The Silent Assassins: Understanding Signature Wrapping Attacks

Overview

A signature wrapping attack is a security vulnerability that can occur in digital signatures. A digital signature is a mathematical mechanism used to ensure the authenticity and integrity of a message or document.

In a signature-wrapping attack, an attacker modifies a signed message in such a way that the signature remains valid, while altering the meaning of the message. This can lead to unexpected repercussions such as unauthorized access to sensitive information or funds.

For example, in a header-wrapping attack, the attacker modifies the header of a signed message while preserving the signature. This can result in the signed message being redirected to a different recipient or being processed differently than intended.

In a body-wrapping attack, the attacker modifies the body of a signed message while preserving the signature. The changed message may reveal sensitive information or cause unauthorized actions.

In a certificate wrapping attack, the attacker replaces a certificate in a signed message with a different certificate that has a valid signature. This can result in the signed message being interpreted as having been signed by a different entity, potentially leading to security vulnerabilities.

TYPES

There are several types of signature wrapping attacks, including:

1. Header wrapping – Header wrapping modifies the header of a signed message while preserving the signature.
2. Body wrapping – It involves altering the body of a signed message while preserving the signature.
3. Certificate wrapping – Certificate wrapping involves replacing a certificate in a signed message with a different certificate that has a valid signature.
4. Timestamp wrapping – Timestamp wrapping changes the timestamp in a signed message, while preserving the signature.
5. Hash function collision attack – A hash function collision attack finds two different messages with the same hash value and uses one of the messages to replace the other in a signed message.
6. Key substitution attack – This type of attack involves replacing the public key in a signed message with a different key that has a valid signature.

PAYLOAD

A payload in the context of a signature wrapping attack refers to the modified message that the attacker creates. The payload is designed to preserve the validity of the digital signature while changing the meaning or intention of the original message.

Several types of payloads can be used in a signature wrapping attack, including:

1. Malicious code – This payload contains malicious code, such as malware, designed to compromise the recipient’s system.
2. Tampered data – A tampered data payload contains altered or falsified data, designed to mislead the recipient.
3. Redirected message – This payload redirects the signed message to a different recipient, potentially exposing sensitive information.
4. Altered message – An altered message payload changes the content or meaning of the signed message, potentially leading to unintended consequences.

It is important to implement proper security measures and use secure algorithms for digital signatures in order to prevent these types of payloads from being delivered. This can help to protect organizations and individuals from the potential harm that can result from a successful signature-wrapping attack.

REAL-TIME EXAMPLE

A real-world example of a signature wrapping attack occurred in 2011 when attackers targeted a certificate authority (CA) in order to perform a certificate wrapping attack. The attackers were able to obtain a valid digital certificate from the CA, which they then used to sign a payload containing malicious software.

This attack allowed the attackers to bypass security measures and deliver the payload to victims since the payload was signed with a valid certificate. The victims’ systems then trusted the payload as if it came from a trusted source, allowing the malware to execute and compromise the systems.

DEMONSTRATION

Consider a scenario where Alice wants to send a signed message to Bob that contains the text “Transfer $100 to Bob’s account”. Alice signs the message using a secure digital signature algorithm, creating a signed message that includes the original text and the signature.

An attacker, Eve, intercepts the signed message and modifies it, creating a new message with the text “Transfer $1000 to Eve’s account” while preserving the signature. The new message, which is now a payload, is then sent by Eve to Bob.

Since the payload has a valid signature, Bob’s system trusts the message as if it came from Alice. Bob processes the message, transferring $1000 to Eve’s account instead of having it transferred to his own account as intended.

This example demonstrates how a signature wrapping attack can lead to unintended consequences and highlights the importance of verifying the authenticity of signed messages before processing them.

HOW DOES IT WORK?

A signature wrapping attack works by modifying a signed message in such a way that the signature remains valid while changing the meaning or intention of the original message. The signature is computed using a secure digital signature algorithm, which generates a signature based on the content of the message.

In order for a signature wrapping attack to be successful, the attacker must modify the message in a specific way that preserves the validity of the signature. This can be executed in several ways.

1. A sender creates a message and signs it using a secure digital signature algorithm, which generates a signature based on the message’s content.
2. The attacker intercepts the signed message and modifies it, creating a new message with different content or intention while preserving the signature.
3. The attacker sends the modified message, also known as the payload, to the recipient.
4.  The recipient’s system trusts the message as if it came from the original sender since the signature is still valid.
5. The recipient processes the message, not realizing that its content or intention has been changed.

PREVENTION

A variety of steps can be taken to prevent signature wrapping attacks.

1. Verify the authenticity of signed messages:
   Before processing a signed message, the recipient should verify the authenticity of the message and the signature. This can be done by checking the certificate chain, the timestamps, and the digital signature algorithm used to sign the message.
2. Use secure digital signature algorithms:
   The digital signature algorithm used to sign messages should be secure and resistant to attacks. For example, algorithms such as RSA, DSA, and ECDSA are considered secure, while algorithms such as MD5 and SHA-1 are no longer considered secure and should be avoided.
3. Implement secure key management practices:
   The private key used to sign messages should be kept secure and protected from theft or loss. This can be achieved through the use of hardware security modules, key escrow systems, or secure key storage practices.
4. Use encrypted communication channels:
   Signatures and messages should be transmitted over encrypted communication channels to prevent eavesdropping and tampering.