CVE-2018-14647 in Python
Summary
by MITRE
Python's elementtree C accelerator failed to initialise Expat's hash salt during initialization. This could make it easy to conduct denial of service attacks against Expat by contructing an XML document that would cause pathological hash collisions in Expat's internal data structures, consuming large amounts CPU and RAM. Python 3.8, 3.7, 3.6, 3.5, 3.4, 2.7 are believed to be vulnerable.
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Analysis
by VulDB Data Team • 05/17/2023
The vulnerability identified as CVE-2018-14647 represents a critical security flaw in Python's elementtree C accelerator component that directly impacts the underlying Expat XML parser. This issue stems from the failure to properly initialize Expat's hash salt during the component's initialization phase, creating a predictable hashing behavior that adversaries can exploit. The vulnerability affects multiple Python versions including 2.7, 3.4, 3.5, 3.6, 3.7, and 3.8, making it a widespread concern across the Python ecosystem. The root cause aligns with CWE-327, which addresses the use of weak cryptographic algorithms or predictable random number generators, and specifically relates to improper initialization of security parameters in hash functions.
The technical exploitation of this vulnerability enables attackers to construct specially crafted XML documents that deliberately trigger pathological hash collisions within Expat's internal hash tables. When the XML parser processes these malicious documents, the collision patterns cause the hash table to degenerate into linked lists, dramatically increasing the time complexity of hash operations from O(1) to O(n) where n represents the number of elements in the colliding bucket. This degradation leads to exponential CPU consumption as the parser must traverse increasingly long chains to resolve hash conflicts, while simultaneously consuming excessive RAM as the hash table structures expand to accommodate the collision chains. The vulnerability operates at the intersection of CWE-400, which addresses resource exhaustion vulnerabilities, and CWE-326, which deals with inadequate encryption strength, as the predictable hash behavior creates a pathway for resource exhaustion attacks.
The operational impact of CVE-2018-14647 extends beyond simple denial of service scenarios to encompass significant system stability and availability concerns. Attackers can leverage this vulnerability to consume massive amounts of computational resources with relatively small input payloads, making it particularly dangerous in environments where XML processing is common such as web services, API endpoints, and document processing systems. The vulnerability's effectiveness is amplified by the fact that it can be triggered through any XML parsing operation that utilizes Python's elementtree module, including those in widely deployed applications like web frameworks, content management systems, and enterprise software that rely on XML for data exchange. This makes the attack surface particularly broad and the potential for widespread disruption significant.
Mitigation strategies for CVE-2018-14647 must address both immediate protection and long-term architectural improvements. The primary recommended solution involves upgrading to Python versions that have patched the vulnerability, specifically Python 3.7.3, 3.6.8, 3.5.7, and 2.7.15, which contain the necessary fixes to properly initialize Expat's hash salt. Organizations should also implement XML input validation and size limiting measures as defensive controls, preventing untrusted XML data from reaching vulnerable parsers. Additionally, deploying rate limiting and resource monitoring mechanisms can help detect and mitigate exploitation attempts. The vulnerability demonstrates the importance of proper cryptographic initialization practices and aligns with ATT&CK technique T1499.004, which covers network denial of service attacks, and T1059.007, which addresses script-based attacks, as the exploitation requires crafting malicious XML content that leverages the underlying hash collision weakness in the parsing infrastructure.