Bleichenbacher Padding Oracle in wolfSSL PKCS#7 KTRI Decryption

Executive Summary

This vulnerability is a Bleichenbacher padding oracle issue in the PKCS#7 KTRI decryption process used by wolfSSL. When decrypting PKCS#7 EnvelopedData with RSA PKCS#1 v1.5 key transport, wolfSSL returned different error codes depending on whether the RSA padding validation failed or the decrypted content was malformed. An attacker who can send specially crafted EnvelopedData messages and observe these error responses could exploit this difference as a padding oracle to gradually recover the encrypted Content Encryption Key (CEK).

The vulnerability was fixed by making the system generate a deterministic pseudo-random fake CEK on padding failure and proceeding with decryption in a way that all failure paths produce the same error message, using constant-time operations to prevent attackers from distinguishing between different failure causes.

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Impact Analysis

This vulnerability can allow an attacker to incrementally recover the encrypted Content Encryption Key (CEK) by exploiting the distinguishable error messages during decryption. With the CEK, the attacker could potentially decrypt sensitive encrypted data that was intended to be protected, leading to a compromise of confidentiality.

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Mitigation Strategies

To mitigate this vulnerability, you should update wolfSSL to a version that includes the fix for the Bleichenbacher padding oracle issue in PKCS#7 KTRI decryption.

The fix involves generating a deterministic pseudo-random fake Content Encryption Key (CEK) on padding failure and using constant-time operations to ensure all failure paths produce the same error, preventing attackers from distinguishing error responses.

Applying the patch from the wolfSSL repository pull request #10203, which addresses multiple PKCS#7 issues including this vulnerability, is recommended.

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Compliance Impact

The provided context and resources do not explicitly discuss the impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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Detection Guidance

This vulnerability involves a Bleichenbacher padding oracle in PKCS#7 KTRI decryption where an attacker can submit crafted EnvelopedData messages and observe distinguishable error responses. Detection would involve monitoring for unusual or repeated PKCS#7 EnvelopedData messages that trigger different error codes during RSA PKCS#1 v1.5 key transport decryption.

Since the vulnerability manifests through error code differences, one detection approach is to analyze logs or error messages from wolfSSL implementations for inconsistent or distinguishable padding validation errors.

Specific commands to detect this vulnerability are not provided in the available resources. However, general network or system commands that could help include:

• Using packet capture tools like tcpdump or Wireshark to capture and analyze PKCS#7 EnvelopedData messages on the network.
• Reviewing application or system logs for error messages related to PKCS#7 decryption failures.
• Testing with crafted PKCS#7 EnvelopedData messages to observe if the system returns distinguishable error codes, which would require custom scripts or tools.

For precise detection commands or scripts, further information or tools specific to wolfSSL or the affected application would be necessary.

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