Cyber Resilience

CVE-2026-55991

Nlnetlabs Unbound 1.22.0 – 1.25.2

Published
22 July 2026
Modified
24 July 2026
Patch / advisory
View advisory
CVSS Score v3.1 5.9
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS Score 0.0024 15th percentile
Risk Priority 45 floored blend · peak EPSS

Summary

CVE-2026-55991 is a medium-severity Signed to Unsigned Conversion Error (CWE-195) vulnerability in Nlnetlabs Unbound. Its CVSS base score is 5.9 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 15th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SA-15 (Development Process, Standards, and Tools) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can trigger a libngtcp2 assertion (if compiled with assertions on) and terminate the entire Unbound process using a single DNS-over-QUIC (DoQ) connection and one normal DNS…

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query. This is caused by an erroneous error value passed to libngtcp2. When 'ngtcp2_conn_writev_stream()' returns 'NGTCP2_ERR_STREAM_DATA_BLOCKED', Unbound continues to call 'ngtcp2_ccerr_set_application_error()' with a '-1' error value. The 'int' literal '-1' is implicitly converted to the function's 'uint64_t error_code' parameter as '0xFFFFFFFFFFFFFFFF'. The follow-on 'ngtcp2_conn_write_connection_close()' serialises that value as a QUIC variable-length integer; because '2^64-1' exceeds the 62-bit varint ceiling, 'ngtcp2_put_uvarintlen()' fails 'assert(n < 4611686018427387904ULL)' and the whole resolver process aborts. A remote, unauthenticated DoQ client can trigger this deterministically with a single QUIC connection by advertising 'initial_max_stream_data_bidi_local = 1' in its transport parameters and sending one DoQ query without ever reading the stream.

CWE(s)
CWE-195

Related Threats

MITRE ATT&CK Enterprise Techniques

T1068 Exploitation for Privilege Escalation Privilege Escalation
Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
T1212 Exploitation for Credential Access Credential Access
Adversaries may exploit software vulnerabilities in an attempt to collect credentials.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2026-33278Same product: Nlnetlabs Unbound
CVE-2026-44687Same product: Nlnetlabs Unbound
CVE-2026-50046Same product: Nlnetlabs Unbound
CVE-2026-52863Same product: Nlnetlabs Unbound
CVE-2026-42534Same product: Nlnetlabs Unbound
CVE-2026-32665Same product: Nlnetlabs Unbound
CVE-2026-32792Same product: Nlnetlabs Unbound
CVE-2026-42955Same product: Nlnetlabs Unbound
CVE-2026-42944Same product: Nlnetlabs Unbound
CVE-2026-40691Same product: Nlnetlabs Unbound

Affected Assets

nlnetlabs
unbound
1.22.0 — 1.25.2

Mitigating Controls

Mitigating Controls (NIST 800-53 r5) AI

SA-11 Developer Testing and Evaluation good match

Developer testing and evaluation can discover conversion errors through static analysis, fuzzing, or targeted unit tests.

SA-15 Development Process, Standards, and Tools partial match

Requiring documented development standards and tools can enforce coding rules that prohibit or safely wrap such casts.

SA-8 Security and Privacy Engineering Principles partial match

Engineering principles can mandate safe integer handling and strong typing to avoid unsafe signed-to-unsigned casts.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices (static analysis, code review, safe-integer standards) directly prevent signed-to-unsigned conversion errors.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing in development can detect conversion-related defects before release.

A.8.25 Secure development life cycle partial match
prevents

Secure development life cycle mandates practices that can catch signed-to-unsigned conversion errors during design and coding.

A.8.26 Application security requirements partial match
prevents

Application security requirements can specify safe integer handling and type-conversion rules.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure system architecture and engineering principles include data-type safety and overflow prevention.

A.8.28 Secure coding partial match
prevents

Secure coding standards directly prohibit unsafe signed-to-unsigned casts and require defensive checks.

References