Vulnerability · Published Mar 8, 2026 · Assigned by VulDB
CVE-2026-3706mkj Dropbear S Range Check curve25519.c unpackneg signature verification
Low 3.7
In plain English
A low-severity insufficient verification of data authenticity in Dropbear from mkj. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 8% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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."
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | dropbear | no fix listed | DEBIAN-CVE-2026-3706 |
| Debian:13 | dropbear | no fix listed | DEBIAN-CVE-2026-3706 |
| Debian:14 | dropbear | 2026.90-1 | DEBIAN-CVE-2026-3706 |
| Ubuntu:22.04:LTS | dropbear | no fix listed | UBUNTU-CVE-2026-3706 |
| Ubuntu:24.04:LTS | dropbear | no fix listed | UBUNTU-CVE-2026-3706 |
| Ubuntu:25.10 | dropbear | no fix listed | UBUNTU-CVE-2026-3706 |
| Ubuntu:26.04:LTS | dropbear | no fix listed | UBUNTU-CVE-2026-3706 |
| Ubuntu:26.10 | dropbear | no fix listed | UBUNTU-CVE-2026-3706 |
Affected products (as reported)
Weakness: CWE-347, CWE-345 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N/E:U/RL:O/RC:C
References
- VDB-349652 | mkj Dropbear S Range Check curve25519.c unpackneg signature verification vdb-entry, technical-description
- VDB-349652 | CTI Indicators (IOB, IOC, IOA) signature, permissions-required
- Submit #765933 | GitHub Dropbear 2025.89 Improper Verification of Cryptographic Signature third-party-advisory
- https://github.com/mkj/dropbear/issues/406 issue-tracking
- https://github.com/mkj/dropbear/pull/407 issue-tracking, patch
- https://github.com/mkj/dropbear/commit/fdec3c90a15447bd538641d85e5a3e3ac981011d patch
- https://github.com/str4d/ed25519-java/issues/82#issue-727629226 issue-tracking
- https://github.com/mkj/dropbear/ product