# CVE-2026-3706: mkj Dropbear S Range Check curve25519.c unpackneg signature verification

> A theoretical signature malleability flaw in Dropbear's curve25519 implementation could allow bypassing signature verification, though it is currently considered unexploitable in SSH.

- **Severity:** Medium 6.3 (`CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:U`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 8 Mar 2026
- **Updated:** 22 Apr 2026
- **Also known as:** DEBIAN-CVE-2026-3706, GHSA-87g2-jprq-4cmc, UBUNTU-CVE-2026-3706

## What is CVE-2026-3706?

CVE-2026-3706 is a medium-severity vulnerability in mkj Dropbear, published on 8 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A vulnerability was determined in mkj Dropbear up to 2025.89. Impacted is the function unpackneg of the file src/curve25519.c of the component S Range Check. This manipulation causes improper verification of cryptographic signature. The attack can be initiated remotely. The attack is considered to have high complexity. The exploitability is considered difficult. The actual existence of this vulnerability is currently in question. Patch name: fdec3c90a15447bd538641d85e5a3e3ac981011d. To fix this issue, it is recommended to deploy a patch. The project maintainer explains: "Signature Malleability is not exploitable in SSH protocol. (...) [A] PoC doesn't exist for SSH implementation, but rather it's against the internal API."

Weakness: CWE-347 (Improper Verification of Cryptographic Signature), CWE-345 (Insufficient Verification of Data Authenticity)

## Which versions are affected by CVE-2026-3706?


## How do I fix CVE-2026-3706?

Apply the vendor's patch.

## Is CVE-2026-3706 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-3706 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-3706 and fix it if it is affected.

What it is: CVE-2026-3706 is a medium-severity vulnerability in mkj Dropbear, published on 8 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-3706 exists.

Affected product: mkj Dropbear. 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-2026-3706/ (Markdown: https://insights.sbomify.com/CVE-2026-3706/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-3706
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-3706
- OSV: https://osv.dev/vulnerability/CVE-2026-3706

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-3706/
Summaries are generated and may contain mistakes; the official description above is the record.