GHSA-f5v8-v6q3-q4h6

Summary

Meridian v2.1.0 (Meridian.Mapping and Meridian.Mediator) shipped with nine defense-in-depth gaps reachable through its public APIs. Two are HIGH severity — the advertised DefaultMaxCollectionItems and DefaultMaxDepth safety caps are silently bypassed on the IMapper.Map(source, destination) overload and anywhere .UseDestinationValue() is configured on a collection-typed property. Four are MEDIUM (constructor invariant bypass, OpenTelemetry stack-trace info disclosure, retry amplification, notification fan-out amplification). Three are LOW (exception message disclosure, dictionary duplicate-key echo, static mediator cache growth under closed-generic types).

All nine are patched in v2.1.1. Upgrade is a drop-in NuGet bump; see the v2.1.1 CHANGELOG for the four behavioural changes (constructor selection, OTel default, publisher fan-out cap, retry caps).

Severity Matrix

| # | Severity | CWE | Finding | Fix | |---|---|---|---|---| | 1 | HIGH | CWE-770 | MappingEngine.TryMapCollectionOntoExisting enumerated the source without enforcing DefaultMaxCollectionItems. Reachable via Mapper.Map<TSrc,TDst>(src, dst) and any .ForMember(..., o => o.UseDestinationValue()) on a collection member through a plain Map(src) call. | Shared cap enforcement helper between MapCollection and TryMapCollectionOntoExisting. | | 2 | HIGH | CWE-674 | Collection-item recursion in the existing-destination path did not increment ResolutionContext.Depth, so self-referential collection graphs could reach stack overflow before DefaultMaxDepth fired. | Depth increments at every collection-item boundary. | | 3 | MEDIUM | CWE-665 | ObjectCreator.CreateWithConstructorMapping always invoked the widest public constructor, silently filling unresolved parameters with default(T) and bypassing narrower-ctor invariants. | Widest-ctor selection now requires every parameter to be bound via explicit ctor mapping, source-name match, or a C# optional default. | | 4 | MEDIUM | CWE-532 | Mediator.MarkActivityFailure emitted the full ex.ToString() (stack + inner chain) to the OpenTelemetry exception.stacktrace activity tag by default, leaking context to any shared trace sink. | Gated on MediatorTelemetryOptions.RecordExceptionStackTrace — opt-in, default false. | | 5 | MEDIUM | CWE-400 | RetryBehavior retried every exception type with unbounded MaxRetries; the exponential-backoff delay overflowed TimeSpan at ~30 attempts. No cancellation exclusion. | Server-side MaxRetriesCap = 10, MaxBackoff = 5 min, OperationCanceledException short-circuit, recommended RetryPolicy.TransientOnly helper. | | 6 | MEDIUM | CWE-400 | TaskWhenAllPublisher started every registered handler concurrently with no bound on fan-out. | New constructor parameter maxDegreeOfParallelism (default 16; -1 restores legacy unbounded). | | 7 | LOW | CWE-209 | Public mapping exceptions leaked FullName of source/destination types and concatenated inner exception messages into top-level property-mapping errors. | Scrubbed to type Name; inner details only via InnerException chain. | | 8 | LOW | CWE-209 | Dictionary materialization threw ArgumentException on duplicate keys, echoing the attacker-supplied key's .ToString(). | Last-write-wins indexer semantics. | | 9 | LOW | CWE-1325 | Static mediator handler caches grow monotonically under closed-generic request types. Doc-only mitigation; no code change — consumers must not allow attacker-controlled runtime type materialization to reach Send, Publish, or CreateStream. | Documented in docs/security-model.md. |

Exploitation

Finding 1 / 2 (headline): A consumer that maps user-supplied collection payloads onto an existing destination list via mapper.Map(userCollection, existingList) — a documented and commonly used AutoMapper-style idiom — processes the full attacker-supplied collection with no size cap and no depth cap. An attacker sending a single request with a large (or self-referential) collection payload can block the worker thread for seconds and exhaust the managed heap or the call stack. Equivalent exposure through .UseDestinationValue() on a collection-typed destination member, reachable via a plain Map(src) call whose destination type default-initializes that member.

Finding 3: A destination type with multiple public constructors that differ only in their parameter-binding invariants (e.g., new UserAccount(string name, Email email) enforcing a non-default Email) could be instantiated with the narrower ctor's invariants silently bypassed if any source field was absent — the widest ctor was always picked, with unbound parameters replaced by default(T).

Findings 4 / 5 / 6: Amplification / information-disclosure vectors described in the matrix above. Each requires moderate integration context (telemetry sink trust, handler count, retry policy) to weaponize, but each is reachable through public APIs without authentication.

Patches

• Meridian.Mapping 2.1.1 (published 2026-04-16)
• Meridian.Mediator 2.1.1 (published 2026-04-16)

Verified via: - GitHub Release assets at https://github.com/UmutKorkmaz/meridian/releases/tag/v2.1.1 - Sigstore attestation (actions/attest-build-provenance@v2 → gh attestation verify green on both .nupkg from the GitHub Release) - NuGet.org indexed both packages within the release workflow run

Workarounds

Users who cannot upgrade immediately may: 1. Avoid mapper.Map(src, dst) and .UseDestinationValue() on collection-typed destination members. 2. Wrap input collection deserialization with an explicit size limit before handing the payload to Meridian. 3. Register TaskWhenAllPublisher with maxDegreeOfParallelism ≤ 16 manually (v2.1.1+ only). 4. Disable OpenTelemetry exception.stacktrace tag emission at the trace exporter level if your trace sink is less trusted than your application.

These are defense-in-depth; the only complete mitigation is upgrading to 2.1.1.

Supported Versions

As of this advisory the supported security branch is 2.1.x. The 2.0.x line (published 2026-04-15) is not receiving the Phase 1 safety-defaults infrastructure needed to carry the HIGH-severity fixes, so 2.0.x is deprecated in favor of 2.1.x. See SECURITY.md for the updated supported-versions table.

Credits

• UmutKorkmaz (reporter and maintainer)

References

• v2.1.1 CHANGELOG section: https://github.com/UmutKorkmaz/meridian/blob/main/CHANGELOG.md#211---2026-04-16
• docs/security-model.md threat model: https://github.com/UmutKorkmaz/meridian/blob/main/docs/security-model.md
• SECURITY.md disclosure policy: https://github.com/UmutKorkmaz/meridian/blob/main/SECURITY.md
• AutoMapper CVE-2026-32933 (motivating precedent for Meridian's safety-defaults)