15 Hidden Android Features You Should Know

While Android has matured by leaps and bounds, it's still going through the refinement phase. Over time, Google has both added and removed many popular features in Android. Sometimes the features are completely removed. However, in many cases, they make it to the Settings page or they are buried under different hidden places inside Android. That's why we have come up with this article where we unearth 15 hidden Android features that are quite interesting and helpful. So, let's go ahead and explore some unique Android features which are available on our Android devices.

Hidden Android Features

Here, we have mentioned several hidden Android features ranging from privacy, security, ease of use and more. Further, we have also added some obscure Android features which were released recently but might have gone under the radar. Now with that said, here are the hidden Android features that you should know and use often.

1. Block Spam Calls

The one feature that I turn on whenever I set up a new Android device is: filter spam calls. It saves me from unwanted calls by telemarketers, fraudsters and spammers. If you use a stock Android device, you must have the Phone app by Google installed as your default dialer. To enable spam call blocking, open the Phone app and tap on the three-dot menu on the top-right corner and open Settings. After that, open "Caller ID and spam" and enable both the toggles. Now, whenever you will receive a call by spammers, the screen won't light up or make any sound. It's almost like DND with Total Silence turned on.

If you are worried that you will miss important calls then fret not. I have been using this feature for a long time and I can vouch that it works flawlessly. While there are other apps like Truecaller with similar features, you should be wary against handing your data to third-party apps, especially given its shady past record.

2. Verification Code Autofill

Many of us have allowed SMS permission to multiple apps for seamless OTP verification. However, this can lead to serious breach as apps can read all your text messages and also build credit profile without your consent. To crack down on this behavior, Google has brought a new API called the SMS Retriever. It allows apps to capture a one-time code without asking for SMS permission. In case, the app developer has not implemented this API, Google will act as a bridge and provide the verification code. That is awesome, right? So to enable this feature, navigate to Settings -> Google -> Verification Code Autofill and enable the toggle. Make sure you have also enabled the Autofill service by Google.

3. Force Dark Mode on all Apps

While the dark mode is slowly becoming the norm, there are still apps like Facebook and WhatsApp which are yet to embrace the dark side. If you want to force dark mode on a range of apps then there is a hidden Android setting that lets you do it. However, you must be on the latest Android 10 build. To enable dark mode for all the apps, navigate to Settings -> About Phone and tap on the Build Number for seven times continuously. A toast notification will show up prompting "Developer Options have been enabled". Now go back, and search for "Override force-dark" in the Settings page. Tap on the first result and then enable the toggle. Just to be sure, restart your device and check if dark mode is working on all apps.

4. Focus Mode

Google has brought a new feature called Focus Mode to Digital Wellbeing with the launch of Android 10. While Digital Wellbeing is great at limiting the screen time of various apps, Focus Mode allows you to block certain apps completely so you can focus on what you are doing. To configure Focus Mode, follow Settings -> Digital Wellbeing -> Focus Mode. Here, you can select apps that you find distracting and can enable Focus Mode straight from the Quick Settings panel.

5. Share WiFi with QR Code

I know the feeling when someone asks you to share your WiFi's password and you can't seem to remember it. If you use a strong password with multiple hashes and characters then it becomes even more tedious to type it out. In such cases, a QR code can help you seamlessly connect to WiFi networks. Thankfully, Android 10 has this not-so-hidden feature under the WiFi Settings page. Open it and you will find the QR code scanner besides the "Add Network" section. Now, scan the QR code and you will be connected in no time.

6. Randomize MAC Address

MAC address is a unique identifier assigned to WiFi-enabled devices including smartphones. Most devices come with a static MAC which can be exploited and used for tracking user movement. So having a random MAC address reduces the chance of tracking and strengthens your privacy. If you are running Android 10 then you can randomize the MAC address from the WiFi Settings page. Tap on the WiFi that you are connected to and open "Privacy". Here, make sure "Use randomized MAC" is your default pick.

