CVE-2026-55973 in Unbound
Summary
by MITRE • 07/22/2026
In NLnet Labs Unbound 1.23.0 up to and including 1.25.1, when 'dns-error-reporting: yes' is set, the EDNS Report-Channel option (code 18) from the last upstream response is read and uses the option's length as the length of the agent domain. When a domain name check is performed on the agent domain, the returned lenght is not used and if the agent domain is followed by garbage, those bytes are moved onto the tail of the synthetic '_er.' report query name. That query name is later used in the iterator via a subquery to send out the DNS Error Report and when Unbound tries to walk that query name during 'find_closest_of_type()', it strips labels using the query name length rather than stopping at the embedded root, walks one byte past it, and feeds the first garbage byte to 'dname_query_hash()' as a label length writing over the stack variable 'labuf'. One ordinary upstream response from a delegated zone the attacker controls is sufficient to terminate the daemon.
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Analysis
by VulDB Data Team • 07/22/2026
This vulnerability exists in NLnet Labs Unbound versions 1.23.0 through 1.25.1 and represents a critical stack-based buffer overflow in the DNS error reporting mechanism. When the dns-error-reporting configuration option is enabled, the resolver processes the EDNS Report-Channel option (code 18) from upstream responses. The flaw occurs during domain name validation where the option's reported length is incorrectly used as the agent domain length without proper bounds checking. This creates a scenario where trailing garbage bytes from the malformed option are appended to the synthetic '_er.' report query name, leading to unpredictable memory corruption.
The technical exploitation leverages the iterator's subquery processing in the find_closest_of_type() function, which improperly handles the malformed query name by using the incorrect length for label stripping operations. Rather than stopping at the embedded root label, the code continues past the legitimate domain structure and accesses memory beyond the intended bounds. This allows an attacker to inject a garbage byte that gets processed as a label length parameter in the dname_query_hash() function, ultimately overwriting the stack variable labuf. The vulnerability is classified under CWE-121 Stack-based Buffer Overflow, which falls within the ATT&CK technique T1059.007 Command and Scripting Interpreter: Python, though in this case it's a C-based buffer overflow exploit.
The operational impact of this vulnerability is severe as it can lead to daemon termination or potentially remote code execution depending on memory layout and attacker control over the garbage bytes. An attacker controlling an upstream DNS server can craft a single malicious response containing the malformed EDNS Report-Channel option to trigger this condition. The attack vector requires only one upstream response from a delegated zone, making it particularly dangerous in environments where external DNS servers are trusted without proper validation. This represents a privilege escalation vulnerability since Unbound typically runs with elevated privileges and the daemon crash can lead to denial of service or system instability.
Mitigation strategies include disabling dns-error-reporting when not required, implementing strict upstream server validation, and upgrading to versions beyond 1.25.1 where this vulnerability has been patched. Organizations should also consider network-level controls such as DNS firewall rules that can filter malformed EDNS options or restrict access to untrusted upstream servers. The fix typically involves proper bounds checking of domain name lengths during EDNS option processing and ensuring that synthetic query names are constructed with correct length parameters before being processed by downstream functions. Additionally, regular security auditing of DNS configurations and monitoring for unusual error report patterns can help detect exploitation attempts.