VU#473698: uClibc, uClibc-ng libraries have monotonically increasing DNS transaction ID


The uClibc and uClibc-ng libraries are vulnerable to DNS cache poisoning due to the use of predicatble DNS transaction IDs when making DNS requests. This vulnerability can allow an attacker to perform DNS cache poisoning attacks against a vulnerable environment.


The uClibc and the Uclibc-ng software are lightweight C standard libraries intended for use in embedded systems and mobile devices. The uClibc library has not been updated since May of 2012. The newer uClibc-ng is the currently maintained fork of uClibc, as announced on the OpenWRT mailing list in July 2014.

Researchers at the Nozomi Networks Security Research Team discovered that all existing versions of uClibc and uClibc-ng libraries are vulnerable to DNS cache poisoning. These libraries do not employ any randomization in the DNS Transaction ID (DNS TXID) field when creating a new DNS request. This can allow an attacker to send maliciously crafted DNS packets to corrupt the DNS cache with invalid entries and redirect users to arbitrary sites. As uClibc and uClibc-ng are used in devices such as home routers and firewalls, an attacker can perform attacks against multiple users in a shared network environment that relies on DNS responses from the vulnerable device.

The DNS cache poisoning scenarios and defenses are discussed in IETF RFC5452.


The lack of DNS response validation can allow an attacker to use unsolicited DNS responses to poison the DNS cache and redirect users to malicious sites.


Develop and apply updates

If your vendor has developed a patched version of software to address this issue, apply the updates provided by your vendor. If you have a forked or customized version of uClibc or uClibc-ng, develop a patch to ensure the dns_lookup function provides adequate randomization of DNS TXID’s while making DNS requests.

Follow security best practices

Consider the following security best-practices to protect DNS infrastructure:

  • Prevent direct exposure of IoT devices and lightweight devices over the Internet to minimize attacks against a caching DNS server.
  • Provide secure DNS recursion service with features such as DNSSEC validation and the interim 0x20-bit encoding as part of enterprise DNS recursion services where applicable.
  • Implement a Secure By Default configuration suitable for your operating environment (e.g., disable caching on embedded IoT devices when an upstream caching resolver is available).


Thanks to the Nozomi Networks Security Research Team for this report

This document was written by Vijay Sarvepalli and Timur Snoke.

Vendor Information

One or more vendors are listed for this advisory. Please reference the full report for more information.

Other Information

CVE IDs: CVE-2022-30295
Date Public: 2022-05-02
Date First Published: 2022-05-09
Date Last Updated: 2022-05-09 19:40 UTC
Document Revision: 2