7. Force Apps to Resize in Split Mode

Android has some unique features like Split Mode which make it quite distinct from iOS. It lets you run two apps at once so you can effortlessly multitask between them. However, not all apps support Split Mode and that's where this hidden Android feature comes into play. With this feature, you can force any app to resize in split mode, irrespective of the app compatibility. So to enable this feature, open Settings and search for "resizable". Open the first result and enable "Force activities to be resizable". Now, restart your device and you can enjoy any app in the Split mode.

8. Default USB Configuration

If you are someone who regularly connects Android devices to PC then this sneaky feature is going to help you a lot. Android 10 now allows you to choose your default USB configuration. If you transfer files regularly then simply choose the desired settings and you are good to go. To configure USB, open Settings and search for "Default USB" and tap on the first result. Here, choose "File transfer" or any other settings based on your preference.

9. Private DNS

While Private DNS was launched with Android Pie, it still remains one of the least talked features of Android. It allows you to encrypt your DNS query so no one can read it, not even your internet service provider. You can find the Private DNS feature in the "Network and Internet" settings page. Here, open "Private DNS" and choose Automatic for Google's DNS or you can also select some other DNS providers as well. I would recommend you to go with Cloudflare's DNS.

10. Device Theming

Device theming has finally arrived on Android, but it's still hidden under the developer options. You can change the accent color, font and icon shape from a handful of options. To find the desired settings page, open Settings and search for "theming". Tap on the top result and customize your Android device as you prefer.

11. Show Taps While Screen Recording

With the release of Android Oreo, Google removed the ability to show taps while screen recording for security reasons. However, the settings to configure taps is still available and hidden under the developer options. Just open the Settings page and search for "taps". Open the first result and enable the toggle. Now, you can record your screen with taps and that's really convenient.

12. Capitalize Words Quickly

If you want to capitalize words quickly then Gboard has this nifty feature which is super helpful and I simply love it. Just select the words and tap the "Shift" button twice to capitalize a chunk of words at once. You can also make it lowercase by similarly double tapping the shift button. And if you just want to keep the initials capital then tap the shift button once. That's pretty great, right? So go ahead and type hassle-free with Gboard.

13. Uninstall Multiple Apps

One of the best hidden Android features is that you can uninstall multiple apps at once. It works on older versions of Android as well so that is great. All you have to do is open the Google Play Store and tap on the hamburger menu and select "My apps and games". Here, switch to the "Installed" section and then tap on "Storage". After that, simply select apps that you want to uninstall and hit that "Free Up" button. Voila, multiple apps just got uninstalled in just one tap.

14. Clear Site Storage Using Chrome

Chrome is notorious for accumulating a large amount of data in background which makes the performance worse and also takes up crucial storage space. If you want to check what websites are eating up your memory space then a hidden Chrome setting can help you with that. Open Chrome on your Android device and head over to its Settings page. Now, navigate to Site Settings and open Storage. Here, you will find all the websites with their storage space. Open any website and tap on the "Delete" icon to finally free up your internal storage from unnecessary cached data.

15. Street View Layer in Google Maps

Street View is an immersive way to explore places and find new landmarks, hotels, restaurants from anywhere around the world. Now, the Street View Layer has been added to Google Maps and it works pretty well. To check if Street View is available in your region, open Google Maps and tap on the "layer" icon on the top-right corner. Now, tap on "Street View" icon and then zoom out to find blue lines on the map. Finally, tap on the blue lines and Street View will show up for that place. That's cool, right? So go ahead and check out Street View to find some hidden gems around the world.

Note: The blue lines show up wherever street view is available.

@EVERYTHING NT

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OWASP Web 2.0 Project Update

Some of you likely recall the talk back in 2016 or so of updating the OWASP Foundation website to not appear so much like a...well, a wiki.  That talk was carried forward into 2017 and 2018 and, with each year, the proposal got pushed ahead as there were other, deeper projects to tackle.  With the arrival of 2019 and a firm project plan under the guidance of Mike McCamon, Executive Director, we are finally moving toward a functioning, modern website that will be a whole lot less...wiki-like.  The journey has been circuitous and, while we are not anywhere near complete, we have a set plan in place to bring it to fruition within the calendar year (second quarter of the year, actually).

