Medium

Prompt Hijacking in oatpp-mcp SSE Session IDs

IdentifiersCVE-2025-6515CWE-340

CVE-2025-6515 affects the oatpp-mcp implementation of the Model Context Protocol (MCP) for the Oat++ C++ web framework. In the SSE transport flow, oatpp-mcp generates the session ID from an instance pointer, making the identifier predictable, non-unique, and not cryptographically secure. Because MCP clients use this session ID in subsequent POST requests, an attacker who can predict or obtain a valid session ID can submit requests that the server associates with another client session. JFrog described this as a form of 'prompt hijacking': the attacker can pre-generate or guess session IDs, wait for them to be reassigned to legitimate clients, and then inject malicious MCP requests or prompts that the server processes as though they originated from the victim session. The vulnerable condition is rooted in insecure generation and reuse of session identifiers in the original SSE-based MCP implementation.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation allows session hijacking of legitimate MCP client connections. An attacker can cause the oatpp-mcp server to accept malicious requests under the victim's session context and return attacker-influenced or malicious responses to the legitimate client. In AI-agent or MCP-driven workflows, this can poison prompt/response exchanges, alter tool outputs, and potentially induce downstream actions based on falsified server responses. The issue compromises session integrity and trust in MCP communications, though the provided content does not establish direct server-side code execution.

Mitigation

If you can’t patch tonight, do this now.

If immediate remediation is not possible, restrict network access to the oatpp-mcp server to trusted clients and segments to reduce the ability of remote attackers to probe or pre-allocate session IDs. Add client-side validation using unpredictable event IDs and accept only responses that match the originating event ID. Monitor for abnormal connection churn, repeated session establishment/teardown, and suspicious POST activity that may indicate session ID harvesting or hijacking attempts. Where feasible, prefer MCP transports/specifications that require secure session handling over the legacy SSE approach.

Remediation

Patch, then assume compromise.

Update or modify oatpp-mcp so that session IDs are generated using a cryptographically secure random number generator and are unique, non-reusable, and carry at least 128 bits of entropy, as recommended by JFrog. Implement the newer MCP transport requirements that mandate secure session identifiers rather than relying on the older SSE behavior. Ensure freed session IDs are not recycled in a predictable manner and review all MCP server-side session management logic for identifier generation, lifetime, and reuse.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

ACTIVITY FEED

Recent activity

6 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

6 SOURCESView more in app

the hacker newsNews

Oct 23, 2025

ThreatsDay Bulletin: $176M Crypto Fine, Hacking Formula 1, Chromium Vulns, AI Hijack & More

A session hijacking / response-injection flaw in the Oat++ MCP (oatpp-mcp) Server-Sent Events (SSE) transport where predictable/reused session IDs can be abused to hijack an active session and inject malicious responses.

cso onlineNews

Oct 22, 2025

Prompt hijacking puts MCP-based AI workflows at risk

A vulnerability in oatpp-mcp caused by guessable session IDs, enabling session hijacking/prompt hijacking in MCP communications and allowing attackers to inject malicious requests or prompts into legitimate client sessions.

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Threat actor evidence

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Associated malware

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Detection signatures

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Vendor-by-vendor mapping

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Social activity4