## https://sploitus.com/exploit?id=1875515F-1163-510B-A697-82A204A481CB
# CVE-2026-5950 - BIND 9 Resolver DoS
Research notes and defensive guidance for CVE-2026-5950, an unbounded resend loop vulnerability in the BIND 9 resolver state machine.
## Overview
CVE-2026-5950 affects recursive BIND 9 resolvers. Under specific retry and bad-server handling conditions, a remote unauthenticated attacker may trigger repeated resend behavior that can cause severe resource exhaustion.
This repository is intentionally defensive. It does not publish exploit payloads or operational attack steps.
## Key Facts
| Field | Value |
| --- | --- |
| CVE | CVE-2026-5950 |
| Product | ISC BIND 9 |
| Component | Resolver |
| Weakness | CWE-606: Unchecked Input for Loop Condition |
| Severity | Medium |
| CVSS v3.1 | 5.3 |
| Vector | `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L` |
| Impact | Availability / resource exhaustion |
| Exploitable | Remotely |
| Authentication | Not required |
## Affected Versions
According to the ISC advisory, the affected ranges are:
| Branch | Affected versions | Fixed version |
| --- | --- | --- |
| BIND 9.18 | `9.18.36` through `9.18.48` | `9.18.49` |
| BIND 9.20 | `9.20.8` through `9.20.22` | `9.20.23` |
| BIND 9.21 | `9.21.7` through `9.21.21` | `9.21.22` |
| BIND 9.18-S | `9.18.36-S1` through `9.18.48-S1` | `9.18.49-S1` |
| BIND 9.20-S | `9.20.9-S1` through `9.20.22-S1` | `9.20.23-S1` |
## Impact
The vulnerability can affect recursive resolver availability through resource exhaustion. ISC notes that resolvers are affected, while authoritative services are believed to be unaffected by this specific issue.
## Defensive Checks
Use the helper script to classify a local BIND version:
```bash
python3 scripts/check_bind_version.py 9.20.22
python3 scripts/check_bind_version.py 9.20.23
python3 scripts/check_bind_version.py 9.18.48-S1
```
Expected output:
```text
9.20.22: affected - upgrade to 9.20.23
9.20.23: not affected by the listed vulnerable range
9.18.48-S1: affected - upgrade to 9.18.49-S1
```
## Recommended Remediation
Upgrade to the fixed release closest to the currently deployed branch:
- `9.18.49`
- `9.20.23`
- `9.21.22`
- `9.18.49-S1`
- `9.20.23-S1`
ISC states that there are no known workarounds in the public advisory.
## Repository Contents
- [docs/advisory.md](docs/advisory.md) - structured advisory notes.
- [docs/detection.md](docs/detection.md) - defensive version and environment checks.
- [scripts/check_bind_version.py](scripts/check_bind_version.py) - safe local version classifier.
## Researcher Credit
The ISC advisory acknowledges Billy Baraja (BielraX) for bringing this vulnerability to ISC's attention.
## References
- [ISC advisory: CVE-2026-5950](https://kb.isc.org/docs/cve-2026-5950)
- [NVD: CVE-2026-5950](https://nvd.nist.gov/vuln/detail/CVE-2026-5950)
- [CVE record: CVE-2026-5950](https://vulners.com/cve/CVE-2026-5950)