CVE-2019-11755

What it actually does

The flaw bypasses S/MIME signature validation by accepting an outer SignedData layer around an inner encrypted message, allowing a signature to appear valid even when the signer lacked access to the plaintext or had stripped a prior signature.

Why it matters to the business

Users may be tricked into trusting forged or altered encrypted email content, increasing risk of successful social engineering or data tampering in email workflows; the issue affects an extremely common mail client with high ecosystem prevalence.

How an attack unfolds

An attacker constructs an S/MIME message containing an inner encrypted layer wrapped by an outer SignedData layer that was never authorized for the encrypted content, then delivers the message so that the vulnerable client incorrectly renders the signature as valid.

How to fix it

Upgrade Mozilla Thunderbird to version 68.1.1 or later; no other mitigations are documented.

How to confirm the fix worked

Confirm installed Thunderbird version is 68.1.1 or newer; test signature display behavior against a crafted S/MIME sample that uses nested encryption and outer SignedData.

Typical attack path

1. Craft S/MIME message with inner encryption and unauthorized outer SignedData
2. Deliver the message via email to the target mailbox
3. Victim opens the message in vulnerable Thunderbird
4. Client incorrectly displays the signature as valid

How to detect it

Review mail server and client logs for inbound S/MIME messages containing nested encryption layers; correlate with user reports of unexpected valid signatures on encrypted mail.

What your security team should do

• Immediately upgrade all Thunderbird instances to 68.1.1 or later
• Audit email workflows that rely on S/MIME signatures for integrity
• Educate users to treat S/MIME signature indicators with caution until patches are applied