The upcoming MCP 2026-07-28 specification brings real security improvements — stateless sessions, mandatory OAuth 2.1, issuer validation. But Akamai’s security research team, publishing findings timed to coincide with the spec’s wider discussion, has identified a set of attack surfaces that the improvements don’t address — and in some cases, the redesign creates.

This isn’t a reason to avoid upgrading to the new spec. It’s a reason to treat the upgrade as a security project that requires active implementation choices, not a passive library update.

The Core Tension: Moving Risk Rather Than Eliminating It

Protocol redesigns often shift risk rather than eliminate it. The MCP 2026-07-28 spec’s move away from stateful server-side sessions removes session hijacking as an attack vector. It also moves the responsibility for state management to the client — and that shift creates new exposure if implementations don’t account for it.

The pattern is familiar from web security history: when server-side session tokens were replaced by client-side JWTs and cookies, CSRF and token forgery risks moved to the forefront. When monolithic session stores gave way to stateless distributed systems, replay attacks and token scope validation became critical. MCP is now navigating a similar transition at the protocol layer.

Attack Surface 1: Client-Supplied State Tampering

In the new stateless architecture, servers hand state objects and workflow tracking identifiers to clients. When a multi-step workflow needs to be resumed, the client returns these identifiers to the server to pick up where the work left off.

The assumption baked into this design is that servers will treat these client-returned values as untrusted input and validate them before use. In practice, many implementations won’t — particularly early implementations from teams who are focused on getting the feature working rather than hardening it against adversarial input.

The attack vectors Akamai identifies here are straightforward:

Workflow hijacking: An attacker who can observe or guess a workflow tracking identifier can craft a request returning that identifier under their own session. If the server doesn’t verify that the client returning the identifier is the same client that received it, they can hijack another user’s active workflow — accessing in-progress data, steering the workflow, or triggering its completion.

Cross-tenant data access: In multi-tenant deployments, an attacker accessing another tenant’s workflow state could retrieve data that was never meant to be accessible to them.

Unauthorized actions: Depending on what the workflow involves, hijacking late-stage state could trigger committed actions — file writes, API calls, database modifications — without going through the authorization path that would normally precede them.

These are essentially IDOR vulnerabilities applied to the agentic workflow state layer. The vulnerability class is not new; its appearance in MCP infrastructure is.

Attack Surface 2: Unsigned _meta Object Abuse

The _meta field is a universal envelope in the new spec that lets clients attach arbitrary custom metadata to MCP messages. It travels alongside every request and is intended for extensibility — passing context about the client, the workflow, or the request that the standard fields don’t cover.

The problem is that _meta is unsigned. Without cryptographic verification, a server receiving a _meta object has no way to confirm its contents haven’t been modified since the server last interacted with the client.

If a server uses _meta values for authorization decisions (checking whether the requester has elevated privileges), routing (directing the request to different backend logic), or privilege escalation (granting capabilities based on metadata claims), an attacker can craft a single malicious request with a modified _meta object to abuse these paths.

The attack requires only:

  1. The ability to send a valid MCP request
  2. Knowledge of which _meta keys the server acts on (which may be discoverable through the spec, documentation, or response behavior)
  3. A server implementation that doesn’t verify _meta integrity before acting on it

Akamai’s recommendation is to treat _meta values used for any security-relevant decision as untrusted and require cryptographic attestation, or avoid using _meta for those decisions entirely.

Attack Surface 3: HTTP Header Desync

The new spec introduces Mcp-Method and Mcp-Name HTTP headers that carry routing and observability information. The same information also appears in the JSON-RPC body. When these two representations disagree — either through attacker manipulation or through proxy/middleware transformation — servers that trust one source over the other can be steered into unexpected behavior.

Header desync attacks are well-known in HTTP security. The new MCP headers create a new instance of this pattern specific to MCP infrastructure. HTTP proxies, load balancers, and API gateways may normalize, strip, or modify custom headers in ways that create conflicts with body content, and implementations need to handle these conflicts safely.

Attack Surface 4: Stored XSS in MCP Apps

The MCP Apps extension, which becomes first-class in the 2026-07-28 spec, enables servers to render UI elements directly to clients. If server-rendered content includes user-influenced or externally-supplied data that isn’t properly escaped, an attacker can inject malicious scripts that execute when a victim’s client renders the affected content.

The stored XSS vector specifically: an attacker saves malicious content to a server (through a tool call, resource submission, or other write path). When a victim client invokes any tool or resource that causes the server to render that content, the attacker’s script executes in the victim’s client context.

This is a foundational web security issue applied to MCP’s new rendering capability. The appropriate mitigation is well-established — content escaping, Content Security Policy headers, and treating all user-influenced content as untrusted — but teams implementing MCP Apps features need to actively apply these protections.

Attack Surface 5: Async Task Denial-of-Service

The spec’s support for long-running asynchronous tasks creates a denial-of-service vector that’s specific to the async execution model. An attacker submits computationally expensive tasks through valid MCP calls, then disconnects. The server continues executing the submitted tasks, consuming CPU, memory, and I/O, without any ongoing connection from the requester.

This is particularly concerning in multi-agent architectures. A single attacker controlling multiple agent identities can submit expensive async tasks across many sessions simultaneously. Each individual submission may fall within normal rate limits; the aggregate load across all sessions can push servers into degraded or unavailable states.

Existing CVEs in Deployed MCP Servers

Beyond the spec-level risks, Akamai’s research also identified CVEs in existing deployed MCP server implementations:

  • CVE-2025-66335: SQL injection vulnerability in Apache Doris MCP server integration
  • Unauthenticated access vulnerabilities in Apache Pinot MCP server configurations

These are implementation-level CVEs in specific MCP server packages, separate from the spec-level risks. Teams using Apache Doris or Pinot integrations through MCP should check their exposure and apply available patches.

Putting It Together: A Risk-Based Upgrade Approach

The Akamai findings don’t argue against upgrading to 2026-07-28 — the spec improvements are genuine and meaningful. They argue for treating the upgrade as a security project rather than a maintenance task.

When you upgrade your MCP infrastructure to the new spec:

For client state management: audit every place your server hands state objects to clients and receives them back. Implement validation — at minimum, verify that the client returning a state identifier is the same client that received it. For high-value workflows, add cryptographic signing.

For _meta handling: identify all places your server uses _meta values for security-relevant decisions. Either add cryptographic attestation for those values or remove the dependency on _meta for those decisions.

For header handling: implement normalization logic that detects and handles conflicts between Mcp-Method/Mcp-Name headers and JSON-RPC body content. Reject or log conflicting requests rather than silently trusting one source.

For MCP Apps: if you’re implementing server-rendered UI features, apply standard XSS protections from day one. Don’t ship a first version and plan to add security later.

For async tasks: implement per-identity rate limiting and task quotas. Set execution time limits and queue size limits. Log all async task submissions with requester identity for incident investigation.

The deadline is July 28. There’s time to upgrade and implement these mitigations before the final spec goes live — but not much time to do both casually.

Sources

  1. Akamai — One Fluke, 3 Pattern: MCP Back-End Vulnerabilities
  2. Akamai — What is Model Context Protocol
  3. Security Boulevard — Akamai report details MCP security weaknesses
  4. Akamai — Preventing Command Injection and SQLi attacks over MCP
  5. MCP Blog — 2026-07-28 Release Candidate

Researched by Searcher → Analyzed by Analyst → Written by Writer Agent (Sonnet 4.6). Full pipeline log: subagentic-20260625-2000

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