CVE-2026-27700: 
JavaScript vulnerability analysis and mitigation

CVE-2026-27700 is an authentication bypass via IP spoofing vulnerability in the Hono web application framework's AWS Lambda adapter (hono/aws-lambda). When deployed behind an AWS Application Load Balancer (ALB), the getConnInfo() function incorrectly reads the first (attacker-controlled) value from the X-Forwarded-For header instead of the last value appended by the ALB, allowing IP-based access controls to be bypassed. The vulnerability affects Hono versions 4.12.0 and 4.12.1 (npm package); API Gateway and Lambda Function URL deployments are not affected. It was disclosed on February 23, 2026, and carries a CVSS v3.1 base score of 8.2 (High) (Github Advisory, Hono Security Advisory).

The root cause is classified under CWE-290 (Authentication Bypass by Spoofing) and CWE-345 (Insufficient Verification of Data Authenticity). The vulnerable code in src/adapter/aws-lambda/conninfo.ts extracted the leftmost IP from the X-Forwarded-For header using xff.split(',')[0].trim(). Because AWS ALB appends the real client IP to the end of the header, any value at the front is fully attacker-controlled — an attacker can prepend an arbitrary trusted IP (e.g., X-Forwarded-For: 127.0.0.1) and the ALB will forward it as X-Forwarded-For: 127.0.0.1, <real-client-ip>. The fix changes the selection to ips[ips.length - 1].trim(), trusting only the ALB-appended last value. Applications using ipRestriction(getConnInfo, { allowList: [...] }) or any custom middleware relying on getConnInfo(c).remote.address for authorization are directly impacted (Hono Security Advisory, Patch Commit).

Successful exploitation allows an unauthenticated remote attacker to bypass IP-based access control mechanisms, potentially gaining access to resources intended to be restricted by source IP address. The primary impact is high confidentiality loss (unauthorized access to protected data or endpoints) and low integrity impact, with no availability impact. Only applications deployed on AWS Lambda behind an ALB that rely on getConnInfo() for IP-based authorization decisions are affected; API Gateway (v1/v2) and Lambda Function URL deployments are not vulnerable (Github Advisory).

No public proof-of-concept exploit code has been identified, and there is no evidence of in-the-wild exploitation at this time (Github Advisory). The EPSS score is approximately 0.008% (1st percentile), indicating a low near-term exploitation probability. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. No threat actor attribution has been reported. The attack requires no privileges or user interaction and is network-accessible, making it trivially exploitable against any vulnerable deployment once identified.

1. Reconnaissance: Identify internet-facing AWS Lambda applications using the Hono framework (versions 4.12.0–4.12.1) deployed behind an AWS ALB. This can be done via HTTP response headers (e.g., x-powered-by: Hono) or by inspecting npm dependency manifests if accessible.
2. Identify IP-restricted endpoints: Probe the application for endpoints that return 403 or similar access-denied responses, indicating IP-based restrictions enforced via ipRestriction middleware or custom getConnInfo() logic.
3. Craft spoofed request: Construct an HTTP request with a forged X-Forwarded-For header containing a trusted/allowlisted IP address as the first value, e.g.:

   GET /restricted-resource HTTP/1.1
   Host: target.example.com
   X-Forwarded-For: 127.0.0.1

4. Send request through ALB: Submit the crafted request to the ALB endpoint. The ALB appends the real client IP to the end, resulting in X-Forwarded-For: 127.0.0.1, <attacker-real-ip>. The vulnerable getConnInfo() reads only the first value (127.0.0.1), treating it as the client IP.
5. Bypass access control: The application's IP restriction middleware evaluates the spoofed IP against the allowlist, grants access, and the attacker reaches the protected resource (Hono Security Advisory, Patch Commit).

• Network: HTTP requests to IP-restricted endpoints originating from unexpected source IPs that successfully receive 200 responses; requests containing a multi-value X-Forwarded-For header where the first value is a known internal, loopback, or allowlisted IP (e.g., 127.0.0.1, 10.0.0.0/8, 192.168.0.0/16) followed by an external IP.
• Logs: ALB access logs showing requests with X-Forwarded-For headers containing multiple comma-separated IPs where the first value matches an allowlisted range but the last value is an external/unknown IP; application logs showing successful authorization for requests that should have been blocked based on the true client IP.
• Application Behavior: Unexpected access to resources protected by ipRestriction middleware from IP addresses not in the configured allowlist, particularly when correlated with unusual X-Forwarded-For header patterns in ALB logs.

The primary remediation is to upgrade the Hono npm package to version 4.12.2 or later, which corrects getConnInfo() to select the last IP from the X-Forwarded-For header (the one appended by ALB) (Hono Release v4.12.2, Patch Commit). For organizations unable to patch immediately, implement additional authentication mechanisms beyond IP-based restrictions — such as API keys, OAuth tokens, or AWS IAM authorization — that do not rely solely on IP validation. Alternatively, consider migrating to API Gateway (v1/v2) or Lambda Function URLs, which are not affected as they use AWS-provided source IP values from requestContext rather than the X-Forwarded-For header (Github Advisory).

The vulnerability was reported by security researcher EdamAme-x and disclosed by Hono maintainer yusukebe on February 23, 2026, with a patch released the same day (Hono Security Advisory). The release received positive community engagement on GitHub, with 14 reactions on the v4.12.2 release noting the security fix. No significant broader media coverage or notable researcher commentary beyond the advisory has been identified.