Freshly discovered bug in OpenPGP.js undermines whole point of encrypted comms

Security researchers are sounding the alarm over a fresh flaw in the JavaScript implementation of OpenPGP (OpenPGP.js) that allows both signed and encrypted messages to be spoofed.

Discovered by Codean Labs' Edoardo Geraci and Thomas Rinsma, the vulnerability essentially undermines the core purpose of using public key cryptography to secure communications.

Tracked as CVE-2025-47934 (8.7 - high), the vulnerability stems from the openpgp.verify and openpgp.decrypt functions. The advisory posted to the library's GitHub repo states that a maliciously modified message can be passed to one of these functions and return a result indicating a valid signature without actually being signed.

The researchers said a full write-up of the vulnerability, complete with a proof of concept (PoC) exploit, is "coming soon." It's common practice to delay disclosing PoCs to allow users time to patch affected products.

The affected versions are 5.0.1 to 5.11.2 and 6.0.0-alpha.0 to 6.1.0. Users are advised to upgrade to either 5.11.3 or 6.1.1 as soon as possible to fix the problem. Versions 4.x aren't affected.

There is no PoC just yet, but the advisory offers up some details about how the attack, which affects both signed (inline) messages and signed-and-encrypted messages, could play out.

The underlying issue is that OpenPGP.js trusts the signing process without properly verifying it. According to the advisory: "In order to spoof a message, the attacker needs a single valid message signature (inline or detached) as well as the plaintext data that was legitimately signed, and can then construct an inline-signed message or signed-and-encrypted message with any data of the attacker's choice, which will appear as legitimately signed by affected versions of OpenPGP.js."

It means any signed message can be spoofed, allowing an attacker to substitute their own data, while the message appears legitimately signed.

For signed-and-encrypted messages, an attacker would need first to acquire a valid signature and encrypt a message using that signature.

Daniel Huigens, cryptography team lead at Proton and head maintainer of OpenPGP.js, said in the advisory that until they can upgrade, users should scrutinize any ostensibly signed message they receive and verify each signature as a detached one.

For signed-and-encrypted messages, Huigens suggested verifying their legitimacy in two steps. First, call openpgp.decrypt without verificationKeys, and then pass the returned signatures and a new message containing decrypted data to openpgp.verify.

The most notable user of OpenPGP is encrypted email provider Proton Mail. The team behind it maintains the library, and the technology is used to offer end-to-end encryption for its users.

As of 2023, Proton Mail had more than 100 million accounts registered. It's not known how many of these accounts are active, but the figure offers some sense of how many people rely on OpenPGP every day.

Various other email services support the OpenPGP standard either natively or with a little extra software tacked on.

Microsoft Outlook supports it, for example, provided users install an add-in such as gpg4o or Gpg4win, although Outlook has its own encryption capabilities via S/MIME or Microsoft Purview Message Encryption.

Many others, most of them open source and a little niche, however, support the standard straight out of the box. ®