GHSA-m6wq-39m7-9xpp Metalama allows local code execution through DLL planting in shared temporary directories

• NuGet Metalama.Framework: >= 2026.1.0-preview, < 2026.1.19 → fixed in 2026.1.19 • NuGet Metalama.Tool: >= 2026.1.0-preview, < 2026.1.19 → fixed in 2026.1.19 • NuGet Metalama.Framework: < 2025.1.18 → fixed in 2025.1.18 • NuGet Metalama.Tool: < 2025.1.18 → fixed in 2025.1.18 • NuGet Metalama.Tool: >= 2026.0.0-preview, < 2026.0.25 → fixed in 2026.0.25 • NuGet Metalama.Framework: >= 2026.0.0-preview, < 2026.0.25 → fixed in 2026.0.25

Summary Metalama loaded and executed compile-time assemblies, auxiliary executable tools, and embedded helper assemblies from predictable temporary directories. On Linux and macOS the default cache location was the world-writable system temporary directory (`Path.GetTempPath()`, i.e. `/tmp`), and previously-generated artifacts were trusted based on the presence of a sentinel/`.completed` marker file rather than on any integrity verification. A local, unprivileged user on a shared machine or CI/build agent can pre-create the deterministic cache directory structure and plant a malicious DLL together with the matching sentinel file. When a higher-privileged user (or build/service account) subsequently runs a build, Metalama finds the sentinel, skips regeneration, and loads and executes the attacker-supplied assembly inside the compiler / IDE / MSBuild process. Affected load-and-execute paths: • the cache of compile-time assemblies that Metalama generates from your aspects; • auxiliary executable tools that Metalama extracts to a local directory and then launches; • embedded .NET helper assemblies that Metalama extracts to a local directory and loads into the build / IDE process. Impact Arbitrary code execution in the security context of the building user — i.e. a local privilege escalation across user accounts on shared POSIX systems (multi-user developer machines, CI/build agents). The attacker obtains code execution as the victim without holding the victim's privileges, and can read source/secrets, tamper with build output (supply-chain injection), or disrupt builds. Affected configurations • Linux and macOS, default configuration — affected. The shipped default uses the world-writable system temp directory. • Windows, default configuration — not affected. The default per-user temporary directory is protected by OS ACLs, so cross-user planting is not possible. • The `METALAMA_TEMP` environment variable is a deployer-controlled override and is out of scope: pointing it at an insecure (e.g. world-writable or attacker-owned) location is a deployment misconfiguration, not a defect in the product. Severity CVSS v3.1: `AV:L/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:H` = 8.2 (High) for the affected POSIX default configuration (Scope:Changed reflects the cross-user privilege escalation). CVSS v4.0 cross-check: `CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:P/VC:L/VI:H/VA:L/SC:L/SI:H/SA:H` = 6.0 (Medium), which more granularly attributes low impact to the cache itself and high impact to the subsequent build process. The honest range is therefore Medium-High depending on the scoring model and platform. Fix Caches and extraction directories were relocated from the world-writable system temp directory to per-user application-data locations (`$XDG_CACHE_HOME` / `~/.cache/Metalama` on POSIX), relying on OS-enforced per-user ownership as the trust boundary. Fixed in 2026.1.19. Weaknesses • CWE-377: Insecure Temporary File • CWE-427: Uncontrolled Search Path Element Affected packages • Metalama.Framework < 2026.1.19, • Metalama.Tool < 2026.1.19 Internal tracking: metalama/Metalama#1650.