Low [CVE-2026-50046] Denial of Service due to freed pointer dereference in DNS-over-TLS handling
This low-severity Red Hat Linux advisory covers CVE-2026-50046 affecting Red Hat Hardened Images, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8.
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Summary
In NLnet Labs Unbound 1.15.0 up to and including 1.25.1, the TLS server name used for DNS-over-TLS (DoT) forwarded queries is tied to a struct's ('serviced_query') lifetime but also referenced by another struct ('waiting_tcp').
When the owning struct is jostled out of the mesh while the DoT TCP stream is still handshaking it frees the storage behind the referenced string and if the TLS stream then errors out, it dereferences the freed pointer. The dereference is read-only and the practical impact is a daemon crash resulting in denial of service.
A malicious actor that knows a DoT forwarding/stub Unbound's configuration could exploit the vulnerability by quering records in the appropriate zone while keeping Unbound uder pressure so that the jostle logic kicks in.
If answers for the vulnerable zone are slow, the likelihood of jostling such queries is higher, although the timing of the jostle needs to be precise. Requirements for a vulnerable Unbound is the existence of a stub/forward zone configured for DoT together with a configured '#authname' suffix on the server identification.
The connectivity to the server needs to exhibit a transient failure at the correct time in order to kick off the vulnerable error path. A use-after-free flaw was found in Unbound's DNS-over-TLS (DoT) forwarded query handling.
- 1.15.0
- 1.25.1
Official advisory · high-confidence parse· fetched 1 month ago·verify at source
- unbound-main-1.25.2-0.1.hum1
- RHSA-2026:43588
Official advisory · high-confidence parse· fetched 1 month ago·verify at source
Mitigation checklist
- To mitigate this issue, avoid configuring Unbound with a `#authname` suffix in DNS-over-TLS (DoT) forwarding or stub zones if not strictly required. If the `#authname` suffix is necessary, restrict network access to the Unbound instance to trusted clients only. This may involve configuring firewall rules to limit inbound connections to the DoT port (typically 853) from known, secure sources. After modifying Unbound's configuration, the service must be reloaded or restarted for the changes to take effect. This action may temporarily interrupt DNS resolution services.
Official advisory · high-confidence parse· fetched 1 month ago·verify at source
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