Researchers from the Horst Goertz Institute (HGI) of the Ruhr-University Bochum (RUB) in Germany have demonstrated an account hijacking attack against Amazon Web Services (AWS) that they believe affects other cloud computing products as well.
The attack uses a technique, known at XML signature wrapping or XML rewriting, that has been known since 2005 and exploits a weakness in the way Web services validate signed requests.
The flaw is located in the WS-Security (Web Services Security) protocol and enables attackers to trick servers into authorizing digitally signed SOAP (Simple Object Access Protocol) messages that have been altered.
"Wrapping attacks aim at injecting a faked element into the message structure so that a valid signature covers the unmodified element while the faked one is processed by the application logic. As a result, an attacker can perform an arbitrary Web Service request while authenticating as a legitimate user," the RUB experts explained in a research paper published in 2009.
The new practical attack against Amazon's cloud infrastructure was demonstrated at the ACM Conference on Computer and Communications Security last week and involved obtaining unauthorized access to an AWS account.
Using XML signature wrapping, the researchers were able to delete and create new images on the customer's EC2 (elastic cloud computing) instance and perform other administrative tasks.
In addition, a separate cross-site scripting (XSS) vulnerability in Amazon's store allowed the team to hijack an AWS session. "We had free access to all customer data, including authentication data, tokens, and even plain text passwords," said Mario Heiderich, who discovered the flaw together with colleagues Juraj Somorovsky and Meiko Jensenone.
"It's a chain reaction. A security gap in the complex Amazon shop always also directly causes a gap in the Amazon cloud," he explained.
Amazon was quick to address both issues, however, the researchers suspect that other cloud computing platform are affected as well. In fact, they already found that Eucalyptus, an open source solutions commonly used for private cloud computing infrastructure, is vulnerable to XML rewriting attacks.
"We find these flaws in nearly every implementation," said Joerg Schwenk, the chair for network and data security at the HGI and coordinator of the research team who identified the Amazon issues. "Other vendors are vulnerable, but we do not know of any other vendors using SOAP for accessing their cloud services," he concluded.
Amazon has recommended security best practices that would protect users from the XML signature wrapping attack which it claims was limited in scope to begin with. "This potential vulnerability involved a very small percentage of all authenticated AWS API calls that use non-SSL endpoints and was not a potentially widespread vulnerability as has been reported," an Amazon spokeswoman said.
The company also disputes the impact of the XSS vulnerability on confidential user data. "The process by which Amazon.com stores customer data would not enable researchers to see and expose information such as passwords or payment information as has been suggested," the spokeswoman added, while stressing that both vulnerabilities have been addressed before impacting any customers.
Schwenk's HGI team, however, also demonstrated a separate attack recently which exploits a vulnerability in the XML Encryption standard to decrypt secure communication between web services.
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