CRL Check Explained: How to Check the Certificate Revocation Status?

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CRL Check

What Is a Certificate Revocation List?

A Certificate Revocation List (CRL) is part of digital security and cryptography. It contains serial numbers of revoked digital certificates reported as null before the scheduled expiry date.

Burdensome reasons for revocation would be a compromise of the private key, a change of the nature of the relationship between the certificate holder and the certificate issuer, or the discovery of errors in the issuance of the certificate.

The CRL, therefore, is maintained and published by the CA but through user notification and system behavior for certificates that have turned out to be untrustworthy.

These systems, while validating the submitted digital certificates, can cross-check the Certificate Revocation List to determine if a given certificate has been revoked, further strengthening the security and robustness of the certificate validation process.

Purpose of Certificate Revocation

Certificates are revoked to make sure that the public key infrastructure will remain trustworthy and secure. The certificate’s revocation is another key part of the lifecycle of digital certificates. The main causes of revocation for a certificate are:

Leaking of Private Key

The certificate will be revoked whenever the private key associated with it is compromised in such a manner that the entity is unable to use the key to make forgeries on its behalf, decrypt valuable information, or sign malicious content.

Change of Circumstance

Many certificates represent quite specific facts about the holder of a certificate and the relation of the certificate holder to the issuer.

If at any stage this changes, for instance, if a person loses his job, or if there is some change of role taking place within organizations, it might very well turn out that the status quo needs re-adjusting by the revocation of a certificate.

Error in Issuance

This includes such cases as the issuance of certificates with wrong data and in violation of policies; revocation, in this case, is necessary for ensuring that there is integrity and accuracy in PKI.

Certificate Transparency Logs vs Certificate Revocation Lists

 Certificate Transparency Logs (CT Logs)Certificate Revocation Lists (CRLs)
PurposeEnhance transparency and accountability in the issuance of SSL/TLS certificates.Provide a list of certificates that are no longer trustworthy.
FunctionLog and publicly disclose all issued certificates to prevent mis issuance.List certificates that have been revoked by the CA due to compromise, policy changes, etc.
StoragePublicly accessible logs maintained by multiple independent log operators.Lists maintained by the issuing Certificate Authority (CA).
AccessibilityEasily accessible and searchable by anyone.Available for download from CA websites or distributed via other means.
Update FrequencyContinuous, real-time updates as new certificates are issued.Periodically updated (e.g., daily or weekly).
Verification ProcessHelps in identifying misissued certificates by cross-referencing logs.Verifies the revocation status of certificates against the CRL.
Information ProvidedFull details of each certificate issued, including serial number, domain name, and issuing CA.Serial numbers of certificates that have been revoked.
Performance ImpactMinimal impact on end-user performance; mainly used for monitoring and audits.Can impact performance due to the need to download and check large lists.
Security FocusPrevents mis issuance and allows detection of unauthorized certificates.Ensures that compromised or otherwise untrustworthy certificates are not used.
DependencyDoes not depend on any revocation mechanisms; works independently.Part of the certificate lifecycle management and relies on regular updates.
Implementation ComplexityRequires CAs to submit certificates to CT logs; browsers need to check logs.Requires periodic distribution and client-side checking of the CRLs.
Real-time CheckingAllows real-time monitoring and detection of certificate issuance.Does not provide real-time status; relies on periodic updates.

Why Would a CA Add Certificates to a CRL?

There are several key reasons why a Certificate Authority (CA) would issue certificates to the Certificate Revocation List (CRL).

The following reasons make it crucial to uphold the integrity, reliability, and security of the digital communication systems that depend on these certificates:

Loss of the Certificate’s Private Key

In case a private key is lost, then the assurance provided by the certificate is rendered useless. This is because the unauthorized party could easily decrypt the sensitive information, create forgery of the certificate holder, or engage in other nefarious acts.

The CA then revokes the compromised certificate and includes it in the CRL, so that everyone will know that the certificate should no longer be accepted.

