Compliance Impact

The vulnerability allows authenticated users to perform SSRF attacks that can lead to cross-tenant data exposure and access to internal services or cloud metadata endpoints. Such unauthorized data exposure and potential access to sensitive internal resources could negatively impact compliance with data protection regulations like GDPR and HIPAA, which require strict controls on personal and sensitive data access and processing.

Because the vulnerability enables attackers to retrieve internal data and reflect it back to them, it increases the risk of data breaches and unauthorized data disclosure, which are critical compliance concerns under these standards.

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Executive Summary

CVE-2026-42261 is an authenticated Server-Side Request Forgery (SSRF) vulnerability in PromptHub versions 0.4.9 to before 0.5.4. It occurs in the POST /api/skills/fetch-remote endpoint, which fetches a user-supplied URL server-side and returns the response body up to 5 MB to the caller.

The vulnerability arises because the SSRF protection function (isPrivateIPv6) fails to block all private or loopback IPv6 addresses due to multiple alternate-but-valid IPv6 representations bypassing the check. This allows attackers to reach internal IPv4 addresses (including loopback, RFC1918, and link-local) via IPv4-mapped IPv6 addresses in hex form or other semantic equivalents of ::1.

Any authenticated user, including those who register on deployments with ALLOW_REGISTRATION=true, can exploit this vulnerability to make the server fetch internal resources and reflect their content back.

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Impact Analysis

This vulnerability can allow attackers to perform internal network reconnaissance by accessing internal services or private IP ranges that should be protected.

Attackers can retrieve sensitive data from internal endpoints, including cloud metadata services, which may lead to cross-tenant data exposure.

Since the server reflects up to 5 MB of response data back to the attacker, it can be used to exfiltrate information from internal systems.

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Detection Guidance

This vulnerability can be detected by monitoring for requests to the authenticated endpoint POST /api/skills/fetch-remote that include user-supplied URLs using IPv4-mapped IPv6 addresses or alternate IPv6 representations of loopback and private IP ranges.

Detection can involve inspecting logs for unusual SSRF attempts where URLs contain IPv6 formats such as ::ffff:7f00:1 (which maps to 127.0.0.1) or other semantic equivalents of ::1.

While no specific commands are provided in the resources, network administrators can use tools like curl or wget to simulate requests to the vulnerable endpoint with crafted IPv6-mapped IPv4 addresses to test if the server reflects responses.

• Example curl command to test SSRF endpoint: curl -X POST -H "Authorization: Bearer <token>" -d '{"url":"http://[::ffff:7f00:1]/"}' https://<prompthub-server>/api/skills/fetch-remote

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Mitigation Strategies

The immediate mitigation step is to upgrade PromptHub to version 0.5.4 or later, where the vulnerability has been patched.

The patch replaces the custom isPrivateIPv6 function with a canonicalization-based check using the ip-address npm package to properly block private and loopback IP addresses in all valid IPv6 representations.

Additionally, if upgrading immediately is not possible, consider restricting access to the POST /api/skills/fetch-remote endpoint to trusted users only and disabling ALLOW_REGISTRATION to prevent arbitrary internet users from registering and exploiting the SSRF.

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