CVE-2019-6477
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Insights vulnerability analysis
Description
A flaw was found in the way bind limited the number of TCP clients that can be connected at any given time. A remote attacker could use one TCP client to send a large number of DNS requests over a single connection, causing exhaustion of the pool of file descriptors available to named, and potentially affecting network connections and the management of files such as log files or zone journal files.
Statement
The patch for CVE-2018-5743 introduced a change in the way bind calculated the number of concurrent connections, from counting the outstanding TCP queries to counting the TCP client connections. However this functionality was not correctly implemented, a attacker could use a single TCP connection to send large number of DNS requests causing denial of service. As per upstream the fix does not help in a situation where a TCP-pipelining client is sending queries at an excessive rate, allowing a backlog of outstanding queries to build up. More details about this is available in the upstream advisory.
This bind flaw can be exploited by a remote attacker (AV:N) by opening large number of simultaneous TCP client connections with the server. The attacker needs to use a server which has TCP-pipelining capability to use one TCP connection to send large number of requests. (AC:L and PR:N) No user interaction is required from the server side (UI:N). The attacker can cause denial of service (A:H) by exhausting the file descriptor pool which named has access to. (S:U)
Mitigation
The vulnerability can be mitigated by disabling server TCP-pipelining:
keep-response-order { any; };
and then restarting BIND. The server restart is necessary because neither a 'reload' nor a 'reconfig' operation will properly reset currently pipelining TCP clients.Disabling TCP-pipelining entirely is completely effective at mitigating the vulnerability with minimal impact to clients that use pipelined TCP connections and with no impact to clients that do not support TCP-pipelining. The majority of Internet client DNS queries are transported over UDP or TCP without use of TCP-pipelining.
Note: This mitigation will only work with bind-9.11 and above.
Additional information
- Bugzilla 1773617: bind: TCP Pipelining doesn't limit TCP clients on a single connection
- CWE-400: Uncontrolled Resource Consumption
- FAQ: Frequently asked questions about CVE-2019-6477
Common Vulnerability Scoring System (CVSS) Score Details
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).
| Red Hat | NVD | |
|---|---|---|
CVSS v3 Base Score | 7.5 | 7.5 |
Attack Vector | Network | Network |
Attack Complexity | Low | Low |
Privileges Required | None | None |
User Interaction | None | None |
Scope | Unchanged | Unchanged |
Confidentiality Impact | None | None |
Integrity Impact | None | None |
Availability Impact | High | High |
CVSS v3 Vector
Red Hat: CVSS:3.0/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
Acknowledgements
Red Hat would like to thank ISC for reporting this issue.
Frequently Asked Questions
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