CVE-2026-29085 is a medium-severity (CVSS 6.5) CWE-74 vulnerability in hono. A fix is available for hono — see the affected versions and patch details below.
Hono: SSE Control Field Injection via CR/LF in writeSSE()
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- 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-2026-29085.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
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.
hononpmDescription
Summary
When using streamSSE() in Streaming Helper, the event, id, and retry fields were not validated for carriage return (\r) or newline (\n) characters.
Because the SSE protocol uses line breaks as field delimiters, this could allow injection of additional SSE fields within the same event frame if untrusted input was passed into these fields.
Details
The SSE helper builds event frames by joining lines with \n. While multi-line data: fields are handled according to the SSE specification, the event, id, and retry fields previously allowed raw values without rejecting embedded CR/LF characters.
Including CR/LF in these control fields could allow unintended additional fields (such as data:, id:, or retry:) to be injected into the event stream.
The issue has been fixed by rejecting CR/LF characters in these fields.
Impact
An attacker could manipulate the structure of SSE event frames if an application passed user-controlled input directly into event, id, or retry.
Depending on application behavior, this could result in injected SSE fields or altered event stream handling. Applications that render e.data in an unsafe manner (for example, using innerHTML) could potentially expose themselves to client-side script injection.
This issue affects applications that rely on the SSE helper to enforce protocol-level constraints.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | hono | all versions | 4.12.4npm install hono@4.12.4 |
Affected Products
honohonoDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for hono, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update hono to 4.12.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-29085 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.
Frequently Asked Questions
Is CVE-2026-29085 in your dependencies?
Find it across npm, including transitive dependencies.