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In July 2020, we introduced a novel attack class called Shadow Attacks. In our recent research, we discovered a new variant of the attack which relies only on an Incremental Update containing a malicious trailer.
A proof-of-concept exploit working on Foxit (Version: 11.0.1.49938) can be downloaded here.

The story so far ...

Shadow attacks are attacks bypassing the integrity protection of digitally signed PDF documents. The attacks abuse two legitimate features in PDF documents which we briefly explain.

Hiding Content

In PDFs, there are multiple techniques to hide content that is not displayed when the document is opened. We, as attackers, usually hide malicious objects without referencing them in the xref section.

Incremental Updates

New content can be appended to a signed PDF document. This is quite dangerous though. The digital signature in PDFs protects a specific range of bytes. Any appended content does not break the signature verification since it is outside this range. As a result, any new Incremental Update does not violate the cryptographic verification of the digital signature.

But, Incremental Updates are quite dangerous since they may completely change the displayed content of the document. In 2019, we showed different techniques based on Incremental Updates – the Incremental Saving Attacks.
As a countermeasure, most vendors warn if additional content is added after signing the document. BUT … not always!!!

There are meaningful use cases where Incremental Updates in digitally signed documents are allowed. For instance, contracts should be signed by multiple parties and each new signature is applied via additional Incremental Update.
Also, PAdES defines Incremental Updates as part of the long-term validation of digitally signed PDFs.
In summary, Incremental Updates are painful from a security perspective. Currently, vendors are trying to estimate whether an Incremental Update is malicious or not by analyzing its content.

Shadow Attacks

Shadow attacks, in general, deceive the PDF applications that an Incremental Update is not malicious. This can be done by providing an Incremental Update with minimal content.
In 2020, we estimated that appending an xref section and a trailer is sufficient to bypass the detection mechanisms of popular applications such as Adobe Reader and Foxit Reader.

Trailer-based Shadow Attack

Three months ago, we tried to reduce the content of the malicious Incremental Update. Our idea was to use only a malicious trailer and still change the content of the entire document when it is opened. Let's see how this can be done.

The Signer's view on the document

If a signer gets the document depicted on the left side, he or she sees the content "Sign the document to get a reward".
The document contains a hidden content depicted as red text – the 4 0 obj containing the text "You are fired. Get out immediately" and an xref section pointing to that object. However, the trailer references another xref section, see (1) and (2). Thus, the red text is never shown.
From the signer's perspective, there is no possibility to detect the hidden content by opening and reviewing the document.
As a result, the signer, for example the company director, signs the document.

The Victim's view on the document

We assume that the attacker receives the signed document and manipulates it.
The attacker appends only a trailer that points to the hidden malicious xref section (the red one). When the victim opens the document, the content "You are fired. Get out immediately" is shown.
However, the digital signature validation does not throw any warning since … well … what could go wrong if only a trailer is appended.

Honest vs. Malicious Trailer

There are small differences between the honest and the malicious trailer– the byte position of the xref section. Now, the trailer points to the hidden xref section.

trailer
<<
/Size 23
/Prev 18735
/Root 13 0 R
]>>
Honest trailer

trailer
<<
/Size 23
/Prev 19192
/Root 13 0 R
]>>
Malicious trailer

Impact and Exploit

We successfully applied the new attack on Foxit Reader (Version: 11.0.1.49938). We promptly reported the vulnerability and provided a Proof-of-Concept (PoC) exploit, known as CVE-2021-40326.
Foxit acknowledged the attack and published a security fix with the new version Foxit Reader 11.1.

We are not aware of any further implementations vulnerable to this attack.
If you think that your application might be vulnerable to the attack, then just download the exploit and test on your own.

Authors of this post

Vladislav Mladenov

Simon Rohlmann

Christian Mainka

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A WordPress plugin with over one million installs has been found to contain a critical vulnerability that could result in the execution of arbitrary code on compromised websites.

The plugin in question is Essential Addons for Elementor, which provides WordPress site owners with a library of over 80 elements and extensions to help design and customize pages and posts.

"This vulnerability allows any user, regardless of their authentication or authorization status, to perform a local file inclusion attack," Patchstack said in a report. "This attack can be used to include local files on the filesystem of the website, such as /etc/passwd. This can also be used to perform RCE by including a file with malicious PHP code that normally cannot be executed."

That said, the vulnerability only exists if widgets like dynamic gallery and product gallery are used, which utilize the vulnerable function, resulting in local file inclusion – an attack technique in which a web application is tricked into exposing or running arbitrary files on the webserver.

The flaw impacts all versions of the addon from 5.0.4 and below, and credited with discovering the vulnerability is researcher Wai Yan Myo Thet. Following responsible disclosure, the security hole was finally plugged in version 5.0.5 released on January 28 "after several insufficient patches."

The development comes weeks after it emerged that unidentified actors tampered with dozens of WordPress themes and plugins hosted on a developer's website to inject a backdoor with the goal of infecting further sites.

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What is cyber crime?

Cybercrime is the use of computers & networks to perform illegal activities such as spreading viruses,online bullying,performing unauthorized electronic fund transfers etc. Most cyber crimes are committed through the internet.
Some cyber crime also be carried out using mobile phones via Sms and online chatting applications.

TYPES OF CYBERCRIME

The following list presents the common types of cybercrimes-

1-Computer Fraud-Intential deception for personal gain via the use of computer system.

2-Privacy Violations-Exposing personal information such as email addresses,phone numbers,account details etc, on social media,websites,etc.

3-Identity theft-Stealing personal information from somebody and impersonating that person.

4-Sharing copyright files/information-This involves distributing copyright protected files such as eBooks and computer program etc.

5-Electronic funds transfer-This involves gaining an unauthorized access to bank computer networks and making illegal funds transferring.

6-Electronic money laundering-This involves the use of the computer to launder money.

7-Atm fraud-This involves intercepting ATM card details such as account numbers and PIN numbers.These details are then used to withdraw funds from the intercepted accounts.

8-Denial of service attack-This involves the use of computers in multiple locations to attack servers with a view of shutting them down.

9-Spam:sending unauthorized emails.

These emails usually contain advertisements.

CYBER LAW

Under The Information Technology Act,2000

CHAPTER XI-OFFENCES-66. Hacking with computer system.

1-whoever with the Intent to cause or knowing that he is likely to cause Wrongfull Loss or Damage to the public or any person Destroys or Deletes or Alter any Information Residing in computer Resource or diminishes its value or utility or affects it injuriously by any means, commits hack.

2-whoever commits hacking shell be punished with imprisonment up to three years, or with fine which may extend up to two lakh rupees,or with both.

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2024年1月27日星期六

Shadow Attacks … The Smallest Attack Vector Ever

The story so far ...

Hiding Content

Incremental Updates

Shadow Attacks

Trailer-based Shadow Attack

The Signer's view on the document

The Victim's view on the document

Honest vs. Malicious Trailer

Impact and Exploit

Authors of this post

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Critical Bug Found In WordPress Plugin For Elementor With Over A Million Installations

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What Is Cybercrime? What Are The Types Of Cybercrime? What Is Cyberlaw In India?

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