Tag Archives: Developer tips

Don’t Reinvent Date Formats

от Божидар Божанов
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Microsoft Exchange has a bug that practically stops email. (The public sector is primarily using Exchange, so many of the institutions I’m responsible for as a minister, have their email “stuck”). The bug is described here, and fortunately, has a solution.

But let me say something simple and obvious: don’t reinvent date formats, please. When in doubt, use ISO 8601 or epoch millis (in UTC), or RFC 2822. Nothing else makes sense.

Certainly treating an int as a date is an abysmal idea (it doesn’t even save that much resources). 202201010000 is not a date format worth considering.

(As a side note, another advice – add automate tests for future timestamps. Sometiimes they catch odd behavior).

I’ll finish with Jon Skeet’s talk on dates, strings and numbers.

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Simple Things That Are Actually Hard: User Authentication

от Божидар Божанов
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You build a system. User authentication is the component that is always there, regardless of the functionality of the system. And by now it should be simple to implement it – just “drag” some ready-to-use authentication module, or configure it with some basic options (e.g. Spring Security), and you’re done.

Well, no. It’s the most obvious thing and yet it’s extremely complicated to get right. It’s not just login form -> check username/password -> set cookie. It has a lot of other things to think about:

  • Cookie security – how to make it so that a cookie doesn’t leak or can’t be forged. Should you even have a cookie, or use some stateless approach like JWT, use SameSite lax or strict?
  • Bind cookie to IP and logout user if IP changes?
  • Password requirements – minimum length, special characters? UI to help with selecting a password?
  • Storing passwords in the database – bcrypt, scrypt, PBKDF2, SHA with multiple iterations?
  • Allow storing in the browser? Generally “yes”, but some applications deliberately hash it before sending it, so that it can’t be stored automatically
  • Email vs username – do you need a username at all? Should change of email be allowed?
  • Rate-limiting authentication attempts – how many failed logins should block the account, for how long, should admins get notifications or at least logs for locked accounts? Is the limit per IP, per account, a combination of those?
  • Captcha – do you need captcha at all, which one, and after how many attempts? Is Re-Captcha an option?
  • Password reset – password reset token database table or expiring links with HMAC? Rate-limit password reset?
  • SSO – should your service should support LDAP/ActiveDirectory authentication (probably yes), should it support SAML 2.0 or OpenID Connect, and if yes, which ones? Or all of them? Should it ONLY support SSO, rather than internal authentication?
  • 2FA – TOTP or other? Implement the whole 2FA flow, including enable/disable and use or backup codes; add option to not ask for 2FA for a particular device for a period of time? Configuring subset of AD/LDAP users to authenticate based on certain group memberships?
  • Force 2FA by admin configuration – implement time window for activating 2FA after a global option is enabled?
  • Login by link – should the option to send a one-time login link be email be supported?
  • XSS protection – make sure no XSS vulnerabilities exist especially on the login page (but not only, as XSS can steal cookies)
  • Dedicated authentication log – keep a history of all logins, with time, IP, user agent
  • Force logout – is the ability to logout a logged-in device needed, how to implement it, e.g. with stateless tokens it’s not trivial.
  • Keeping a mobile device logged in – what should be stored client-side? (certainly not the password)
  • Working behind proxy – if the client IP matters (it does), make sure the X-Forwarded-For header is parsed
  • Capture login timezone for user and store it in the session to adjust times in the UI?
  • TLS Mutual authentication – if we need to support hardware token authentication with private key, we should enable TLS mutual. What should be in the truststore, does the web server support per-page mutual TLS or should we use a subdomain, if there’s a load balancer / reverse proxy, does it support it and how to forward certificate details?
  • Require account activation or let the user login immediately after registration? Require account approval by back-office staff?
  • Initial password setting for accounts created by admins – generate initial password and force changing it on first login? Don’t generate password and start from a password reset flow?
  • Login anomalies – how to detect them and should you inform the user? Should you rely on 3rd party tools (e.g. a SIEM), or have such functionality built-in?

And that’s for the most obvious feature that every application has. No wonder it has been implemented incorrectly many, many times. The IT world is complex and nothing is simple. Sending email isn’t simple, authentication isn’t simple, logging isn’t simple. Working with strings and dates isn’t simple, sanitizing input and output isn’t simple.