TLDR: How Can You Help? 

There are certainly ways in which you can get involved now.  For instance, we are looking for a clean way to get wiki pages into GitHub markdown format for archival.  I have done some work here but there are parsing issues with some of the tools.  Do you know a good tool or have you done similar work?  Also, are you or do you know a good designer, someone familiar with GitHub pages that can provide some useful help and feedback along the way?  A Jekyll expert to help code a theme with a handful of templates would be a great addition.  In addition, we could use website server admins who could help with assigning redirects to maintain search integrity.  Finally, there will be a great many pages to move that we will also eventually need community involvement in.  

So, What Have We Done? 

Thus far we have researched various ideas for standing up a new site, including modifying the current wiki, spinning up our own web server, contracting a third party to host and build a new site, and also using existing infrastructure with our own content to launch a new face for OWASP.  Our discussions led us to a familiar place, one that nearly every developer in the OWASP space is familiar with: GitHub.   

In our conversations with GitHub, it became readily apparent that using the platform would be a win for the Foundation as well as GitHub.  Nearly everyone who runs a project at OWASP (documentation or otherwise) uses GitHub.  Because our target audience is also mostly developers we know that they are also very comfortable with the platform.  And while GitHub has a number of high profile companies using their GitHub Pages, the use of the platform as the basis for the entire website of the number one non-profit foundation in the application security sector is a big draw.

We have run with that GitHub Pages idea and have spent internal manpower on a proof of concept.  This proof of concept is less about the UX of the site than the functionality, the ability to utilize the authentication systems, and the ability to utilize automation to push out changes quickly.

Where Are We Now?

We are doing the final stages of website architecture. We are also planning what needs to be in the site, how the pieces will integrate with current projects and chapters, and how we might utilize the community to integrate the pieces so that we have a visually and functionally cohesive website that spans across multiple repositories.

What Is Next?

We will soon be looking for a modern website design that is responsive and clean.  We will begin using the knowledge gained from our proof of concept to build out the internals of the website and then we will start implementing the highest traffic pages and administrative areas into the new platform.  Once we have the big-ticket items moved we will start looking at what is left and moving over those pieces.  The eventual goal would be to have a new, modern website for the future of OWASP while keeping the wiki as an archive of really useful information.

We hope you are as excited as we are about the future of the OWASP Foundation website and will join us as we move toward a modern web presence.  If you have any questions or would like to volunteer your time, experience or knowledge, please contact me at harold.blankenship@owasp.com

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CEH: Fundamentals Of Social Engineering

Social engineering is a nontechnical method of breaking into a system or network. It's the process of deceiving users of a system and convincing them to perform acts useful to the hacker, such as giving out information that can be used to defeat or bypass security mechanisms. Social engineering is important to understand because hackers can use it to attack the human element of a system and circumvent technical security measures. This method can be used to gather information before or during an attack.

A social engineer commonly uses the telephone or Internet to trick people into revealing sensitive information or to get them to do something that is against the security policies of the organization. By this method, social engineers exploit the natural tendency of a person to trust their word, rather than exploiting computer security holes. It's generally agreed that users are the weak link in security; this principle is what makes social engineering possible.

The most dangerous part of social engineering is that companies with authentication processes, firewalls, virtual private networks, and network monitoring software are still wide open to attacks, because social engineering doesn't assault the security measures directly. Instead, a social-engineering attack bypasses the security measures and goes after the human element in an organization.

Types of Social Engineering-Attacks

There are two types of Social Engineering attacks

Human-Based 

Human-based social engineering refers to person-to-person interaction to retrieve the desired information. An example is calling the help desk and trying to find out a password.

