Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC
Summary
| CVE | CVE-2026-85501 |
|---|---|
| State | PUBLISHED |
| Assigner | NLnet Labs |
| Source Priority | CVE Program / NVD first with legacy fallback |
| Published | 2026-09-16 09:17:06 UTC |
| Updated | 2026-09-23 19:10:46 UTC |
| Description | Novel vulnerabilities to launch algorithmic complexity attacks on DNSSEC have been researched under the term 'ReTrap'. These result in degradation of service when malicious zones are used to serve the algorithmic complexity vulnerabilities. NLnet Labs Unbound up to and including 1.26.0 is vulnerable to some of them. TagTrap, where the triple(Zone, Algo, KeyTag) matching mechanism introduces a significant attack vector when resolvers handle malicious responses containing numerous mismatched DNSKEY, RRSIG, and DS record. DelegationTrap, where constructing the chain-of-trust requires iterative validation of DNSKEY and DS records from the root zone downward. For deeply nested domains, this results in significant computational overhead. NsecTrap, where responses with excessive invalid NSEC records compel the resolver to validate each one. AdditionalTrap, where Unbound by default would try to DNSSEC validate the ADDITIONAL section as well. This can be exploited to waste validation resources by malicious users. |
Risk And Classification
Primary CVSS: v3.1 7.5 HIGH from [email protected]
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS: 0.004820000 probability, percentile 0.388880000 (date 2026-09-24)
Problem Types: CWE-770 | CWE-770 CWE-770: Allocation of Resources Without Limits or Throttling
| Version | Source | Type | Score | Severity | Vector |
|---|---|---|---|---|---|
| 3.1 | [email protected] | Primary | 7.5 | HIGH | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
| 3.1 | [email protected] | Secondary | 5.3 | MEDIUM | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L |
| 3.1 | CNA | CVSS | 5.3 | MEDIUM | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L |
CVSS v3.1 Breakdown
Attack Vector
NetworkAttack Complexity
LowPrivileges Required
NoneUser Interaction
NoneScope
UnchangedConfidentiality
NoneIntegrity
NoneAvailability
HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD Known Affected Configurations (CPE 2.3)
Vendor Declared Affected Products
| Source | Vendor | Product | Version | Platforms |
|---|---|---|---|---|
| CNA | NLnet Labs | Unbound | affected 1.26.1 semver | Not specified |
References
| Reference | Source | Link | Tags |
|---|---|---|---|
| www.nlnetlabs.nl/downloads/unbound/CVE-2026-85501.txt | [email protected] | www.nlnetlabs.nl | Patch, Vendor Advisory |
| CVE Program record | CVE.ORG | www.cve.org | canonical |
| NVD vulnerability detail | NVD | nvd.nist.gov | canonical, analysis |
Vendor Comments And Credit
Discovery Credit
CNA: Zuyao Xu (Nankai University, AOSP Lab) (en)
CNA: Xiang Li (Nankai University, AOSP Lab) (en)
CNA: Qifan Zhang (Palo Alto Networks) (en)
Additional Advisory Data
| Source | Time | Event |
|---|---|---|
| CNA | 2025-05-24T00:00:00.000Z | Issue reported by Zuyao Xu |
| CNA | 2025-05-28T00:00:00.000Z | NLnet Labs shares some concerns about the testing methodology |
| CNA | 2025-08-14T00:00:00.000Z | Zuyao Xu shares updated research results |
| CNA | 2025-08-26T00:00:00.000Z | NLnet Labs shares patch with Zuyao Xu |
| CNA | 2025-09-03T00:00:00.000Z | Zuyao Xu verifies patch |
| CNA | 2026-05-05T00:00:00.000Z | Issue similar to TagTrap reported by Qifan Zhang |
| CNA | 2026-05-07T00:00:00.000Z | NLnet Labs shares patch with Qifan Zhang |
| CNA | 2026-05-08T00:00:00.000Z | Qifan Zhang verifies patch |
| CNA | 2026-09-16T00:00:00.000Z | Fixes released with version 1.26.1 |
Solutions
CNA: This issue is fixed starting with version 1.26.1
There are currently no legacy QID mappings associated with this CVE.