CVE-2026-54287: Hono HTTP Cookie Header Corruption in AWS Lambda Deployments

Technical summary

The vulnerability stems from improper HTTP header normalization in Hono's AWS Lambda integration layer. When ALB or VPC Lattice v2 present multiple Set-Cookie headers to Hono, the framework joins them with commas rather than preserving them as discrete headers. HTTP Set-Cookie headers must never be combined this way because comma is a legitimate delimiter within cookie attribute values (notably in RFC 6265 Expires dates, which use the format 'Wdy, DD Mon YYYY HH:MM:SS GMT'). A client receiving 'Set-Cookie: sessionid=abc; Expires=Mon, 22 Jun 2026 10:00:00 GMT, sessionid2=def' cannot determine where one cookie ends and another begins. The root cause is CWE-116 (Improper Encoding or Escaping of Output), where the joining logic fails to account for cookie syntax. This affects only AWS Lambda deployments using ALB or VPC Lattice v2 frontends.

Business impact

Applications relying on Hono for session management, authentication tokens, or security-related cookies face degraded functionality. Users may experience unexpected logouts, lost preferences, or authentication bypasses if critical cookies are silently dropped by clients. For regulated industries (finance, healthcare, e-commerce), cookie loss can create audit and compliance issues. The impact is particularly acute in multi-tenant or microservices architectures where cookie-based state is essential. Any business dependency on secure session handling via Hono on AWS Lambda warrants immediate assessment.

Affected systems

The vulnerability impacts Hono versions prior to 4.12.25 when deployed as AWS Lambda functions fronted by Application Load Balancer (ALB) or VPC Lattice v2. Other Hono deployment targets (Node.js servers, Cloudflare Workers, Deno, etc.) are not affected because they do not perform the problematic header joining. On-premises deployments using ALB or Lattice v2 are out of scope unless they also invoke Hono in a Lambda context.

Exploitability

Exploitation does not require active attacker intervention—the vulnerability manifests passively during normal operation. Any client receiving a Set-Cookie response from an affected Hono/Lambda/ALB or Lattice v2 configuration will experience cookie mishandling. An attacker cannot directly craft a malicious request to trigger additional damage beyond the inherent header mangling, but the silent cookie loss creates an opening for session hijacking or authentication bypass if the dropped cookies contained security tokens. The low attack complexity and lack of authentication requirements mean widespread exposure in production environments, though the attack surface is limited to organizations using this specific stack.

Remediation

Upgrade Hono to version 4.12.25 or later immediately. The patch corrects the header normalization logic to preserve Set-Cookie headers as discrete values instead of joining them. After patching, redeploy Lambda functions and perform smoke testing of authentication and session flows to confirm cookies are now properly transmitted. Verify client-side cookie handling by checking browser developer tools or application logs for Set-Cookie headers in responses.

Patch guidance

1. Update Hono dependency in your package.json to version 4.12.25 or higher (e.g., 'hono': '^4.12.25'). 2. Run 'npm install' (or yarn/pnpm equivalent) to fetch the patched version. 3. Rebuild and redeploy your Lambda function artifact. 4. Test authentication workflows end-to-end to confirm cookies are correctly split and interpreted by clients. 5. Monitor CloudWatch logs for any unexpected session-related errors post-deployment. No configuration changes or AWS console adjustments are required; the fix is entirely within Hono's code.

Detection guidance

Monitor HTTP responses from your Lambda/ALB or Lattice v2 endpoint for malformed Set-Cookie headers. Use browser developer tools (Network tab) or tcpdump to inspect raw responses; look for multiple cookie values separated by commas within a single Set-Cookie header. Check application logs for unexpected session loss, authentication failures, or cookie-related warnings. In AWS CloudWatch, filter Lambda function logs for errors related to cookie parsing or session management. Correlate cookie-related errors with Hono version numbers to confirm you are running a vulnerable release.

Why prioritize this

This vulnerability scores MEDIUM (5.3 CVSS) because it requires a specific deployment configuration (AWS Lambda + ALB/Lattice v2) and does not directly enable confidentiality breaches. However, the integrity impact is direct: cookies are reliably corrupted or dropped, affecting session security and user experience. Organizations running Hono on this AWS stack should prioritize patching within 1–2 weeks to prevent cookie-related session hijacking or authentication bypass. The passive nature of the flaw means it is active in production right now if you are on an unpatched version.

Risk score, explained

The CVSS 3.1 score of 5.3 (MEDIUM) reflects: Network-accessible attack vector (AV:N), low attack complexity (AC:L), no privilege required (PR:N), no user interaction (UI:N), unchanged scope (S:U), no confidentiality impact (C:N), low integrity impact (I:L reflecting silent data loss), and no availability impact (A:N). The integrity rating is low rather than high because the impact is passive corruption rather than full system compromise; however, for applications heavily dependent on cookie integrity, the actual business risk may be higher than the numeric score suggests.

Frequently asked questions