CVE-2021-25218
Description
A flaw was found in bind. An assertion failure is triggered, resulting in termination of the named server process, if named attempts to respond over UDP with a response that is larger than the current effective interface maximum transmission unit (MTU), and if response-rate limiting (RRL) is active. The highest threat from this vulnerability is to system availability.
Statement
An attacker may abuse the Path MTU discovery (PMTUD) protocol to trick bind into exceeding the interface MTU. Response Rate Limiting (RRL) is not enabled by default for user defined views nor the builtin one, but it is enabled by default for the default builtin CHAOS class view, which bind uses to provide various information.
This issue did not affect the versions of bind as shipped with Red Hat Enterprise Linux 6, 7, and 8 as they provide an older version of the code which does not allow to trigger the assertion.
Mitigation
Disabling RRL in all views, including the builtin CHAOS class view, prevents the faulty assertion from being reached in the vulnerable versions of bind. To do that you can remove `rate-limit` from your named.conf files and provide a replacement for the builtin CHAOS view, like the one below:
view override_bind chaos {
recursion no;
notify no;
allow-new-zones no;
max-cache-size 2M;
zone "version.bind" chaos {
type primary;
database "_builtin version";
};
zone "hostname.bind" chaos {
type primary;
database "_builtin hostname";
};
zone "authors.bind" chaos {
type primary;
database "_builtin authors";
};
zone "id.server" chaos {
type primary;
database "_builtin id";
};
};
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 | 7.5 | 7.5 | N/A |
| Attack Vector | Network | Network | N/A |
| Attack Complexity | Low | Low | N/A |
| Privileges Required | None | None | N/A |
| User Interaction | None | None | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | None | None | N/A |
| Integrity Impact | None | None | N/A |
| Availability Impact | High | High | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart
An attacker that can trigger an assert statement can still lead to a denial of service if the relevant code can be triggered by an attacker, and if the scope of the assert() extends beyond the attacker's own session.
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