Researchers have developed code exploiting several vulnerabilities in PGP (including GPG) for email, and theorized many more which others could build upon. For users who have few—or even no—alternatives for end-to-end encryption, news of these vulnerabilities may leave many questions unanswered.
Digital security trainers, whistleblowers, journalists, activists, cryptographers, industry, and nonprofit organizations have relied on PGP for 27 years as a way to protect email communications from eavesdroppers and ensure the authenticity of messages. If you’re like us, you likely have recommended PGP as an end-to-end encrypted email solution in workshops, trainings, guides, cryptoparties, and keysigning parties. It can be hard to imagine a workflow without PGP once you’ve taken the time to learn it and incorporate it in your communications.
We’ve attempted to answer some important questions about the current state of PGP email security below.
Who is affected, and why should I care?
Since PGP is used as a communication tool, sending messages to others with unpatched clients puts your messages at risk, too. Sending PGP messages to others also increases the risk that they will turn to a vulnerable client to decrypt these messages. Until enough clients are reliably patched, sending PGP-encrypted messages can create adverse ecosystem incentives for others to decrypt them. Balancing the risks of continuing to use PGP can be tricky, and will depend heavily on your own situation and that of your contacts.
Is disabling HTML sufficient?
Turning off sending HTML email will not prevent this attack. For some published attacks, turning off viewing HTML email may protect your messages being leaked to an attacker by you. However, since PGP email is encrypted to both the sender and each recipient, it will not protect these messages from being leaked by anyone else you’ve communicated with. Additionally, turning off HTML email may not protect these messages against future attacks that are discovered which build off of the current vulnerabilities.
Turning off reading HTML email while still sending PGP-encrypted messages encourages others to read these with their own potentially vulnerable clients. This promotes an ecosystem that puts the contents of these messages (as well as any past messages that are decrypted by them) at risk.
I use software that is verified with a PGP signature. Can it be trusted?
Yes! Verifying software signed with PGP is not vulnerable to this class of attack. Package management systems enforcing signature verification (like some distributions of Linux do) are also unaffected.
What are the vulnerabilities?
There are two attacks of concern demonstrated by the researchers:
1. “Direct exfiltration” attack:
This takes advantage of the details of how mail clients choose to display HTML to the user. The attacker crafts a message that includes the old encrypted message. The new message is constructed in such a way that the mail software displays the entire decrypted message—including the captured ciphertext—as unencrypted text. Then the email client’s HTML parser immediately sends or “exfiltrates” the decrypted message to a server that the attacker controls.
2. Ciphertext modification attack:
The second attack abuses the underspecification of certain details in the OpenPGP standard to exfiltrate email contents to the attacker by modifying a previously obtained encrypted email. This second vulnerability takes advantage of the combination of OpenPGP’s lack of mandatory integrity verification combined with the HTML parsers built into mail software. Without integrity verification in the client, the attacker can modify captured ciphertexts in such a way that as soon as the mail software displays the modified message in decrypted form, the email client’s HTML parser immediately sends or “exfiltrates” the decrypted message to a server that the attacker controls. For proper security, the software should never display the plaintext form of a ciphertext if the integrity check does not check out. Since the OpenPGP standard did not specify what to do if the integrity check does not check out, some software incorrectly displays the message anyway, enabling this attack. Furthermore, this style of attack, if paired with an exfiltration channel appropriate to the context, may not be limited to the context of HTML-formatted email.
We have more detail about the specifics of the vulnerabilities and details on mitigations.
What does the paper say about my email client?
Some email clients are impacted more than others, and the teams behind those clients are actively working on mitigating the risks presented. The paper describes both direct exfiltration (table 4, page 11) and backchannels (table 5, page 20) for major email clients. Even if your client has patched current vulnerabilities, new attacks may follow.
But I use [insert email software here] and it’s not on the affected list. Should I care?
While you may not be directly affected, the other participants in your encrypted conversations may be. For this attack, it isn’t important whether the sender or any receiver of the original secret message is targeted. This is because a PGP message is encrypted to each of their keys.
