# CVE-2026-44042: UltraVNC repeater wi_uudecode off-by-one in base64 decode boundary check

> An off-by-one error in UltraVNC repeater's Base64 decoder allows a one-byte stack buffer overflow, potentially leading to code execution or denial of service.

- **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:** No
- **Published:** 1 Jul 2026
- **Updated:** 9 Jul 2026
- **Also known as:** GHSA-gh7j-9w79-xm55

## What is CVE-2026-44042?

CVE-2026-44042 is a low-severity memory corruption vulnerability in uvnc UltraVNC, published on 1 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

UltraVNC repeater through 1.8.2.2 contains an off-by-one error in the Base64 decode helper used for HTTP Basic authentication. In repeater/webgui/webutils.c:817, the wi_uudecode() function checks whether the input length exceeds the output buffer with a strict greater-than comparison (>), while the correct check should be greater-than-or-equal (>=). When strlen(authdata) equals sizeof(decode), the decoded output length (approximately 3/4 of input) does not overflow the buffer in current practice because the outer HTTP request bounds constrain the Authorization header. However, the defective check leaves a latent off-by-one condition that could become exploitable if the buffering constraints change. The current risk is limited to a one-byte write at the boundary of a 1024-byte stack buffer under constrained conditions.

Weakness: CWE-193 (Off-by-one Error)

## Which versions are affected by CVE-2026-44042?

It affects 1 product.


Products: Uvnc Ultravnc.

## How do I fix CVE-2026-44042?

No fix has been published for CVE-2026-44042 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-44042 being exploited?

There is no known exploitation of CVE-2026-44042 in the wild and no public exploit code.

## Other vulnerabilities in uvnc UltraVNC

- [CVE-2019-8264](https://insights.sbomify.com/CVE-2019-8264/): Memory corruption in UltraVNC (critical 9.8)
- [CVE-2019-8265](https://insights.sbomify.com/CVE-2019-8265/): Memory corruption in UltraVNC (critical 9.8)
- [CVE-2019-8266](https://insights.sbomify.com/CVE-2019-8266/): Memory corruption in UltraVNC (critical 9.8)
- [CVE-2019-8280](https://insights.sbomify.com/CVE-2019-8280/): Memory corruption in UltraVNC (critical 9.8)
- [CVE-2018-15361](https://insights.sbomify.com/CVE-2018-15361/): Memory corruption in UltraVNC (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-44042 and fix it if it is affected.

What it is: CVE-2026-44042 is a low-severity memory corruption vulnerability in uvnc UltraVNC, published on 1 Jul 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-2026-44042 in the wild and no public exploit code.

Affected product: uvnc UltraVNC. 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-2026-44042/ (Markdown: https://insights.sbomify.com/CVE-2026-44042/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-44042
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-44042
- OSV: https://osv.dev/vulnerability/CVE-2026-44042

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-44042/
Summaries are generated and may contain mistakes; the official description above is the record.