Metalama is a Roslyn-based open-source patterns and architecture toolkit for C#. Prior to 2025.1.18, 2026.0.25 and 2026.1.19, Metalama.Framework and Metalama.Tool load and execute compile-time assemblies, auxiliary executable tools and embedded helper assemblies from predictable temporary directories. On `Linux` and `macOS`, the default `Path.GetTempPath()` location is world-writable, and existing artifacts are trusted based on sentinel or `.completed` marker files rather than integrity verification. A local unprivileged user on a shared machine or CI/build agent can pre-create the deterministic directory structure and plant a malicious DLL with the matching sentinel. When a higher-privileged developer or build account subsequently builds, Metalama skips regeneration and executes the planted artifact in the compiler, IDE or MSBuild process as the building user. This permits cross-user privilege escalation, reading source and secrets, supply-chain tampering with build output, or disrupting builds. Default Windows per-user temporary directories are protected by ACLs and are not affected. Deployer-selected insecure cache locations are outside the vulnerability's scope. This issue is fixed in versions 2025.1.18, 2026.0.25 and 2026.1.19.
Metalama, a Roslyn-based toolkit integrated with Visual Studio, contains a deserialization vulnerability in its Windows Visual Studio integration from versions 2023.0.101-rc through 2026.1.18. The vulnerability arises from unsafe deserialization of local named-pipe messages without restricting CLR types, allowing a same-user local process to execute arbitrary code within the Visual Studio process. The issue is fixed in Metalama versions 2025.1.18, 2026.0.25, and 2026.1.19, with the maintained Metalama + PostSharp extension fixed in 2026.1.5. Retired extensions require removal or migration.