CVE-2026-54287
hono: AWS Lambda adapter merges multiple `Set-Cookie` headers into one value, dropping cookies on ALB single-header and Lattice
Summary
### 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.
Affected packages
| Ecosystem | Package | Affected versions | Fixed versions |
|---|---|---|---|
| npm | hono | — | 4.12.25 |
Remediation: Upgrade to 4.12.25 or later.
References
Includes data from the GitHub Advisory Database, licensed under CC-BY 4.0.
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EPSS scores provided by the FIRST.org Exploit Prediction Scoring System.