Summary
GnuTLS v3.8.13 may accept certificates containing invalid extensions because certificate validation does not correctly reject those conditions. In deployments relying on this validation, a remote attacker who can present a crafted certificate may bypass intended trust checks and impersonate a service.
GnuTLS v3.8.13 may accept certificates containing invalid extensions because certificate validation does not correctly reject those conditions. In deployments relying on this validation, a remote attacker who can present a crafted certificate may bypass intended trust checks and impersonate a service.
Affected context
This issue occurs in applications and services that use GnuTLS v3.8.13 to validate certificates during TLS authentication. Risk is higher when certificate extensions are treated as sufficient for trust decisions without independent validation of their syntax, semantics, criticality, and consistency with the negotiated security policy.
This issue occurs in applications and services that use GnuTLS v3.8.13 to validate certificates during TLS authentication. Risk is higher when certificate extensions are treated as sufficient for trust decisions without independent validation of their syntax, semantics, criticality, and consistency with the negotiated security policy.
Exploit preconditions
- Attack surface: TLS certificate validation performed by an application using GnuTLS
- Required access: Ability to establish a TLS connection and present a certificate chain or endpoint certificate
- Trigger point: Certificate processing during connection establishment
- Why exploitation is possible: Invalid certificate extensions may not be rejected consistently, allowing a malformed certificate to pass checks that should enforce certificate validity and trust requirements
- Attack surface: TLS certificate validation performed by an application using GnuTLS
- Required access: Ability to establish a TLS connection and present a certificate chain or endpoint certificate
- Trigger point: Certificate processing during connection establishment
- Why exploitation is possible: Invalid certificate extensions may not be rejected consistently, allowing a malformed certificate to pass checks that should enforce certificate validity and trust requirements
Why this happens
The underlying bug is incomplete or incorrect validation of certificate extensions during certificate verification. This maps to CWE-295: Improper Certificate Validation, where an application accepts certificate data without adequately checking whether it conforms to required rules or security policy. The insecure pattern is treating a certificate as acceptable when extension encoding, meaning, criticality, or applicability has not been fully validated.
The underlying bug is incomplete or incorrect validation of certificate extensions during certificate verification. This maps to CWE-295: Improper Certificate Validation, where an application accepts certificate data without adequately checking whether it conforms to required rules or security policy. The insecure pattern is treating a certificate as acceptable when extension encoding, meaning, criticality, or applicability has not been fully validated.
Potential impact
- An attacker may impersonate a trusted service
- TLS clients may establish connections to an unintended endpoint
- Confidential data could be exposed through interception
- Authentication or authorization decisions based on certificate identity could be bypassed
- Downstream systems may inherit incorrect trust decisions
- An attacker may impersonate a trusted service
- TLS clients may establish connections to an unintended endpoint
- Confidential data could be exposed through interception
- Authentication or authorization decisions based on certificate identity could be bypassed
- Downstream systems may inherit incorrect trust decisions
General mitigation guidance
Mitigation generally focuses on ensuring that certificate validation rejects malformed, unsupported, or semantically invalid extensions by default.
Common defensive patterns include:
- Use certificate-validation APIs with strict, standards-compliant verification behavior
- Treat unsupported critical extensions and invalid extension combinations as validation failures
- Reduce trust in externally supplied certificate metadata until the complete chain and policy checks succeed
- Apply explicit hostname, identity, key-usage, validity-period, and chain-verification checks where appropriate
- Run network-facing services with least privilege and limit the impact of a compromised trust decision
- Centralize TLS policy enforcement rather than allowing inconsistent application-specific validation
Mitigation generally focuses on ensuring that certificate validation rejects malformed, unsupported, or semantically invalid extensions by default.
Common defensive patterns include:
- Use certificate-validation APIs with strict, standards-compliant verification behavior
- Treat unsupported critical extensions and invalid extension combinations as validation failures
- Reduce trust in externally supplied certificate metadata until the complete chain and policy checks succeed
- Apply explicit hostname, identity, key-usage, validity-period, and chain-verification checks where appropriate
- Run network-facing services with least privilege and limit the impact of a compromised trust decision
- Centralize TLS policy enforcement rather than allowing inconsistent application-specific validation
How teams detect issues like this
Organizations can look for certificate-validation weaknesses through:
- Static analysis and dependency review for custom or incomplete certificate-checking logic
- Code review signals such as ignored verification results, permissive fallback paths, or acceptance of unknown extensions
- TLS interoperability tests using malformed, unsupported, and contradictory certificate extensions
- Policy enforcement requiring hostname verification, chain validation, and rejection of invalid critical extensions
- Runtime monitoring for unexpected certificate identities, issuer changes, or validation failures that are suppressed
Automated security and dependency-review tools may help identify affected library usage and weak validation patterns.
Organizations can look for certificate-validation weaknesses through:
- Static analysis and dependency review for custom or incomplete certificate-checking logic
- Code review signals such as ignored verification results, permissive fallback paths, or acceptance of unknown extensions
- TLS interoperability tests using malformed, unsupported, and contradictory certificate extensions
- Policy enforcement requiring hostname verification, chain validation, and rejection of invalid critical extensions
- Runtime monitoring for unexpected certificate identities, issuer changes, or validation failures that are suppressed
Automated security and dependency-review tools may help identify affected library usage and weak validation patterns.
Sources
| Source | Link |
|---|---|
| gnutls. | gnutls.com |
| gist. | gist.github.com/ |
| github. | github.com/ |
