CVE-2026-39410 is a medium-severity (CVSS 4.8) Improper Input Validation vulnerability in hono. A fix is available for hono — see the affected versions and patch details below.
Hono has a non-breaking space prefix bypass in cookie name handling in getCookie()
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-2026-39410.
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.
How urgent is this, really
CVE-2026-39410 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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
A discrepancy between browser cookie parsing and parse() handling allows cookie prefix protections to be bypassed.
Cookie names that are treated as distinct by the browser may be normalized to the same key by parse(), allowing attacker-controlled cookies to override legitimate ones.
Details
Browsers follow RFC 6265bis and only trim SP (0x20) and HTAB (0x09) from cookie names. Other characters, such as the non-breaking space (U+00A0), are preserved as part of the cookie name.
For example, the browser treats the following cookies as distinct:
"dummy-cookie"
"\u00a0dummy-cookie"
However, parse() previously used JavaScript's trim(), which removes a broader set of characters including U+00A0. As a result, both names are normalized to:
"dummy-cookie"
This mismatch allows attacker-controlled cookies with a U+00A0 prefix to shadow or override legitimate cookies when accessed via getCookie().
Impact
An attacker who can set cookies (e.g., via a man-in-the-middle on a non-secure page or other injection vector) can bypass cookie prefix protections and override sensitive cookies.
This may lead to:
- Bypassing
__Secure-and__Host-prefix protections - Overriding cookies that rely on the Secure attribute
- Session fixation or session hijacking depending on application usage
This issue affects applications that rely on getCookie() for security-sensitive cookie handling.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | hono | all versions | 4.12.12npm install hono@4.12.12 |
Detection & 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 4.12.12 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39410 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-2026-39410 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-39410. 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-2026-39410 in your dependencies?
O3 Security finds CVE-2026-39410 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.