Cessation of Operation

If the certificate holder stopped the activities, for instance, a company that is no longer in business or a website that has been closed down, the certificate is no longer required.

To prevent the occurrences of any misuse of the certificate in the future, it is revoked and issued to the CRL.

Violation of CA Policy

The certificate holders are required to follow the rules and regulation of the CA which regulate the use of the certificate.

These policies include the requirement to revoke the certificate in case it is used for other than intended purposes or in case of the private key leak.

This action helps to prevent the misuse of certificates and their usage only within the prescribed guidelines of the CA.

Why a Certificate Revocation List Is Necessary?

A CRL or Certificate Revocation List is among one of the most essential elements of the digital security or public key infrastructure. Here are several reasons why a CRL is necessary:

Mitigating Security Breaches

If a certificate is somehow invalidated or the private key is gained by a third party in some way then the security and integrity of the encrypted communications or signed data becomes compromised.

Certification Authorities can therefore include the compromised certificate on CRLs so that users and systems know not to rely on it again, effectively excluding the attackers from exploiting the compromised case.

Ensuring Trustworthiness

Since a digital certificate system is only as trustworthy as the certificates that it will issue, the validity of those issued certificates becomes a fundamental block of the system.

For instance, if the CA realizes any cause that would make it not to trust a certificate, for instance, the CA realized the certificate was issued in error or is involved in any fraudulent activities, putting the certificate on CRL can help to continue to maintain general populace’s trust on the CA services, as well as the PKI environment.

Maintaining Compliance

In terms of security, several field and regulatory mechanisms impose the necessity to rotate or invalidate the compromised or otherwise invalid certificates.

A CRL serves the purpose of assisting organizations in meeting these legal and regulatory demands, which could potentially cause legal problems if not met; meanwhile assuring the soundness of these organizations’ activities.

Enhancing Security Posture

This is very important because the certificates which are no longer valid can be withdrawn while the compromised ones have adverse impacts on the security of an organization.

It makes sure it only accepts certificates that it can verify are correct and reliable within its networks and systems, thus minimizing its security threats.

Preventing Unauthorized Access

Certificates often may be revoked if the employee leaves an organization or if their responsibilities change to avoid vulnerability to their former status on the internal systems.

Such certificates can be revoked, and including them on a CRL helps to prevent situations where an old employee, for example, or an employee who no longer requires access to certain resources, still has access.

How to Check the Certificate Revocation Status?

The CA adds the serial number of the end-entity certificate to the Certificate Revocation List (CRL).

To initiate a check, you will need the following tools:

  • OpenSSL
  • Serial number of the end-entity certificate
  • Downloaded Certificate Revocation List (CRL)

The URL of the CRL is encoded in end-entity certificates. To retrieve the URL, use the following command:

openssl x509 -in cert.crt -noout -text | grep crl

Alternatively, you can decode the certificate online. Once you have the URL, download the CRL using the command below:

wget [URL of CRL]

Next, retrieve the serial number of the end-entity certificate with this command:

openssl x509 -in cert.crt -noout -serial

After obtaining the serial number, you can check if the certificate is on the CRL by running the following command:

openssl crl -inform DER -text -in [name of downloaded CRL] | grep [serial number of the certificate]

For example:

openssl crl -inform DER -text -in COMODORSADomainValidationSecureServerCA.crl | grep B1B9D6CF7E84F8E11AA7710D48815907

If there is no output, the certificate is not added to the CRL.

If the output shows the serial number, the certificate is on the CRL.

This method helps in verifying the revocation status of a certificate, ensuring that only valid certificates are trusted.

Advantages of Client-Based Certificate Revocation Status Checks

Real-Time Validation

OCSP based checks offer results within a few seconds of its run time, where the client checks the status of a certificate by contacting OCSP responder or using the available CRLs.

This ensures that any change that may be made to the validity of the certificate, for example by revocation, is quickly identified and addressed.

