CVE-2021-36369 is an improper-authentication flaw (CWE-287) in the client-side SSH code of Dropbear through version 2020.81: Dropbear checks the authentication methods a server claims to support in a way that does not follow the SSH RFCs, so a malicious or compromised SSH server can manipulate that check to steer the client's login process, bypass extra protections such as FIDO2 security keys or SSH-Askpass prompts, and abuse a forwarded SSH agent to authenticate to a different server without the user noticing; NVD's own analysis rates this CVSS v3.1 7.5 (high).
Vulnerability summary
7.5
High
1.9%
EPSS score (FIRST)
No
CISA KEV catalog
2022-10-12
Published
Vulnerability ID: CVE-2021-36369 · Source: NVD
Affected components
| Component | CPE | Version range |
|---|---|---|
| Dropbear SSH (client) | cpe:2.3:a:dropbear_ssh_project:dropbear_ssh:*:*:*:*:*:*:*:* | Dropbear through 2020.81 (fixed in 2022.82) |
What does this mean for affected devices?
CVE-2021-36369 (CWE-287, improper authentication) affects Dropbear, the lightweight SSH client and server widely embedded in routers, NAS devices and other resource-constrained firmware as a smaller alternative to OpenSSH. NVD's description states: "An issue was discovered in Dropbear through 2020.81. Due to a non-RFC-compliant check of the available authentication methods in the client-side SSH code, it is possible for an SSH server to change the login process in its favor. This attack can bypass additional security measures such as FIDO2 tokens or SSH-Askpass. Thus, it allows an attacker to abuse a forwarded agent for logging on to another server unnoticed" (NVD).
NVD's own Primary analysis scores this CVSS v3.1 7.5 (high), vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N — network-reachable, low attack complexity, no privileges or user interaction required, with high impact to integrity (the manipulated login flow) and no direct confidentiality or availability impact recorded in the vector itself (NVD).
The fix was developed and discussed in Dropbear's own GitHub repository, tracked as pull request #128 against mkj/dropbear, and shipped in the DROPBEAR_2022.82 release (Dropbear GitHub releases).
Debian's LTS security team confirmed the same root cause and impact in its advisory for Debian 10 (buster): "Due to a non-RFC-compliant check of the available authentication methods in the client-side SSH code, it was possible for an SSH server to change the login process in its favor. This attack can bypass additional security measures such as FIDO2 tokens or SSH-Askpass. Thus, it allows an attacker to abuse a forwarded agent for logging on to another server unnoticed," fixed in dropbear 2018.76-5+deb10u2 (Debian LTS announce).
This CVE is not listed in the CISA Known Exploited Vulnerabilities catalog as of catalog version 2026.11.06, and FIRST scores the probability of exploitation at 0.01864 (79.373th percentile) as of 2026-11-06 (CISA KEV, FIRST EPSS).
Devices or firmware images bundling Dropbear through 2020.81 as an SSH client — especially where users rely on agent forwarding and expect FIDO2 or SSH-Askpass prompts to stop a hostile relay server — should be updated to Dropbear 2022.82 or later, or to a distribution package that has backported the fix.
Evidence
- Dropbear through 2020.81 performs a non-RFC-compliant check of available authentication methods in client-side SSH code, letting a malicious server bypass FIDO2/SSH-Askpass protections and abuse a forwarded agent. — NVD
- NVD's own Primary analysis scores this CVSS v3.1 7.5 (high), vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N. — NVD
- The fix was tracked as pull request #128 in the mkj/dropbear GitHub repository and shipped in the DROPBEAR_2022.82 release. — Dropbear GitHub releases
- Debian's LTS security team republished the same description and shipped a fix in dropbear 2018.76-5+deb10u2 for Debian 10 (buster). — Debian LTS announce
- Not present in the CISA KEV catalog as of catalog version 2026.11.06; EPSS score is 0.01864 (79.373th percentile) as of 2026-11-06. — FIRST EPSS
Questions about this page?
Related advisories
Sources
Frequently asked questions
Does having FIDO2 security keys or SSH-Askpass configured protect against CVE-2021-36369?
No — that is the point of the flaw. NVD's description states the non-RFC-compliant authentication-method check specifically lets a malicious SSH server bypass additional security measures such as FIDO2 tokens or SSH-Askpass, because it manipulates which authentication method the client believes is required.
What does an attacker actually gain by exploiting CVE-2021-36369?
By steering the client's login process, the attacker can abuse a forwarded SSH agent to authenticate to a different downstream server, unnoticed by the user, per NVD and the Debian LTS advisory.
Which Dropbear release fixes CVE-2021-36369?
The fix shipped in the DROPBEAR_2022.82 release; the underlying change was developed as pull request #128 in the mkj/dropbear GitHub repository, per Dropbear's own release notes.
Is CVE-2021-36369 a server-side or client-side Dropbear vulnerability?
It is in Dropbear's client-side SSH code — NVD's description specifically attributes the non-RFC-compliant authentication-method check to "the client-side SSH code," meaning the Dropbear client (dbclient), not the Dropbear SSH server, performs the flawed check.