Computer-Based 

​Computer-based social engineering refers to having computer software that attempts to retrieve the desired information. An example is sending a user an email and asking them to reenter a password in a web page to confirm it. This social-engineering attack is also known as phishing.

Human-Based Social Engineering

Human-Based further categorized as follow:

Impersonating an Employee or Valid User

In this type of social-engineering attack, the hacker pretends to be an employee or valid user on the system. A hacker can gain physical access by pretending to be a janitor, employee, or contractor. Once inside the facility, the hacker gathers information from trashcans, desktops, or computer systems.

Posing as an Important User

In this type of attack, the hacker pretends to be an important user such as an executive or high-level manager who needs immediate assistance to gain access to a computer system or files. The hacker uses intimidation so that a lower-level employee such as a help desk worker will assist them in gaining access to the system. Most low-level employees won't question someone who appears to be in a position of authority.

Using a Third Person

Using the third-person approach, a hacker pretends to have permission from an authorized source to use a system. This attack is especially effective if the supposed authorized source is on vacation or can't be contacted for verification.

Calling Technical Support

Calling tech support for assistance is a classic social-engineering technique. Help desk and technical support personnel are trained to help users, which makes them good prey for social-engineering attacks.

Shoulder Surfing 

Shoulder surfing is a technique of gathering passwords by watching over a person's shoulder while they log in to the system. A hacker can watch a valid user log in and then use that password to gain access to the system.

Dumpster Diving

Dumpster diving involves looking in the trash for information written on pieces of paper or computer printouts. The hacker can often find passwords, filenames, or other pieces of confidential information.

Computer-Based Social Engineering

Computer-based social-engineering attacks can include the following:

• Email attachments
• Fake websites
• Pop-up windows

Insider Attacks

If a hacker can't find any other way to hack an organization, the next best option is to infiltrate the organization by getting hired as an employee or finding a disgruntled employee to assist in the attack. Insider attacks can be powerful because employees have physical access and are able to move freely about the organization. An example might be someone posing as a delivery person by wearing a uniform and gaining access to a delivery room or loading dock. Another possibility is someone posing as a member of the cleaning crew who has access to the inside of the building and is usually able to move about the offices. As a last resort, a hacker might bribe or otherwise coerce an employee to participate in the attack by providing information such as passwords.

Identity Theft

A hacker can pose as an employee or steal the employee's identity to perpetrate an attack. Information gathered in dumpster diving or shoulder surfing in combination with creating fake ID badges can gain the hacker entry into an organization. Creating a persona that can enter the building unchallenged is the goal of identity theft.

Phishing Attacks

Phishing involves sending an email, usually posing as a bank, credit card company, or other financial organization. The email requests that the recipient confirm banking information or reset passwords or PINs. The user clicks the link in the email and is redirected to a fake website. The hacker is then able to capture this information and use it for financial gain or to perpetrate other attacks. Emails that claim the senders have a great amount of money but need your help getting it out of the country are examples of phishing attacks. These attacks prey on the common person and are aimed at getting them to provide bank account access codes or other confidential information to the hacker.

Online Scams

Some websites that make free offers or other special deals can lure a victim to enter a username and password that may be the same as those they use to access their work system.
The hacker can use this valid username and password once the user enters the information in the website form. Mail attachments can be used to send malicious code to a victim's system, which could automatically execute something like a software keylogger to capture passwords. Viruses, Trojans, and worms can be included in cleverly crafted emails to entice a victim to open the attachment. Mail attachments are considered a computer-based social-engineering attack.

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How To Create Fake Email Address Within Seconds

How To Create Fake Email Address Within Seconds

Email address is a type of identification by which an email receiver identifies the person who sends mail to him/her. That's why while creating an email address you need to enter your personal details that must be valid. However, what if we tell you that you can create an email address that doesn't require any validation of personal details and that email address gets permanently deleted after your work is done. So here we have a method To Create Fake Email Address. By this, you can create a fake email address that will get auto-deleted after ten minutes. Just follow the below steps to proceed.

