# CVE-2026-77860: 'serve-expired' can bypass Unbound 'wait-limit'

> Attackers can bypass DNSBomb protections in Unbound by exploiting a double decrement bug in the serve-expired feature, leading to resource exhaustion.

- **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 16 Sep 2026
- **Also known as:** AZL-101718, DEBIAN-CVE-2026-77860, ECHO-a935-81b8-4eae, GHSA-8483-8qxh-jcf2, UBUNTU-CVE-2026-77860

## What is CVE-2026-77860?

CVE-2026-77860 is a low-severity denial of service vulnerability in Nlnetlabs Unbound, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In NLnetLabs Unbound 1.20.0 up to and including 1.26.0, a vulnerability on the 'serve-expired' code path can cause a double decrement on the 'wait-limit' counter per client IP essentially bypassing one of the counter measures that was introduced for DNSBomb (CVE-2024-33655). A malicious actor can exploit this by controlling an authoritative zone with short TTL, so cached entries expire quickly. Each 'slow' query, one the attacker's authoritative never answers, is followed by one query for an expired cached name, which is answered immediately via the 'serve-expired' path and decrements the counter twice. This second query was named 'pump'. By alternating slow queries and pumps, the attacker keeps the per-client counter at or below the configured 'wait-limit' indefinitely, and can hold an arbitrary number of pending queries from a single source IP, up to the global mesh quota (num-queries-per-thread); eventually bypassing one of the counter measures introduced for DNSBomb (CVE-2024-33655). This vulnerability is present on the 'serve-expired' code path.

Weakness: CWE-675 (Multiple Operations on Resource in Single-Operation Context)

## Which versions are affected by CVE-2026-77860?

It affects 1 product.


Products: Nlnetlabs Unbound.

## How do I fix CVE-2026-77860?

Apply the vendor's patch.

## Is CVE-2026-77860 being exploited?

There is no known exploitation of CVE-2026-77860 in the wild and no public exploit code.

## Other vulnerabilities in Nlnetlabs Unbound

- [CVE-2026-81642](https://insights.sbomify.com/CVE-2026-81642/): Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY (critical 9.1)
- [CVE-2026-33278](https://insights.sbomify.com/CVE-2026-33278/): Possible arbitrary code execution during DNSSEC validation (critical 9.1)
- [CVE-2026-42944](https://insights.sbomify.com/CVE-2026-42944/): Heap overflow with multiple NSID, COOKIE, PADDING EDNS options (high 8.7)
- [CVE-2026-42959](https://insights.sbomify.com/CVE-2026-42959/): Crash during DNSSEC validation of malicious content (high 8.7)
- [CVE-2026-82717](https://insights.sbomify.com/CVE-2026-82717/): CNAME synthesis could lead to heap corruption (high 8.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-77860 and fix it if it is affected.

What it is: CVE-2026-77860 is a low-severity denial of service vulnerability in Nlnetlabs Unbound, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-77860 in the wild and no public exploit code.

Affected product: Nlnetlabs Unbound. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-77860/ (Markdown: https://insights.sbomify.com/CVE-2026-77860/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-77860
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77860
- OSV: https://osv.dev/vulnerability/CVE-2026-77860

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-77860/
Summaries are generated and may contain mistakes; the official description above is the record.