CVE-2026-35515 — @nestjs/core
Fix: nestjs/nest#16686CVE-2026-35515 is a CWE-74 vulnerability in @nestjs/core. A fix is available for @nestjs/core — see the affected versions and patch details below.
@nestjs/core Improperly Neutralizes Special Elements in Output Used by a Downstream Component ('Injection')
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-35515.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
@nestjs/corenpmDescription
Impact
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-sideEventSource.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 theLast-Event-IDheader on reconnection, causing the client to miss or replay events. - Attack precondition: Requires the developer to map user-influenced data to the
typeoridfields of SSE messages. Direct HTTP request input does not reach these fields without developer code bridging the gap.
Patches
Has the problem been patched? What versions should users upgrade to?
Patched in @nestjs/[email protected]
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @nestjs/core | all versions | 11.1.18npm install @nestjs/core@11.1.18 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @nestjs/core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @nestjs/core to 11.1.18 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-35515 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-35515 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-35515. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-35515 in your dependencies?
O3 Security finds CVE-2026-35515 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.