution within the context of the victim's build process. For instance, an attacker could replace standard binaries such as /bin/sh or other commonly executed paths with malicious scripts that exfiltrate sensitive data. Since builds often have access to secrets mounted into the environment for authentication purposes, this attack vector provides a direct path for attackers to read these credentials and compromise downstream systems that rely on artifacts produced by the build pipeline.
Beyond secret theft, the vulnerability facilitates broader supply chain attacks where output artifacts are altered or builds are deliberately hung to cause denial of service. The flaw affects both regular snapshotters and lazy-pulling mechanisms like stargz, meaning it is not limited to traditional full-layer downloads but extends to systems that pull layers on demand. This broadens the attack surface significantly across various container runtime configurations. From a classification perspective, this issue aligns with CWE-345 Insufficient Verification of Data Authenticity and falls under MITRE ATT&CK techniques related to Supply Chain Compromise and Cache Poisoning. The lack of integrity checks allows an adversary to inject malicious payloads into the build context without detection by standard verification procedures.
Mitigation strategies must focus on enforcing strict cryptographic validation of layer contents against their advertised identifiers before any caching or snapshotting operations occur. Developers should ensure that BuildKit is updated to versions where this validation logic has been implemented, ensuring that the computed hash of each downloaded layer matches the DiffID specified in the image manifest. In environments with shared caches, it is crucial to isolate cache stores per tenant or project to limit the blast radius if a single malicious build occurs. Additionally, implementing content-addressable storage verification at the registry level can provide an additional layer of defense by ensuring that only images with verified integrity are pulled into the environment. Regular auditing of base image sources and using trusted registries further reduces the risk of encountering such crafted artifacts during automated builds.