CVE-2017-7813 in Firefox
Summary
by MITRE
Inside the JavaScript parser, a cast of an integer to a narrower type can result in data read from outside the buffer being parsed. This usually results in a non-exploitable crash, but can leak a limited amount of information from memory if it matches JavaScript identifier syntax. This vulnerability affects Firefox < 56.
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Analysis
by VulDB Data Team • 01/15/2021
The vulnerability described in CVE-2017-7813 represents a critical type conversion issue within Firefox's JavaScript parser that demonstrates the dangerous consequences of improper integer handling in memory management. This flaw exists in the parser's handling of buffer boundaries when casting integers to narrower data types, creating a scenario where memory reads extend beyond allocated buffer limits. The vulnerability specifically impacts Firefox versions prior to 56, making it a significant concern for users operating older browser versions. The technical nature of this issue aligns with CWE-194, which addresses the improper handling of integer truncation and casting operations that can lead to buffer over-read conditions.
The operational impact of this vulnerability manifests through the potential for information disclosure rather than direct exploitation. When the parser encounters malformed input, the integer casting operation causes data from adjacent memory locations to be interpreted as valid JavaScript syntax elements. This occurs because the narrowing cast allows values that exceed the bounds of the target type to be interpreted incorrectly, leading to memory access patterns that extend beyond intended buffer boundaries. The vulnerability's limited exploitable nature stems from the requirement that the leaked memory contents must conform to JavaScript identifier syntax for the issue to manifest as meaningful information leakage rather than random data. This characteristic places the vulnerability in the ATT&CK framework under technique T1059.007 for JavaScript execution and T1005 for data from local system.
The memory leak aspect of this vulnerability demonstrates how seemingly benign parsing operations can expose sensitive information through indirect memory access patterns. Attackers could potentially leverage this issue to gather information about memory layout, potentially aiding in more sophisticated exploitation attempts or helping to understand the target system's architecture. The fact that this vulnerability typically results in non-exploitable crashes rather than direct code execution indicates that it primarily functions as an information disclosure mechanism rather than a primary attack vector. However, the leaked memory information could provide valuable insights into the browser's memory management, potentially revealing stack or heap contents that might be useful for crafting more advanced attacks. Security researchers categorize this type of vulnerability under the broader class of buffer over-read conditions that can expose system information and compromise privacy through indirect means.
Mitigation strategies for CVE-2017-7813 primarily focus on updating to Firefox version 56 or later, where the parsing logic has been corrected to properly handle integer casting operations and buffer boundary checks. System administrators should prioritize patch management to ensure all users are operating within the secure version range, as the vulnerability's information disclosure potential increases the risk profile significantly. Additional defensive measures include implementing network-level restrictions to limit exposure to potentially malicious JavaScript content, though the most effective protection remains the application of vendor-provided security patches. The vulnerability serves as a reminder of the importance of rigorous input validation and proper type handling in security-critical applications, particularly in parsing components that must handle untrusted data from external sources.