CVE-2017-17689
Description
From CVE.org
The S/MIME specification allows a Cipher Block Chaining (CBC) malleability-gadget attack that can indirectly lead to plaintext exfiltration, aka EFAIL.
Statement
The research paper talks about use of HTML as a back channel to create an oracle for modified encrypted emails. HTML emails which use external links like "" can cause security issues if they are honored by the MUAs. Due to flaws in MIME parsers many MUAs seem to concatenate decrypted HTML mine parts which makes it easy to plan such snippets in HTML emails. Please refer to https://lists.gnupg.org/pipermail/gnupg-users/2018-May/060315.html about how GnuPG can mitigate this flaw.
For Thunderbird, this vulnerability was known as CVE-2018-5162 and resolved in 52.8.
Mitigation
The easiest way to mitigate this vulnerability is not to use HTML emails. If you really need to use them ensure that MUA clients disable external links embedded in HTML emails. For example in thunderbird email client, Edit->Preferences->Privacy->Disable "Allow remote content in messages".
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.3 | 5.9 | N/A |
| Attack Vector | Network | Network | N/A |
| Attack Complexity | High | High | N/A |
| Privileges Required | None | None | N/A |
| User Interaction | Required | None | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | High | High | N/A |
| Integrity Impact | None | None | N/A |
| Availability Impact | None | None | N/A |
Vector
Red Hat: CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:N/A:N
NVD: CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
Understanding the Weakness (CWE)
Confidentiality
Technical Impact: Read Application Data
Frequently Asked Questions
Not sure what something means? Check out our Security Glossary.
Want to get errata notifications? Sign up here.
