CVE-2026-27700 is a high-severity (CVSS 8.2) CWE-290 vulnerability in hono. A fix is available for hono — see the affected versions and patch details below.
Hono is Vulnerable to Authentication Bypass by IP Spoofing in AWS Lambda ALB conninfo
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-2026-27700.
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 the AWS Lambda adapter (hono/aws-lambda) behind an Application Load Balancer (ALB), the getConnInfo() function incorrectly selected the first value from the X-Forwarded-For header.
Because AWS ALB appends the real client IP address to the end of the X-Forwarded-For header, the first value can be attacker-controlled.
This could allow IP-based access control mechanisms (such as the ipRestriction middleware) to be bypassed.
Details
In ALB environments, AWS appends the actual client IP address to the end of any existing X-Forwarded-For header value. However, the previous implementation of getConnInfo() extracted the leftmost IP address:
address = xff.split(',')[0].trim()
If a client sent:
X-Forwarded-For: <spoofed-ip>
ALB would forward:
X-Forwarded-For: <spoofed-ip>, <real-client-ip>
Since the implementation selected the first value, the spoofed IP address was trusted. This affected applications using:
ipRestriction(getConnInfo, { allowList: [...] })
or any custom middleware relying on getConnInfo(c).remote.address for authorization decisions.
The issue only affects deployments using the AWS Lambda adapter behind an ALB. API Gateway (v1/v2) and Lambda Function URLs are not affected, as they use AWS-provided source IP values from requestContext.
Impact
An unauthenticated remote attacker could bypass IP-based access restrictions by supplying a crafted X-Forwarded-For header. This may allow access to resources that were intended to be restricted by IP address.
Only applications deployed behind an ALB and relying on getConnInfo() for IP-based authorization are affected.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | hono | ≥ 4.12.0&&< 4.12.2 | 4.12.2npm install hono@4.12.2 |
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 4.12.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27700 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.
Frequently Asked Questions
Is CVE-2026-27700 in your dependencies?
Find it across npm, including transitive dependencies.