A critical vulnerability, "CVE-2026-49859," has been reported in Deno, the JavaScript/TypeScript runtime, which allows bypassing network access restrictions. If exploited, an attacker-controlled script could gain unauthorized access to internal network resources that should otherwise be restricted, potentially leading to data theft or serving as a foothold for further attacks. All developers and operators using Deno must immediately assess the impact and implement the recommended countermeasures.
Vulnerability Overview and Scope of Impact
CVE-2026-49859 is a vulnerability present in Deno versions prior to 2.8.1, stemming from insufficient validation of network restrictions. When Deno's `fetch()` function is called, the destination hostname is checked against the `--deny-net` rule, but the IP address to which that hostname resolves was not re-verified.
Due to this flaw, attackers can use a cleverly crafted domain name to pass the hostname check while resolving to an IP address that should actually be denied (e.g., a private IP address on an internal network), thereby completely bypassing network restrictions. This has been described as a fundamental defect in Deno's network security implementation.
Specific Impacts and Attack Scenarios
If this vulnerability is exploited, the following severe impacts are possible:
1. **Internal Network Reconnaissance**: Attackers could exploit Deno applications to scan internal network IP addresses, identifying available services or vulnerable targets.
2. **Sensitive Data Exfiltration**: Confidential information could be transmitted externally via Deno applications from systems that should ideally be isolated from external access, such as internal databases or API endpoints.
3. **Foothold for Further Attacks**: After establishing access to internal systems, there's a risk of the vulnerability serving as a foothold for more advanced cyberattacks (e.g., privilege escalation, malware deployment).
This mechanism is particularly concerning because attackers can bypass security controls without requiring additional privileges or complex exploitation techniques.
Actions Engineers Should Take Immediately
The most direct countermeasure for this vulnerability is to update Deno to version 2.8.1 or later. The developers have implemented an IP address re-validation mechanism in this version, fixing the vulnerability.
Additionally, if your Deno application is exposed to the public or accepts requests from external sources, it is strongly recommended to consider additional defense measures at the web server level. Below is an example Nginx configuration to block access to private IP addresses. This can prevent unintended requests from your Deno application to internal networks from being routed by the server.
```nginx
# プライベートIPアドレス範囲へのアクセスを拒否するNginx設定例
# Denoアプリケーションのアップストリーム設定内、またはlocationブロック内で使用
# IPv4プライベートアドレス範囲
set_real_ip_from 10.0.0.0/8;
set_real_ip_from 172.16.0.0/12;
set_real_ip_from 192.168.0.0/16;
# localhostをブロックする場合（Denoが同一ホスト上の内部サービスにアクセスする場合）
# set_real_ip_from 127.0.0.1/8;
# Denoアプリケーションのlocationブロック
location /your-deno-app/ {
# 以下はupstream_passの前に挿入して、リクエストが内部IPに向けられていないか確認
# $hostまたは$upstream_addrがプライベートIPに解決される可能性を考慮
# Deno自体が不正なリクエストを生成した場合に有効
# ここでは例として、$remote_addrがプライベートIPだった場合に拒否する
# 実際には、Denoからのリクエスト先IPを検証する必要があるため、より高度なWAFルールやOSレベルのファイアウォールが推奨されます。
# このNginx設定は、Denoアプリケーションがプロキシとして動作し、外部からのリクエストを内部IPに転送するようなシナリオで役立ちます。
# if ($remote_addr ~ "^(10\.|172\.(1[6-9]|2[0-9]|3[0-1])\.|192\.168\.|127\.)") {
# return 403; # Private IPからのアクセスを拒否
# }
# もしDenoがリバースプロキシ配下で動作し、バックエンドへのリクエストを制御したい場合
# 通常はDenoのバージョンアップが最優先ですが、多層防御として検討
proxy_pass http://your_deno_backend_upstream;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}
# WAF (Web Application Firewall) の導入も強力な対策です。
# OWASP ModSecurity Core Rule Set (CRS) のようなWAFルールセットは、
# SSRF (Server-Side Request Forgery) などのネットワーク制限回避攻撃を検出・ブロックできます。
```Furthermore, please review and implement the following security best practices:
1. **Principle of Least Privilege for Deno Applications**: Restrict the execution environment's permissions to the minimum necessary, preventing the Deno process from accessing unnecessary network resources or file systems.
2. **Network Segmentation**: Isolate networks where critical internal systems operate from where Deno applications run to prevent lateral movement of attacks.
3. **Regular Vulnerability Scanning**: Conduct regular vulnerability scans for all software in use (including OS, middleware, and libraries) to detect and address known vulnerabilities early.
4. **WAF Implementation**: Implement a Web Application Firewall (WAF) to add a layer that automatically detects and blocks malicious request patterns and suspicious communications.
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