CVE-2026-54287 Overview
CVE-2026-54287 affects Hono, a web application framework supporting multiple JavaScript runtimes. The vulnerability impacts deployments on AWS Lambda using the Application Load Balancer (ALB) single-header response format or the VPC Lattice v2 response format. Hono versions prior to 4.12.25 join multiple Set-Cookie headers into a single comma-separated value. Because commas appear naturally inside cookie attributes such as Expires dates, clients cannot reliably split the combined value back into discrete cookies. As a result, browsers and HTTP clients silently drop or misparse cookies, leading to broken session state and potential integrity issues. The flaw is categorized under [CWE-116] Improper Encoding or Escaping of Output.
Critical Impact
Cookies set by Hono applications on AWS Lambda may be dropped or misparsed by clients, breaking authentication state and weakening session integrity guarantees.
Affected Products
- Hono web framework versions prior to 4.12.25
- Hono deployments behind AWS Application Load Balancer using single-header response mode
- Hono deployments behind AWS VPC Lattice v2
Discovery Timeline
- 2026-06-22 - CVE-2026-54287 published to the National Vulnerability Database
- 2026-06-22 - Last updated in NVD database
Technical Details for CVE-2026-54287
Vulnerability Analysis
The vulnerability stems from how Hono serializes HTTP response headers when adapting to AWS Lambda integrations. AWS Application Load Balancer in single-header mode and VPC Lattice v2 both represent response headers as a flat map of name to single value. Hono concatenates multiple Set-Cookie headers into one value separated by commas. The HTTP specification permits header field combining with commas for most headers, but Set-Cookie is explicitly exempt because its attribute syntax already contains commas. For example, Expires=Wed, 21 Oct 2026 07:28:00 GMT includes a comma after the weekday. When two such cookies are joined, the resulting value becomes ambiguous and unparseable by RFC 6265 compliant clients.
Root Cause
The root cause is improper output encoding [CWE-116] in the AWS Lambda response adapters within Hono. The framework treats Set-Cookie like any other multi-valued header rather than emitting it through the platform-specific multi-value mechanism such as the ALB multiValueHeaders field. The serialization path does not preserve header semantics required by RFC 6265.
Attack Vector
The issue is reachable over the network without authentication or user interaction. An attacker does not directly exploit the framework. Instead, any response that issues multiple cookies, such as a session cookie alongside a CSRF token, produces a malformed Set-Cookie value. Clients drop the affected cookies silently, which can weaken anti-CSRF protections, cause authentication failures, or expose applications to logic flaws that assume cookies persist across requests. Refer to the GitHub Security Advisory GHSA-j6c9-x7qj-28xf for further technical context.
Detection Methods for CVE-2026-54287
Indicators of Compromise
- HTTP responses from Hono Lambda functions containing a single Set-Cookie header with multiple Expires= or Max-Age= attributes joined by commas.
- Application logs showing session re-establishment, unexpected logouts, or repeated CSRF token issuance for the same client.
- Browser developer tools reporting only one cookie stored when multiple were expected from a Hono endpoint.
Detection Strategies
- Inventory Lambda functions using the Hono framework and compare installed versions against 4.12.25 using package manifests such as package.json and package-lock.json.
- Inspect ALB target group configurations to identify routes that rely on single-header response mode rather than multiValueHeaders.
- Run synthetic transactions against production endpoints and validate that each expected cookie appears as a discrete Set-Cookie header in the raw HTTP response.
Monitoring Recommendations
- Capture and review HTTP response headers at the load balancer or CDN layer for malformed Set-Cookie values.
- Alert on spikes in authentication failures or session reset events tied to specific Lambda functions.
- Track Hono framework versions across CI/CD pipelines and flag deployments that pin versions earlier than 4.12.25.
How to Mitigate CVE-2026-54287
Immediate Actions Required
- Upgrade Hono to version 4.12.25 or later across all AWS Lambda deployments.
- Audit affected applications for broken authentication or CSRF flows that may have occurred while the vulnerable version was in production.
- Rotate session identifiers and CSRF tokens for any user sessions established during the exposure window.
Patch Information
The issue is fixed in Hono 4.12.25. The maintainers published remediation details in the GitHub Security Advisory GHSA-j6c9-x7qj-28xf. Update via the package manager, for example npm install hono@4.12.25 or pnpm update hono.
Workarounds
- Switch ALB integrations from single-header response mode to multiValueHeaders so cookies are emitted as separate entries.
- Avoid setting more than one cookie per response from affected Hono routes until the upgrade is applied.
- Move cookie issuance to a reverse proxy or edge function that correctly preserves discrete Set-Cookie headers.
# Configuration example
npm install hono@^4.12.25
# Verify installed version
npm ls hono
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
