Splunk MCP Server app: Deserialization of Untrusted Data
CVE-2026-76404 is a critical-severity deserialization of untrusted data vulnerability in Splunk MCP Server app from Splunk, with a CVSS 3.1 base score of 9.1 out of 10. It can be exploited remotely over the network by an attacker with administrator privileges, without any action from a user. The record lists version 1.2.1 as the first without the flaw, so updating Splunk MCP Server app to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.75% chance of exploitation within 30 days, higher than 56% of scored CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Update Splunk MCP Server app to 1.2.1 or a later version.
- Update Model Context Protocol Server to 1.2.1 or a later version.
- Read the advisory on advisory.splunk.com for workarounds and exact fixed builds.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Splunk | Splunk MCP Server app | 1.2 up to, but not including, 1.2.1 | 1.2.1 |
| Splunk | Model Context Protocol Server | before 1.2.1 | 1.2.1 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- HighAdministrator-level access is required first.
- User interaction
- NoneNo action by a victim is required.
- Scope
- ChangedThe attack can reach beyond the vulnerable component into other systems.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H. Scored by CNA (cisco.com).
Official description
In Splunk MCP Server app versions below 1.2.1, a user who holds the "admin" Splunk role could execute arbitrary commands on the underlying operating system. The vulnerability is possible because of missing input validation in the app's credential management component, which deserializes stored data without checking whether the content is of the expected type.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Deserialization of Untrusted Data
The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.
How developers prevent this class of flaw
- If available, use the signing/sealing features of the programming language to assure that deserialized data has not been tainted. For example, a hash-based message authentication code (HMAC) could be used to ensure that data has not been modified.
- When deserializing data, populate a new object rather than just deserializing. The result is that the data flows through safe input validation and that the functions are safe.
- Explicitly define a final object() to prevent deserialization.
More Deserialization of Untrusted Data vulnerabilities (CWE-502)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Recent Deserialization of Untrusted Data vulnerabilities
- 9.8 Critical severity
- 8.8 High severity
- 8.4 High severity
- 7.8 High severity
- 7.8 High severity
- 7.8 High severity
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