CVE-2011-1756 in Citadel
Summary
by MITRE
modules/xmpp/serv_xmpp.c in Citadel 7.86 and earlier does not properly detect recursion during entity expansion, which allows remote attackers to cause a denial of service (memory and CPU consumption) via a crafted XML document containing a large number of nested entity references, a similar issue to CVE-2003-1564.
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Analysis
by VulDB Data Team • 11/13/2021
The vulnerability described in CVE-2011-1756 represents a critical flaw in the Citadel messaging system version 7.86 and earlier, specifically within the XML processing module located at modules/xmpp/serv_xmpp.c. This issue manifests as an insufficient recursion detection mechanism during entity expansion operations, creating a pathway for remote attackers to exploit the system through carefully crafted XML documents. The vulnerability is particularly concerning because it enables attackers to consume excessive system resources, leading to denial of service conditions that can severely impact system availability and performance.
The technical implementation of this vulnerability stems from the improper handling of XML entity expansion processes within the Citadel XMPP server component. When processing XML documents containing deeply nested entity references, the system fails to implement adequate recursion depth checking mechanisms. This allows an attacker to construct XML payloads with numerous nested entity references that, when processed by the vulnerable software, trigger excessive memory allocation and CPU consumption patterns. The flaw operates by exploiting the standard XML parsing behavior where entity references can be recursively expanded, but the system lacks proper safeguards to prevent unlimited expansion sequences.
From an operational impact perspective, this vulnerability creates significant security risks for organizations relying on Citadel messaging systems. The denial of service conditions can result in complete service unavailability, affecting communication infrastructure and potentially disrupting business operations. Attackers can consume system resources rapidly through relatively small malicious XML documents, making this attack vector particularly efficient and difficult to detect. The resource exhaustion can manifest as memory leaks, high CPU utilization, or complete system crashes, depending on the target system configuration and resource constraints.
This vulnerability aligns with CWE-400, which categorizes the issue as an "Uncontrolled Resource Consumption" or "Resource Exhaustion" problem, specifically related to improper handling of XML entity expansion. The attack pattern follows the methodology outlined in the ATT&CK framework under the T1499.004 technique for "Endpoint Denial of Service" and potentially T1595.001 for "Network Denial of Service" when the attack targets network infrastructure components. The similarity to CVE-2003-1564 demonstrates a persistent pattern in XML processing vulnerabilities where entity expansion recursion is not properly constrained, indicating a fundamental weakness in XML parser implementations that has persisted across multiple years.
Organizations should implement immediate mitigations including upgrading to Citadel versions that address this vulnerability, implementing XML parsing restrictions, and deploying network-level controls to monitor and limit XML traffic patterns. System administrators should also consider implementing rate limiting mechanisms for XML processing, configuring memory limits for XML parsing operations, and establishing monitoring protocols to detect unusual resource consumption patterns that may indicate exploitation attempts. Additionally, regular security assessments should verify that XML processing components properly validate and constrain entity expansion to prevent similar vulnerabilities from manifesting in other software components.