Metalama: Unsafe deserialization over Metalama's Visual Studio inter-process named pipe enables local code executionGHSA-h26j-4vp7-x9w2

Summary

Metalama uses a local named-pipe channel to communicate between its Visual Studio processes — the Visual Studio UI process (devenv), the out-of-process Roslyn code-analysis process, and the CodeLens process. This channel deserialized messages from the pipe without restricting which CLR types could be instantiated. A local process able to write to the pipe can send a crafted message that instantiates arbitrary .NET types — a deserialization primitive in the same class as BinaryFormatter (CWE-502), and a well-known gadget-chain code-execution vector.

The flaw is present, via two successive serializers, in every Metalama release from the first public version (2023.0) up to and including 2026.1.18, and is fixed in 2026.1.19.

The pipe name is predictable: Metalama_{role}_{processId}_{hash(PackageVersion)}, derived from the public package version and a locally-enumerable process id. This channel is used by the Visual Studio integration only (Windows named pipes); it is not used for command-line / MSBuild builds.

Mechanisms

JSON era — Metalama 2023.0 through 2026.0.x (Newtonsoft.Json). The pipe formatter enabled polymorphic type embedding:

formatter.JsonSerializer.TypeNameHandling = TypeNameHandling.All;   // ServiceEndpoint.cs (early), later BaseEndpoint.cs

The accompanying SerializationBinder (added later for multi-version assembly support) only remapped assembly names; its BindToType resolved any type name from the wire (assembly.GetType(typeName)) with no type allow-list. TypeNameHandling.All is the canonical unsafe-deserialization setting and has publicly available gadget chains (e.g. ysoserial.net), so this era is at least as exploitable as the later one. (Introduced by commit 0554368737, first shipped in release/2023.0.101-rc.)

MessagePack era — Metalama 2026.1.0 through 2026.1.18. The 2026.1 line migrated the pipe serialization to MessagePack (PR #1299, commit 117b607805, 2026-01-29), using typeless MessagePack (TypelessObjectResolver), which likewise embeds CLR type names on the wire and instantiates arbitrary types on receipt — again with no allow-list.

Both mechanisms share the same channel, the same predictable pipe name, the same impact, and the same access preconditions.

Impact

Potential arbitrary code execution inside the Visual Studio process that hosts the affected component (the UI process, the code-analysis process, or the CodeLens process), with full loss of confidentiality, integrity and availability within that process.

Mitigating factor (limits practical severity to Medium): the pipe is created with the default Windows ACL (no explicit PipeSecurity), which permits only same-user processes to write. All of the Visual Studio processes involved run as the same logged-in developer, so exploitation requires the attacker to already be executing code as that same user. There is no privilege escalation and no crossing of a security boundary; the practical value is code execution inside the developer's Visual Studio session (lateral movement / persistence within an already same-user-compromised context). This is why the practical severity is Medium despite the CVSS base score.

Severity

CVSS v3.1: AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H = 7.3 (High) base score; practical severity Medium, because the attacker gains no privilege beyond what they already had (same user, same Visual Studio session).

CVSS v4.0 cross-check: CVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N = 7.0 (High). The Subsequent-System metrics are None because no security boundary is crossed.

Affected products and remediation

All affected products are part of the Visual Studio integration.

The PostSharp extension was a separate, Metalama-free product before the 2024.1 merge (Metalama.Vsx issue #33843), so earlier PostSharp versions are not affected. GitHub has no Visual Studio Marketplace ecosystem, so only the NuGet package appears in the structured "Affected products" list.

Fix

Fixed in Metalama 2026.1.19 (commit d97f52e1b5, PR #1651): the type-name deserialization is wrapped in RpcContractMessagePackOptions, which enforces a [RpcContract] type allow-list on the wire; pipe access is restricted to the current user. The fix reaches the maintained Visual Studio extension in version 2026.1.5. The retired Caravela and Metalama extensions will not be fixed.