CVE intelligence and bounded remediation
CVE-2021-20230 — Stunnel Stunnel security vulnerability
A flaw was found in stunnel before 5.57, where it improperly validates client certificates when it is configured to use both redirect and verifyChain options. This flaw allows an attacker with a certificate signed by a Certificate Authority, which is not the one accepted by the stunnel server, to access the tunneled service instead of being redirected to the address specified in the redirect option. The highest threat from this vulnerability is to confidentiality.
- Severity
- High
- CVSS
- 7.5 (3.1)
- Published
- 2021-02-23
- CISA KEV
- Not currently listed
- Ecosystem
- software/application
- Weaknesses
- CWE-295
Affected products
- stunnel / stunnel
Matched remediation archetype
Cryptography, certificate, signature, and channel validation
This catalog composition supplies bounded fallback guidance. Explicitly reviewed curated workflows load with the complete record below.
Check exposure
- Inventory affected algorithms, key uses, trust stores, certificate validation settings, random sources, and plaintext channels across clients and services.
- Determine which secrets, identities, signatures, or data protections depend on the affected primitive or validation path.
- Check debug, compatibility, fallback, and hostname or audience override settings in build and runtime configuration.
Remediate safely
- Use a maintained platform cryptographic API with approved algorithms, modes, parameters, randomness, and full peer identity validation.
- Remove insecure fallback and validation bypasses; separate keys by purpose and load them from managed secret storage.
- Plan rotation or reissuance for affected keys, certificates, tokens, hashes, or ciphertext and document compatibility sequencing.
Authoritative sources
Complete CVE record and remediation plan
The detailed catalog view below loads this exact record, its source evidence, and the full seven-phase agentic change plan.