We have done a poor job in building the frameworks and tools to help us with all those things. We can’t really ignore them, we have to think about them actively and take conscious, informed decisions.

The post Simple Things That Are Actually Hard: User Authentication appeared first on Bozho's tech blog.

Obtaining TLS Client Certificates In Spring Integration

от Божидар Божанов
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Spring Integration is a very powerful and extensible framework for, well, integrations. But sometimes it’s not trivial how to get some information that yo need. In my case – a certificate used for mutual authentication in a TLS (syslog over TLS) connection. You have a Java method that receives a Message and ideally you’d want to get the certificate chain used by the client to authenticate itself (e.g. you may need to extract the CN).

Fortunately, Spring Integration is flexible. And it can be done, but it’s a bit convoluted. I’ll use XML notation, but the same can be achieved through Java config.

<bean id="nioConnectionSupport" class="com.yourcompany.util.net.TLSMutualNioConnectionSupport">
        <constructor-arg ref="sslContextSupport" />
        <constructor-arg value="false" />
<bean id="interceptorFactoryChain" class="org.springframework.integration.ip.tcp.connection.TcpConnectionInterceptorFactoryChain">
        <property name="interceptors">
            <bean class="com.yourcompany.util.net.TLSSyslogInterceptorFactory" />

<int-ip:tcp-connection-factory id="tlsConnectionFactory" type="server" port="${tcp.tls.port}"
                                   using-nio="true" nio-connection-support="nioConnectionSupport"
                                   single-use="false" interceptor-factory-chain="interceptorFactoryChain" />

The sslContextSupport would typically be a org.springframework.integration.ip.tcp.connection.DefaultTcpSSLContextSupport or a custom implementation (e.g. if you want to use a “blind” trust store)

Then you’d need the two classes. You can check them at their respective gists: TLSSyslogInterceptorFactory and TLSMutualNioConnectionSupport.

What do these classes do? The TLSSyslogInterceptorFactory sets a new header for the message that contains the client ceritficates. The TLSMutualNioConnectionSupport class sets the “wantClientAuth” option on the SSL Engine. There is another option – “needClientAuth” which would for client authentication, rather than just support it. Depending on the use case you can use one or the other.

Then you can obtain the certificates at your handler method via:

Certificate[] certificates = (Certificate[]) message.getHeaders().get(TLSSyslogInterceptorFactory.TLS_CLIENT_CERTIFICATES);

A small tip I wanted to share to help the next one trying to achieve that.

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Every Serialization Framework Should Have Its Own Transient Annotation

от Божидар Божанов
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We’ve all used dozens of serialization frameworks – for JSON, XML, binary, and ORMs (which are effectively serialization frameworks for relational databases). And there’s always the moment when you need to exclude some field from an object – make it “transient”.

So far so good, but then comes the point where one object is used by several serialization frameworks within the same project/runtime. That’s not necessarily the case, but let me discuss the two alternatives first:

  • Use the same object for all serializations (JSON/XML for APIs, binary serialization for internal archiving, ORM/database) – preferred if there are only minor differences between the serialized/persisted fields. Using the same object saves a lot of tedious transferring between DTOs.
  • Use different DTOs for different serializations – that becomes a necessity when scenarios become more complex and using the same object becomes a patchwork of customizations and exceptions

Note that both strategies can exist within the same project – there are simple objects and complex objects, and you can only have a variety of DTOs for the latter. But let’s discuss the first option.

If each serialization framework has its own “transient” annotation, it’s easy to tweak the serialization of one or two fields. More importantly, it will have predictable behavior. If not, then you may be forced to have separate DTOs even for classes where one field differs in behavior across the serialization targets.

For example the other day I had the following surprise – we use Java binary serialization (ObjectOutputStream) for some internal buffering of large collections, and the objects are then indexed. In a completely separate part of the application, objects of the same class get indexed with additional properties that are irrelevant for the binary serialization and therefore marked with the Java transient modifier. It turns out, GSON respects the “transient” modifier and these fields are never indexed.

In conclusion, this post has two points. The first is – expect any behavior from serialization frameworks and have tests to verify different serialization scenarios. And the second is for framework designers – don’t reuse transient modifiers/annotations from the language itself or from other frameworks, it’s counterintuitive.

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