CVE-2026-48516
MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings
MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, InterfaceLookupFormatter<TKey,TElement> constructs an internal Dictionary<TKey, IGrouping<TKey,TElement>> with the default equality comparer instead of the security-aware comparer supplied by options.Security.GetEqualityComparer<TKey>(). This formatter omission allows hash-collision CPU denial of service against ILookup<TKey,TElement> even when the application has opted into the untrusted-data security posture This vulnerability is fixed in 2.5.301 and 3.1.7.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- High
- Privileges needed
- None
- User interaction
- None
What an attacker can affect
- Confidentiality
- None
- Integrity
- None
- Availability
- Low
Exploit likelihood
What each source reports
CVE List Score used
MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings
MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, InterfaceLookupFormatter<TKey,TElement> constructs an internal Dictionary<TKey, IGrouping<TKey,TElement>> with the default equality comparer instead of the security-aware comparer supplied by options.Security.GetEqualityComparer<TKey>(). This formatter omission allows hash-collision CPU denial of service against ILookup<TKey,TElement> even when the application has opted into the untrusted-data security posture This vulnerability is fixed in 2.5.301 and 3.1.7.
- Record
CVE-2026-48516- CVSS 4.0
- 6.3 Medium
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N - Weakness
- CWE-407
- Updated
- 2026-11-02
EUVDEUVD-2026-38380
MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, InterfaceLookupFormatter<TKey,TElement> constructs an internal Dictionary<TKey, IGrouping<TKey,TElement>> with the default equality comparer instead of the security-aware comparer supplied by options.Security.GetEqualityComparer<TKey>(). This formatter omission allows hash-collision CPU denial of service against ILookup<TKey,TElement> even when the application has opted into the untrusted-data security posture This vulnerability is fixed in 2.5.301 and 3.1.7.
- Record
EUVD-2026-38380- CVSS 4.0
- 6.3 Medium
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N - Updated
- 2026-11-02
GitHub AdvisoriesGHSA-q2h6-ghwm-5qm8
MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings
Summary
InterfaceLookupFormatter<TKey,TElement> constructs an internal Dictionary<TKey, IGrouping<TKey,TElement>> with the default equality comparer instead of the security-aware comparer supplied by options.Security.GetEqualityComparer<TKey>().
Other hash-based collection formatters use the security-aware comparer when MessagePackSecurity.UntrustedData is configured. This formatter omission allows hash-collision CPU denial of service against ILookup<TKey,TElement> even when the application has opted into the untrusted-data security posture.
Impact
Applications are affected when they deserialize untrusted payloads into schemas containing ILookup<TKey,TElement> with a key type for which attacker-controlled hash collisions are feasible.
Under the default comparer, many colliding keys can degrade dictionary insertion from amortized constant time to quadratic behavior. A payload of colliding keys can consume CPU for a disproportionate amount of time. This bypasses the mitigation that developers intentionally enabled by using MessagePackSecurity.UntrustedData.
Affected components
- Package:
MessagePack - API:
InterfaceLookupFormatter<TKey,TElement>.Create - Data type:
ILookup<TKey,TElement> - Finding ID:
MESSAGEPACKCSHARP-041
Patches
Fixes are prepared and will be released in coordinated patch versions.
Upgrade guidance:
- Upgrade
MessagePackto the patched version for your release line. - Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.
The fix should create the internal dictionary with options.Security.GetEqualityComparer<TKey>(), matching the sibling dictionary and lookup formatter behavior.
Workarounds
Patching is recommended.
Until a patched version is available, avoid exposing ILookup<TKey,TElement> in DTOs that deserialize untrusted data. Use collection shapes that are already protected by the security-aware comparer path, or validate and cap collection sizes at the transport boundary.
Resources
MESSAGEPACKCSHARP-041:InterfaceLookupFormattermissing security comparer- CWE-407: Inefficient Algorithmic Complexity
- Record
GHSA-q2h6-ghwm-5qm8- CVSS 4.0
- 6.3 Medium
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N - Weakness
- CWE-407
- Updated
- 2026-11-02
NVD
MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, InterfaceLookupFormatter<TKey,TElement> constructs an internal Dictionary<TKey, IGrouping<TKey,TElement>> with the default equality comparer instead of the security-aware comparer supplied by options.Security.GetEqualityComparer<TKey>(). This formatter omission allows hash-collision CPU denial of service against ILookup<TKey,TElement> even when the application has opted into the untrusted-data security posture This vulnerability is fixed in 2.5.301 and 3.1.7.
