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NLnet Labs NSD: Stack buffer overflow when parsing SVCB/HTTPS records

CVE-2026-101338High 8.7

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Summary

NSD, an authoritative DNS server by NLnet Labs, has a stack buffer overflow (CWE-121) in parsing SVCB and HTTPS resource records that contain ECH parameters. An attacker who can supply a specially crafted zone file can overwrite the stack and write data beyond the buffer limit. The vendor states this can be exploited to execute arbitrary commands. Zone files written by NSD itself, including those for secondary zones, are not affected; the zone must be fed to NSD directly, for example from an operator-controlled provisioning system.

Affected

  • NSD 4.10.0 up to and including 4.15.2
  • Default configuration is not affected according to the CVE record
  • The vendor's range is wider than the one listed by VulDB (4.15.0 to 4.15.2); the vendor's range applies
  • Distribution packages: status unknown, no fixed version found; do not assume your OS package is patched

Fixed in

  • NSD 4.15.3 and all later versions
  • For NSD 4.15.2 only: vendor patch patch_CVE-2026-101338.diff (apply with patch -p1, then make install)

Workaround

  • No workaround documented by the vendor; the only stated solution is the upgrade or the patch
  • General risk reduction (not from the vendor): restrict who can supply or modify zone files and review automated provisioning pipelines that write zone content

Exploitation

  • CISA SSVC assessment: exploitation none, automatable no, technical impact total
  • No in-the-wild exploitation found; not listed in CISA KEV at the time of research
  • No public proof of concept as of publication
  • VulDB labels the issue '0-Day' with an estimated exploit price; this is a market estimate, not evidence of exploitation

Our recommendation

  • Check whether NSD is running and whether it is primary for any zone or accepts zone files from third parties or provisioning systems
  • If it is primary or accepts external zone input, upgrade to NSD 4.15.3 or later now; check distribution packages separately
  • If it is only secondary, schedule the upgrade together with the other NSD CVEs published the same day (CVE-2026-101339, CVE-2026-101341, CVE-2026-101337)
  • Restrict write access to zone files and monitor changes to them until patched

Public exploits

PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.

Research sources

Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 04:08.

Original description (NVD)

When a SVCB or HTTPS RR has filled the wire format RDATA up to the brim, another ECH parameter may overwrite the stack, and once beyond the limit, the base64 decoder can write attacker controlled data of unlimited size. Zone files which are written out by NSD itself, such as for zones for which the NSD instance is secondary, will not be formatted like this. The zone file must be fed to NSD directly, (but it could come, for example, from an operator controlled provisioning system).

NVD references (1)

Sources

The tracker combines official, freely usable data sources – updated continuously:

This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.

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