Also Read: Fixing NET::ERR_CERT_REVOKED_Certificate Error in Chrome and Firefox

The availability of information as and when the real-time checks are conducted ensures that clients can deny access to certificates that have been revoked, a situation that can cost time in the event that the latest status information has not been updated.

Enhanced Security

Client based certificate status check is more secure for the servers and it also tends towards directly checking the validity of the certificate.

This eliminates the dangers of accepting certificates that have been compromised, or certificates that have been revoked thus ensuring continued, secure communications.

Furthermore, the use of external servers for revocation status was reduced, thus minimizing points of failure or potential malicious exploitation since the status is not controlled from another server.

Reduced Dependency on Intermediaries

Implementing validation independently of a CA or OCSP responder minimizes the dependency on these external agents that can be performed client-side checks.

This decentralization adds the strength of the security system, as the validation can go on even the center authority being offline.

Starting from the concerns, Enriched schedule coverage, Lowered dictation from external services, Higher reliability from presented sources, and the increased autonomy in validation processes make it such that the clients exist in some capacity not expected to rely solely on the availability and performance of these services.

Improved Performance

Here, the client-based checks can bring large improvements in the use of caching memories.

Information obtained from the last few OCSP requests or CRLs stored locally enable a faster certificates validation process than that from external servers which incurs longer latency.

Validation can also be run locally and this balances load distribution, the risk of bottlenecking can arise when there is high demand for a single OCSP server.


It is also important to note that clients can set various revocation checking policies depending on the general security to be employed and the amount of speed required by the stakeholder.

This is because configuration flexibility suggests necessary tactics: The client can choose the rate at which OCSP should be queried, or how long the CRL data must be cached.

Such tailored policies ensure that the validation strategies to be applied are well aligned to the level of sensitivity that the transaction contains, or the environment that the certificates will be used in.

Drawbacks of Client-Based Certificate Revocation Status Checks

Resource Intensive

When performing status checks on client-based certificates, revocation status may consume resources, especially where there are numerous certificates in use.

Each client is to perform these checks, which may cause computational load and affect the efficiency of the client system may include the CPU, memory, and bandwidth requirements for the cooperation application. This may negatively impact other activities and cause decreased performance by the user.

Dependency on Network Availability

Such checks may check for the availability of OCSP responders or download CRLs over the network as and when required. This has an implication that in cases where the network connectivity is poor or very slow, timely revocation check is not possible.

This reliance on dependable internet connectivity can at times be a huge disadvantage especially where the network is likely to experience frequent outages thereby delaying or possibly having the validation fail.

Inconsistent Implementation

The efficiency and dependability of client-based implementations differ greatly from each other. It also shows that not all clients have it implemented in the same way, resulting in further vulnerabilities.

Another issue with CRLs is that some clients are not able to conduct revocation checks or they simply disregard them, which leads to possible risks.

This lack of consistency gives organizations the wrong perception of security and that opens up more vulnerabilities that would have been addressed by having server-based tests for all five checklists.

Scalability Issues

If the number of clients increases the total load on the OCSP responders or if there is a need to distribute or update CRLs then scalability can become an issue.

Growing traffic demands that are possible to control in large-scale networks may overload the network components and cause performance degradation.

It becomes a problem for logistics as well as technical grounds on how to store up-to-date revocation information within a numerous number of clients.

Privacy Concerns

Such checks carried out from the client-side should also be of concern as every revocation request comes with information about the certificate being authenticated and therefore may be used to expose the user’s activities.

When not anonymized sufficiently, such queries can be intercepted by unauthorized parties, resulting in violation of the user’s privacy rights.

Privacy can be protected by implementing revocation checks but this poses some challenges due to the added measures needed to prevent eavesdropping.


Strengthen the protection of your data transmission and improve compliance with the certificate management standards that require performing checks of the revocation status of certificates.

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Janki Mehta

Janki Mehta

Janki Mehta is a Cyber-Security Enthusiast who constantly updates herself with new advancements in the Web/Cyber Security niche. Along with theoretical knowledge, she also implements her practical expertise in day-to-day tasks and helps others to protect themselves from threats.

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