Note:  The method we are discussing is just for a known purpose and should not be used for any illegal purposes, as we will be not responsible for any wrongdoing.

How To Create Fake Email Address Within Seconds

The method of creating a fake email address is very simple and easy as these are based on online websites that will provide you a free email address without taking any personal details from you.

#1 10 Minute Mail

10 Minute Mail

10 Minute Mail will provide you a temporary e-mail address. Any e-mails sent to that address will show automatically on the web page. You can read them, click on links, and even reply to them. The email address will expire after 10 minutes.

#2 GuerrillaMail

Guerrillamail

It is one of the most user-friendly ones out there, with this, you can get disposable email ID easily. You need to enter the details, and the fake email ID will be generated. Moreover, this also lets you send emails with attachment up to 150MB. You will be provided with a temporary email address which you can use to verify some websites which require the email address.

#3 Mailinator

Mailinator

Mailinator is a free, Public, Email System where you can use any inbox you want. You will be given a Mailinator address which you can use anytime a website asks for an email address. The public emails you will receive will be auto-deleted after few hours of receiving.

#4 MailDrop

MailDrop

Maildrop is a great idea when you want to sign up for a website but you are concerned that they might share your address with advertisers. MailDrop is powered by some of the spam filters created by Heluna, used in order to block almost all spam attempts before they even get to your MailDrop inbox. This works the same way like Mailinator in which you will be given a temporary Email address which you can use to verify sites etc.

#5 AirMail

AirMail

AirMail is a free temporary email service, you are given a random email address you can use when registering to new websites or test-driving untrusted services. All emails received by AirMail servers are displayed automatically in your online browser inbox.

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OSWA™

"The OSWA™-Assistant is a self-contained, no Operating System required, freely downloadable, standalone toolkit which is solely focused on wireless auditing. As a result, in addition to the usual WiFi (802.11) auditing tools, it also covers Bluetooth and RFID auditing. Using the toolkit is as easy as popping it into your computer's CDROM and making your computer boot from it!" read more...

Website: http://oswa-assistant.securitystartshere.org

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DOWNLOAD SENTRY MBA V1.4.1 – AUTOMATED ACCOUNT CRACKING TOOL

Sentry MBA is an automated account cracking tool that makes it one of the most popular cracking tools. It is used by cybercriminals to take over user accounts on major websites. With Sentry MBA, criminals can rapidly test millions of usernames and passwords to see which ones are valid on a targeted website. The tool has become incredibly popular — the Shape Security research team sees Sentry MBA attack attempts on nearly every website we protect.  Download Sentry MBA v1.4.1 latest version.

FEATURES

Sentry MBA has a point-and-click graphical user interface, online help forums, and vibrant underground marketplaces to enable large numbers of individuals to become cybercriminals. These individuals no longer need advanced technical skills, specialized equipment, or insider knowledge to successfully attack major websites.

Sentry MBA attack has three phases,

• Targeting and attack refinement
• Automated account check
• Monetization

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How To Spoof PDF Signatures

One year ago, we received a contract as a PDF file. It was digitally signed. We looked at the document - ignoring the "certificate is not trusted" warning shown by the viewer - and asked ourselfs:

"How do PDF signatures exactly work?"

We are quite familiar with the security of message formats like XML and JSON. But nobody had an idea, how PDFs really work. So we started our research journey.

Today, we are happy to announce our results. In this blog post, we give an overview how PDF signatures work and on top, we reveal three novel attack classes for spoofing a digitally signed PDF document. We present our evaluation of 22 different PDF viewers and show 21 of them to be vulnerable. We additionally evaluated 8 online validation services and found 6 to be vulnerable.

In cooperation with the BSI-CERT, we contacted all vendors, provided proof-of-concept exploits, and helped them to fix the issues and three generic CVEs for each attack class were issued: CVE-2018-16042, CVE-2018-18688, CVE-2018-18689.


