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CVE-2026-61704 details a DNS Rebinding Server-Side Request Forgery (SSRF) bypass in the `link-preview-js` library, affecting versions prior to 4.0.4. This vulnerability arises from an incomplete fix for a previously reported SSRF issue, CVE-2026-43897, which was thought to be resolved in version 4.0.1. However, the implemented mitigation, which uses the `resolveDNSHost` function, has a fundamental flaw.
The root cause is a race condition in the DNS resolution and validation process. The `resolveDNSHost` function validates a single resolved IP address against a blocklist of private or loopback ranges. If this validation passes, the library proceeds to make the final HTTP `fetch()` request. Critically, this subsequent request is made against the original, user-supplied hostname instead of the IP address that was just validated. The connection is not “pinned” to the validated IP.
This architectural mismatch allows an attacker to bypass the protection. By controlling a DNS server, an attacker can configure a domain to resolve to a benign public IP address during the initial validation phase. Then, for the subsequent connection, the same domain resolves to a loopback (e.g., 127.0.0.1) or an internal IP address. This technique is known as DNS rebinding. It tricks the application into connecting to internal network resources, successfully bypassing the SSRF protection. This flaw also affects how the library handles HTTP redirects.
The impact is severe, as it allows a remote, unauthenticated attacker to force the server to make requests to internal services. This could include sensitive endpoints like cloud metadata APIs, internal administrative dashboards, or other backend services not exposed to the public internet. Exploitation could lead to the exfiltration of sensitive internal data.
DailyCVE Form:
Platform: link-preview-js
Version: < 4.0.4
Vulnerability: DNS Rebinding SSRF
Severity: High (CVSS 7.5)
date: 2026-09-03
Prediction: Already Patched (4.0.4)
What Undercode Say:
Analytics show that while the original CVE-2026-43897 was patched in 4.0.1, the fix was incomplete. The `resolveDNSHost` mitigation, intended to prevent the attack, was itself flawed. This is a classic case of a security control failing due to a mismatch between validation and execution contexts. The library validates an IP but fails to enforce that the connection uses that validated IP. Attackers are actively probing for this weakness, as evidenced by detected exploitation attempts.
Exploit: (Educational Purposes!)
A Proof-of-Concept (PoC) to demonstrate this vulnerability in a controlled, local environment would involve the following steps:
1. Setup a Malicious DNS Server: Configure a DNS server you control to resolve a specific domain (e.g., malicious.attacker.com) to different IP addresses based on the source or timing.
2. Configure the Target: Point the `link-preview-js` library to use your malicious DNS server.
3. Initial Validation: The `resolveDNSHost` function will query your DNS server for malicious.attacker.com. Your server responds with a safe, public IP address (e.g., 1.1.1.1). This passes the initial SSRF check.
4. Final Request: The library proceeds to make the `fetch()` request. However, it uses the original hostname malicious.attacker.com. Your DNS server now responds with an internal IP address (e.g., `127.0.0.1` or `169.254.169.254` for a cloud metadata service).
The following conceptual bash commands illustrate the DNS rebinding technique (not an actual exploit code for the library):
Simulate initial validation request (returns public IP) dig malicious.attacker.com @<attacker-dns> +short Output: 1.1.1.1 Simulate the final fetch request (returns internal IP) The library uses the same domain, but the DNS response changes. dig malicious.attacker.com @<attacker-dns> +short Output: 127.0.0.1
Protection:
The primary protection against this specific CVE is to upgrade to `link-preview-js` version 4.0.4 or later, which fixes the DNS rebinding vulnerability. As a general best practice for SSRF prevention, applications should implement network-layer hardening to restrict outbound traffic. This includes:
Implementing firewall rules to block egress traffic to private IP ranges (e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 127.0.0.0/8).
Avoiding reliance on DNS-based blocklists alone, as they can be bypassed.
Considering the use of a dedicated HTTP client that can be configured to connect only to explicitly allowed IP addresses.
Impact:
Successful exploitation of this vulnerability allows an attacker to pivot from the public internet into an organization’s internal network. The server running the vulnerable library becomes a proxy for accessing internal resources. This can lead to:
Data Exfiltration: Accessing sensitive data from internal services like cloud metadata endpoints (e.g., AWS IMDS), configuration servers, or databases.
Network Reconnaissance: Mapping internal network infrastructure by scanning for open ports and services.
Lateral Movement: Using the compromised server as a foothold to attack other internal systems.
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Sources:
Reported By: github.com
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