# CVE-2025-6515: Reuse of session IDs in oatpp-mcp leads to session hijacking and prompt hijacking by remote attackers

> Remote attackers can hijack MCP sessions by guessing predictable session IDs, allowing them to intercept and modify server responses.

- **Severity:** Medium 6.8 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 20 Oct 2025
- **Updated:** 20 Oct 2025
- **Also known as:** GHSA-rvw9-mm8q-456q

## What is CVE-2025-6515?

CVE-2025-6515 is a medium-severity authentication bypass vulnerability in oatpp-mcp, published on 20 Oct 2025. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

The MCP SSE endpoint in oatpp-mcp returns an instance pointer as the session ID, which is not unique nor cryptographically secure. This allows network attackers with access to the oatpp-mcp server to guess future session IDs and hijack legitimate client MCP sessions, returning malicious responses from the oatpp-mcp server.

Weakness: CWE-330 (Use of Insufficiently Random Values)

## Which versions are affected by CVE-2025-6515?


## How do I fix CVE-2025-6515?

No fix has been published for CVE-2025-6515 yet. Limit exposure of the affected software until one is available.

## Is CVE-2025-6515 being exploited?

There is no known exploitation of CVE-2025-6515 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-6515 and fix it if it is affected.

What it is: CVE-2025-6515 is a medium-severity authentication bypass vulnerability in oatpp-mcp, published on 20 Oct 2025. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2025-6515 in the wild and no public exploit code.

Affected product: oatpp-mcp. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-6515/ (Markdown: https://insights.sbomify.com/CVE-2025-6515/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-6515
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-6515
- OSV: https://osv.dev/vulnerability/CVE-2025-6515

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-6515/
Summaries are generated and may contain mistakes; the official description above is the record.