Full results are available in the master thesis of Karsten Meyer zu Selhausen, in our security report, and on our website.

Digitally Signed PDFs? Who the Hell uses this?

Maybe you asked yourself, if signed PDFs are important and who uses them.
In fact, you may have already used them.
Have you ever opened an Invoice by companies such as Amazon, Sixt, or Decathlon?
These PDFs are digitally signed and protected against modifications.
In fact, PDF signatures are widely deployed in our world. In 2000, President Bill Clinton enacted a federal law facilitating the use of electronic and digital signatures in interstate and foreign commerce by ensuring the validity and legal effect of contracts. He approved the eSign Act by digitally signing it.
Since 2014, organizations delivering public digital services in an EU member state are required to support digitally signed documents, which are even admissible as evidence in legal proceedings.
In Austria, every governmental authority digitally signs any official document [§19]. In addition, any new law is legally valid after its announcement within a digitally signed PDF.
Several countries like Brazil, Canada, the Russian Federation, and Japan also use and accept digitally signed documents.
According to Adobe Sign, the company processed 8 billion electronic and digital signatures in the 2017 alone.

Crash Course: PDF and PDF Signatures

To understand how to spoof PDF Signatures, we unfortunately need to explain the basics first. So here is a breef overview.

PDF files are ASCII files. You can use a common text editor to open them and read the source code.

PDF header. The header is the first line within a PDF and defines the interpreter version to be used. The provided example uses version PDF 1.7. 

PDF body. The body defines the content of the PDF and contains text blocks, fonts, images, and metadata regarding the file itself. The main building blocks within the body are objects. Each object starts with an object number followed by a generation number. The generation number should be incremented if additional changes are made to the object.

In the given example, the Body contains four objects: Catalog, Pages, Page, and stream. The Catalog object is the root object of the PDF file. It defines the document structure and can additionally declare access permissions. The Catalog refers to a Pages object which defines the number of the pages and a reference to each Page object (e.g., text columns). The Page object contains information how to build a single page. In the given example, it only contains a single string object "Hello World!".

Xref table. The Xref table contains information about the position (byte offset) of all PDF objects within the file.

Trailer. After a PDF file is read into memory, it is processed from the end to the beginning. By this means, the Trailer is the first processed content of a PDF file. It contains references to the Catalog and the Xref table.

How do PDF Signatures work?

PDF Signatures rely on a feature of the PDF specification called incremental saving (also known as incremental update), allowing the modification of a PDF file without changing the previous content.

 

As you can see in the figure on the left side, the original document is the same document as the one described above. By signing the document, an incremental saving is applied and the following content is added: a new Catalog, a Signature object, a new Xref table referencing the new object(s), and a new Trailer. The new Catalog extends the old one by adding a reference to the Signature object. The Signature object (5 0 obj) contains information regarding the applied cryptographic algorithms for hashing and signing the document. It additionally includes a Contents parameter containing a hex-encoded PKCS7 blob, which holds the certificates as well as the signature value created with the private key corresponding to the public key stored in the certificate. The ByteRange parameter defines which bytes of the PDF file are used as the hash input for the signature calculation and defines 2 integer tuples: 
a, b : Beginning at byte offset a, the following b bytes are used as the first input for the hash calculation. Typically, a 0 is used to indicate that the beginning of the file is used while a b is the byte offset where the PKCS#7 blob begins.
c, d : Typically, byte offset c is the end of the PKCS#7 blob, while c d points to the last byte range of the PDF file and is used as the second input to the hash calculation.

According to the specification, it is recommended to sign the whole file except for the PKCS#7 blob (located in the range between a b and c).

Attacks

During our research, we discovered three novel attack classes on PDF signatures:

1. Universal Signature Forgery (USF)
2. Incremental Saving Attack (ISA)
3. Signature Wrapping Attack (SWA)

In this blog post, we give an overview on the attacks without going into technical details. If you are more interested, just take a look at the sources we summarized for you here.

