CVE-2026-54287: Hono: AWS Lambda adapter merges multiple `Set-Cookie` headers into one value, dropping cookies on ALB single-header and Lattice
Summary
On AWS Lambda, the ALB single-header response and the VPC Lattice v2 response join multiple Set-Cookie headers into one comma-separated value. Because commas also appear inside cookie attributes (for example Expires dates), clients cannot split the value back into individual cookies and silently drop or misparse them.
Details
Per RFC 6265, each cookie must be its own Set-Cookie header line, and commas may appear inside attribute values. Joining cookies with ", " collides with those commas, producing a value that clients cannot reliably split. Only ALB single-header mode and VPC Lattice v2 are affected; API Gateway v1/v2 and ALB with multi-value headers enabled already use an array and are unaffected.
Impact
A client may receive only one of the cookies, a malformed cookie, or none. Session, CSRF, or preference cookies can silently fail to apply, breaking sessions or forcing re-authentication. This affects applications that set multiple cookies per response and run on AWS Lambda behind an ALB in single-header mode (the default) or VPC Lattice v2.
Other sources
Hono is a Web application framework that provides support for any JavaScript runtime. Prior to 4.12.25, on AWS Lambda, the ALB single-header response and the VPC Lattice v2 response join multiple Set-Cookie headers into one comma-separated value. Because commas also appear inside cookie attributes (for example Expires dates), clients cannot split the value back into individual cookies and silently drop or misparse them. This vulnerability is fixed in 4.12.25.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/honoto a version that resolves this vulnerability.Fixed in 4.12.25 - Upgrade
Upgrade
Honoto a version that resolves this vulnerability.Fixed in 4.12.25
Event History
Frequently Asked Questions
What is the severity of CVE-2026-54287?
The severity of CVE-2026-54287 is classified as medium with a score of 5.3.
How do I fix CVE-2026-54287?
To fix CVE-2026-54287, ensure that your application correctly handles multiple Set-Cookie headers according to the expected format.
What systems are affected by CVE-2026-54287?
CVE-2026-54287 affects applications using AWS Lambda with the ALB single-header response or VPC Lattice v2.
What type of vulnerability is CVE-2026-54287?
CVE-2026-54287 is categorized under Cross-Site Request Forgery (CSRF) vulnerabilities.
What potential impact does CVE-2026-54287 have on applications?
The impact of CVE-2026-54287 can lead to clients being unable to properly parse multiple Set-Cookie headers, potentially affecting session management.