CVE-2019-12401 in Solr
Summary
by MITRE
Solr versions 1.3.0 to 1.4.1, 3.1.0 to 3.6.2 and 4.0.0 to 4.10.4 are vulnerable to an XML resource consumption attack (a.k.a. Lol Bomb) via it?s update handler.?By leveraging XML DOCTYPE and ENTITY type elements, the attacker can create a pattern that will expand when the server parses the XML causing OOMs.
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Analysis
by VulDB Data Team • 08/18/2020
The vulnerability identified as CVE-2019-12401 represents a critical XML external entity processing flaw in Apache Solr versions ranging from 1.3.0 through 1.4.1, 3.1.0 through 3.6.2, and 4.0.0 through 4.10.4. This issue falls under the category of XML external entity injection attacks, specifically exploiting the XML parsing mechanisms within the Solr update handler component. The vulnerability is particularly dangerous because it allows attackers to craft malicious XML payloads that can trigger excessive memory consumption during XML processing, leading to resource exhaustion and potential system instability.
The technical implementation of this vulnerability relies on the exploitation of XML DOCTYPE and ENTITY declaration elements that are processed by Solr's XML parser. When an attacker submits a malicious XML document containing carefully crafted ENTITY definitions, the parser recursively expands these entities during processing, creating exponential memory consumption patterns. This particular attack pattern is commonly referred to as a "Lol Bomb" due to the way it causes the parser to consume memory in a manner that resembles a denial of service attack. The vulnerability occurs because Solr's XML processing does not properly restrict the depth or expansion of XML entities, allowing recursive entity references to consume system resources without adequate bounds checking.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as it can lead to complete system compromise and service disruption. When the XML parser encounters maliciously constructed entity expansions, it consumes memory rapidly until the system runs out of available resources, resulting in out-of-memory errors and process termination. This resource exhaustion can affect not only the Solr service itself but also the underlying operating system, potentially causing cascading failures that impact other applications running on the same host. The vulnerability is particularly concerning in enterprise environments where Solr serves as a critical component for search and data processing services, as it can be exploited by attackers to disrupt business operations and potentially gain further access to the system.
Mitigation strategies for this vulnerability should focus on implementing proper XML parsing restrictions and input validation measures. Organizations should immediately upgrade to patched versions of Solr that address this specific XML entity expansion issue, as the vulnerability has been resolved in later releases. Additionally, administrators should configure Solr to disable external entity processing entirely by setting appropriate parser configurations and implementing strict XML schema validation. The implementation of rate limiting and resource quotas for XML processing operations can also help prevent exploitation attempts. From a cybersecurity perspective, this vulnerability aligns with CWE-611 (Improper Restriction of XML External Entity Reference) and maps to ATT&CK technique T1499.004 (Endpoint Denial of Service) in the enterprise attack framework, highlighting the need for comprehensive security controls that address both the immediate vulnerability and broader system resilience requirements.
The remediation process should include thorough security auditing of all Solr installations to identify affected versions and ensure proper patching. System administrators should also implement monitoring solutions that can detect unusual XML processing patterns and memory consumption spikes that may indicate exploitation attempts. Network segmentation and access controls should be enforced to limit exposure of vulnerable Solr instances to untrusted networks, while regular security assessments should verify that XML processing configurations remain secure against similar attack vectors.