Sending PGP messages to others also increases the risk that your recipients will turn to a vulnerable client to decrypt these messages. Until enough clients are reliably patched, sending PGP-encrypted messages can create adverse ecosystem incentives for others to decrypt them.
Does this mean PGP is broken?
The weaknesses in the underlying OpenPGP standard (specifically, OpenPGP’s lack of mandatory integrity verification) enable one of the attacks given in the paper. Despite its pre-existing weaknesses, OpenPGP can still be used reliably within certain constraints. When using PGP to encrypt or decrypt files at rest, or to verify software with strict signature checking, PGP still behaves according to expectation.
OpenPGP also uses underlying cryptographic primitives such as SHA-1 which are no longer considered safe and lacks the benefits of Authenticated Encryption (AE), and signatures can be trivially stripped from messages. In time, newer standards will have to be developed which address these more fundamental problems in the specification. Unfortunately, introducing fixes to introduce authenticated encryption without also rotating keys to strictly enforce usage constraints will make OpenPGP susceptible to backwards-compatibility attacks. This will have to be addressed in any future standard.
In short, OpenPGP can be trusted to a certain degree. For long-term security of sensitive communications, we suggest you migrate to another end-to-end encrypted platform.
What should I do about PGP software on my computer?
In general, keeping PGP (or GPG) on your system should be safe from the known exploits, provided that it is disconnected from email as described above. Some Linux systems depend on GPG for software verification, and PGP is still useful for manually verifying software. Uninstalling your PGP software may make your keys inaccessible and prevent you from decrypting past messages in some instances, as well.
Can my previous emails be read by an attacker?
If the PGP-encrypted contents of previous emails are sent to you in new emails using this attack and you open that email in an unpatched email client with PGP software enabled, then yes. For viewing your archive of encrypted emails, we recommend using the command line.
What if I keep getting PGP emails?
You can decrypt these emails via the command line. If you prefer not to, notify your contacts that PGP is, for the time being, no longer safe to use in email clients and decide whether the conversation can continue over another end-to-end encrypted platform, such as Signal.
Going forward, what should I look out for?
We will be following this issue closely in the coming weeks. Authors of email clients and PGP plugins are working actively to patch this vulnerability, so you should expect updates forthcoming. For the latest updates, you can follow https://sec.eff.org/blog or https://www.eff.org/issues/security.
Is there a replacement for sending end-to-end encrypted messages?
There is no secure, vetted replacement for PGP in email.
There are, however, other end-to-end secure messaging tools that provide similar levels of security: for instance, Signal. If you need to communicate securely during this period of uncertainty, we recommend you consider these alternatives.
I don’t have other end-to-end encrypted messaging options available. PGP is my only option. Can I still use it?
Unfortunately, we cannot recommend using PGP in email clients until they have been patched, both on your device and your recipient’s device. The timeline for these patches varies from client to client. We recommend disconnecting PGP from your email client until the appropriate mitigations have been released. Stay tuned to https://sec.eff.org/blog or https://www.eff.org/issues/security for more info.
I don’t want to use the command line. Surely there’s a usable alternative. Can’t you recommend something else?
It’s very difficult to assess new software configurations in such a short timeframe. Some email clients are more vulnerable to this attack than others. However, using these email clients can have the effect of putting others at risk. We suggest decrypting archived emails with the command line, and moving to another end-to-end platform for conversations, at least until we are confident that the PGP email ecosystem has been restored to its previous level of security.
I only use PGP in the command line. Am I affected?
Yes and no. As we currently understand, if you are using PGP solely for file encryption, without email, there are no known exfiltration channels to send the file contents to an attacker. However, the contents may still have been modified in transit in a way that you won’t necessarily be able to see, depending on how the implementer of the specific PGP software chose to do things. This is due to the integrity downgrade aspect of the vulnerability.
Additionally, if you are using PGP to encrypt a message sent over email and your recipient uses a vulnerable email client, your correspondences are at risk of decryption. As it’s likely that many people use an email client to access PGP-encrypted emails, it’s important to clarify with your recipients that they have also disabled PGP in their email clients, or are using an unaffected client.
If you must continue sensitive correspondences, we highly recommend switching to a vetted end-to-end encryption tool.