Latest / The Digital Warfare Podcast / EP. 174: Rejetto HFS CVE-2026-61500 Exploited for Admin Access and Remote Code
Transcript
- Digital Warfare: You're listening to the Digital Warfare Podcast. Today we're looking at a vulnerability where the weakness is not a missing password check. It is something deeper: predictability. The vulnerability is CVE-2026-61500. It affects Regetto HTTP file server, commonly known as HFS, and attackers are now exploiting it in real-world activity. The floor can allow an unauthenticated remote attacker to recover the cryptographic material protecting an HFS administrator session, forge a valid admin cookie, and ultimately execute code on the server. Here's how it works. HFS versions 3.0.0 through 3.2.0 generated their session cookie signing key using JavaScript's math.random function. That is the first problem. Math.random is designed for ordinary pseudo-random operations. It is not designed to generate cryptographic secrets, but the vulnerability goes further. The same underlying pseudo-random number generator also produced values exposed to unauthenticated users during the server's login process. That means an attacker does not initially need a username-password combination. They can interact with the login interface and collect a small number of generated values. Those values reveal enough information about the server's random number generator for an attacker to mathematically reconstruct its internal state. Once that internal state is known, the attacker can reproduce the sequence of values generated by the server, and from there, recover the session cookie signing key. That signing key is critical. A server uses it to determine whether a session cookie is legitimate. If an attacker knows the signing key, they no longer have to steal an administrator's existing session, they can create their own. The attacker generates a synthetic session containing administrator identity information, signs it with the recovered key, and sends it back to the server. HFS accepts the Forge cookie as legitimate. At that point, an attacker has administrator-level access without ever knowing the administrator's password, but administrative access is only part of the chain. HFS includes a legitimate configuration feature called server underscore code that allows server-side JavaScript to be configured. An attacker with forged administrative access can abuse that functionality to execute code on the underlying HFS host. That turns a predictable random number generator into unauthenticated remote code execution, and there is now evidence of real-world exploitation. Security telemetry began detecting exploitation attempts on October 1st. That means organizations should no longer treat this as a theoretical vulnerability. The affected range is straightforward. HFS versions 3.0.0 through 3.2.0 are vulnerable. The first fixed release is 3.2.1. The security change replaces the weak random number generation used for sensitive values with cryptographically secure alternatives. For defenders, the first priority is obvious. Upgrade to HFS 3.2.1 or later. But if the server was internet-facing while vulnerable, patching alone is not enough. Investigate. Review access logs for repeated unauthenticated login requests, followed by unexpected authenticated administrator activity. Inspect the configuration for unfamiliar server underscore code entries. Review newly created accounts. Check for unexpected processes. Inspect outbound connections. And look for file system changes that cannot be explained by normal administration. Organizations should also invalidate existing sessions after upgrading. And if there is evidence of compromise, Rotate credentials and secrets accessible from the HFS host. The broader lesson here is important. Security does not only depend on encryption algorithms, it depends on how the secrets behind those algorithms are created. A cryptographically strong signature provides very little protection if the signing key itself can be predicted. Randomness is security infrastructure, session identifiers, API tokens, encryption keys, password reset links, authentication challenges. All of them depend on values attackers cannot predict. CVE-2026-61500 demonstrates what happens when that assumption fails. The attacker does not break the cryptography. They predict the secret. They forge trust. And then they use legitimate administrative functionality to take control. If your organization runs Regetto HFS, verify the version now. Patch vulnerable systems. Investigate previously exposed servers. And remember, sometimes the most dangerous vulnerability is not the code that checks the credential, it is the code that creates the credential in the first place. This has been your Digital Warfare Daily Brief. Stay secure. We'll see you tomorrow.