CVE-2021-3716
Description
A flaw was found in nbdkit due to to improperly caching plaintext state across the STARTTLS encryption boundary. A MitM attacker could use this flaw to inject a plaintext NBD_OPT_STRUCTURED_REPLY before proxying everything else a client sends to the server, potentially leading the client to terminate the NBD session. The highest threat from this vulnerability is to system availability.
Statement
Red Hat Enterprise Linux 7 is not affected by this flaw as it ships an old version of nbdkit (v1.8.0) that does not include support for NBD Structured Replies.
Mitigation
In general, it is recommended to use forced TLS (--tls=require) to avoid MitM attacks in the first place. If opportunistic encryption must be used (--tls=yes), ensure that clients that want to use TLS will request structured replies after the encryption is enabled. If this cannot be done, apply network filtering (e.g., firewall, TCP wrappers, etc.) or use a Unix domain socket instead of TCP to ensure that untrusted clients cannot connect to nbdkit as a MitM attacker.
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).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 3.5 | 3.1 | N/A |
| Attack Vector | Network | Network | N/A |
| Attack Complexity | Low | High | N/A |
| Privileges Required | Low | Low | N/A |
| User Interaction | Required | None | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | None | None | N/A |
| Integrity Impact | None | None | N/A |
| Availability Impact | Low | Low | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:L
NVD: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L
Understanding the Weakness (CWE)
Integrity,Confidentiality
Technical Impact: Gain Privileges or Assume Identity
If an attackers can spoof the endpoint, the attacker gains all the privileges that were intended for the original endpoint.
Acknowledgements
This issue was discovered by Eric Blake (Red Hat).
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