CVE-2026-35515

Patched in @nestjs/[email protected]

What kind of vulnerability is it? Who is impacted?

SseStream._transform() interpolates message.type and message.id directly into Server-Sent Events text protocol output without sanitizing newline characters (\r, \n). Since the SSE protocol treats both \r and \n as field delimiters and \n\n as event boundaries, an attacker who can influence these fields through upstream data sources can inject arbitrary SSE events, spoof event types, and corrupt reconnection state. Spring Framework's own security patch (6e97587) validates these same fields (id, event) for the same reason.

Actual impact:

• Event spoofing: Attacker forges SSE events with arbitrary event: types, causing client-side EventSource.addEventListener() callbacks to fire for wrong event types.
• Data injection: Attacker injects arbitrary data: payloads, potentially triggering XSS if the client renders SSE data as HTML without sanitization.
• Reconnection corruption: Attacker injects id: fields, corrupting the Last-Event-ID header on reconnection, causing the client to miss or replay events.
• Attack precondition: Requires the developer to map user-influenced data to the type or id fields of SSE messages. Direct HTTP request input does not reach these fields without developer code bridging the gap.

CVE-2026-35515 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (11.1.18); upgrading removes the vulnerable code path.

Kodem intelligence

Has the problem been patched? What versions should users upgrade to?

1. What is CVE-2026-35515? CVE-2026-35515 is a medium-severity security vulnerability in @nestjs/core (npm), affecting versions <= 11.1.17. It is fixed in 11.1.18.
2. How severe is CVE-2026-35515? CVE-2026-35515 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of @nestjs/core are affected by CVE-2026-35515? @nestjs/core (npm) versions <= 11.1.17 is affected.
4. Is there a fix for CVE-2026-35515? Yes. CVE-2026-35515 is fixed in 11.1.18. Upgrade to this version or later.
5. Is CVE-2026-35515 exploitable, and should I be worried? Whether CVE-2026-35515 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-35515 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-35515? Upgrade @nestjs/core to 11.1.18 or later.