Universal Signature Forgery (USF)

The main idea of Universal Signature Forgery (USF) is to manipulate the meta information in the signature in such a way that the targeted viewer application opens the PDF file, finds the signature, but is unable to find all necessary data for its validation.

Instead of treating the missing information as an error, it shows that the contained signature is valid. For example, the attacker can manipulate the Contents or ByteRange values within the Signature object. The manipulation of these entries is reasoned by the fact that we either remove the signature value or the information stating which content is signed.
The attack seems trivial, but even very good implementations like Adobe Reader DC preventing all other attacks were susceptible against USF.

Incremental Saving Attack (ISA)

The Incremental Saving Attack (ISA) abuses a legitimate feature of the PDF specification, which allows to update a PDF file by appending the changes. The feature is used, for example, to store PDF annotations, or to add new pages while editing the file.

The main idea of the ISA is to use the same technique for changing elements, such as texts, or whole pages included in the signed PDF file to what the attacker desires.
In other words, an attacker can redefine the document's structure and content using the Body Updates part. The digital signature within the PDF file protects precisely the part of the file defined in the ByteRange. Since the incremental saving appends the Body Updates to the end of the file, it is not part of the defined ByteRange and thus not part of the signature's integrity protection. Summarized, the signature remains valid, while the Body Updates changed the displayed content.
This is not forbidden by the PDF specification, but the signature validation should indicate that the document has been altered after signing.

Signature Wrapping Attack (SWA)

Independently of the PDFs, the main idea behind Signature Wrapping Attacks is to force the verification logic to process different data than the application logic.

In PDF files, SWA targets the signature validation logic by relocating the originally signed content to a different position within the document and inserting new content at the allocated position. The starting point for the attack is the manipulation of the ByteRange value allowing to shift the signed content to different loctions within the file.

On a very technical level, the attacker uses a validly signed document (shown on the left side) and proceeds as follows:

• Step 1 (optional): The attacker deletes the padded zero Bytes within the Contents parameter to increase the available space for injecting manipulated objects.
• Step 2: The attacker defines a new /ByteRange [a b c* d] by manipulating the c value, which now points to the second signed part placed on a different position within the document.
• Step 3: The attacker creates a new Xref table pointing to the new objects. It is essential that the byte offset of the newly inserted Xref table has the same byte offset as the previous Xref table. The position is not changeable since it is refer- enced by the signed Trailer. For this purpose, the attacker can add a padding block (e.g., using whitespaces) before the new Xref table to fill the unused space.
• Step 4: The attacker injects malicious objects which are not protected by the signature. There are different injection points for these objects. They can be placed before or after the malicious Xref table. If Step 1 is not executed, it is only possible to place them after the malicious Xref table.
• Step 5 (optional): Some PDF viewers need a Trailer after the manipulated Xref table, otherwise they cannot open the PDF file or detect the manipulation and display a warning message. Copying the last Trailer is sufficient to bypass this limitation.
• Step 6: The attacker moves the signed content defined by c and d at byte offset c*. Optionally, the moved content can be encapsulated within a stream object. Noteworthy is the fact that the manipulated PDF file does not end with %%EOF after the endstream. The reason why some validators throw a warning that the file was manipulated after signing is because of an %%EOF after the signed one. To bypass this requirement, the PDF file is not correctly closed. However, it will be still processed by any viewer.

Evaluation

In our evaluation, we searched for desktop applications validating digitally signed PDF files. We analyzed the security of their signature validation process against our 3 attack classes. The 22 applications fulfill these requirements. We evaluated the latest versions of the applications on all supported platforms (Windows, MacOS, and Linux).

Authors of this Post

Vladislav Mladenov
Christian Mainka
Karsten Meyer zu Selhausen
Martin Grothe
Jörg Schwenk

Acknowledgements

Many thanks to the CERT-Bund team for the great support during the responsible disclosure.
We also want to acknowledge the teams which reacted to our report and fixed the vulnerable implementations.

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