- Record
CVE-2026-48516- CVSS 4.0
- 6.3 Medium
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - Weakness
- CWE-407
- Updated
- 2026-11-02
OSV
MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings
MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, InterfaceLookupFormatter<TKey,TElement> constructs an internal Dictionary<TKey, IGrouping<TKey,TElement>> with the default equality comparer instead of the security-aware comparer supplied by options.Security.GetEqualityComparer<TKey>(). This formatter omission allows hash-collision CPU denial of service against ILookup<TKey,TElement> even when the application has opted into the untrusted-data security posture This vulnerability is fixed in 2.5.301 and 3.1.7.
- Record
CVE-2026-48516- CVSS 4.0
- 6.3 Medium
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N - Weakness
- CWE-407
- Updated
- 2026-11-02
OSVGHSA-q2h6-ghwm-5qm8
MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings
Summary
InterfaceLookupFormatter<TKey,TElement> constructs an internal Dictionary<TKey, IGrouping<TKey,TElement>> with the default equality comparer instead of the security-aware comparer supplied by options.Security.GetEqualityComparer<TKey>().
Other hash-based collection formatters use the security-aware comparer when MessagePackSecurity.UntrustedData is configured. This formatter omission allows hash-collision CPU denial of service against ILookup<TKey,TElement> even when the application has opted into the untrusted-data security posture.
Impact
Applications are affected when they deserialize untrusted payloads into schemas containing ILookup<TKey,TElement> with a key type for which attacker-controlled hash collisions are feasible.
Under the default comparer, many colliding keys can degrade dictionary insertion from amortized constant time to quadratic behavior. A payload of colliding keys can consume CPU for a disproportionate amount of time. This bypasses the mitigation that developers intentionally enabled by using MessagePackSecurity.UntrustedData.
Affected components
- Package:
MessagePack - API:
InterfaceLookupFormatter<TKey,TElement>.Create - Data type:
ILookup<TKey,TElement> - Finding ID:
MESSAGEPACKCSHARP-041
Patches
Fixes are prepared and will be released in coordinated patch versions.
Upgrade guidance:
- Upgrade
MessagePackto the patched version for your release line. - Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.
The fix should create the internal dictionary with options.Security.GetEqualityComparer<TKey>(), matching the sibling dictionary and lookup formatter behavior.
Workarounds
Patching is recommended.
Until a patched version is available, avoid exposing ILookup<TKey,TElement> in DTOs that deserialize untrusted data. Use collection shapes that are already protected by the security-aware comparer path, or validate and cap collection sizes at the transport boundary.
Resources
MESSAGEPACKCSHARP-041:InterfaceLookupFormattermissing security comparer- CWE-407: Inefficient Algorithmic Complexity
- Record
GHSA-q2h6-ghwm-5qm8- CVSS 4.0
- 6.3 Medium
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N - Weakness
- CWE-407
- Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
MessagePackNuGet |
before 2.5.301 | 2.5.301 |
MessagePackNuGet |
3.0 up to 3.1.7 | 3.1.7 |
MessagePackNuGet |
before 2.5.301, plus listed versions 0.1.0-beta0.2.0-beta0.2.1-beta0.2.2-beta0.2.3-beta Show 45 moreShow less0.3.0-beta0.4.00.4.10.4.20.5.00.6.00.6.10.7.00.7.20.8.00.8.10.8.20.8.30.8.40.8.51.0.01.0.11.0.21.0.31.1.01.1.11.1.1.11.1.21.2.01.2.0.11.2.0.21.2.11.2.21.2.31.3.01.3.11.3.1.11.3.21.3.31.4.01.4.11.4.21.4.31.4.41.5.01.5.0.11.5.0.21.5.11.6.01.6.0.1 |
2.5.301 |
MessagePackNuGet |
3.0 up to 3.1.7, plus listed versions 3.0.111-alpha3.0.129-beta3.0.134-beta3.0.208-rc-00013.0.3 Show 10 moreShow less3.0.3003.0.3083.0.54-alpha3.1.03.1.13.1.23.1.33.1.43.1.53.1.6 |
3.1.7 |
History
| Recorded | Changed | Sources |
|---|---|---|
| first indexed | CVE List, EUVD, GitHub Advisories, NVD, OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2026-48516 is a medium vulnerability (CVSS 6.3) in Messagepack, published June 22, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.40%, the 35th percentile. Fixed in MessagePack 2.5.301.
References
Left out here: 70 versions. The JSON record has them.