CVE-2011-1753 in ejabberd
Summary
by MITRE
expat_erl.c in ejabberd before 2.1.7 and 3.x before 3.0.0-alpha-3, and exmpp before 0.9.7, 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 identified as CVE-2011-1753 represents a critical denial of service flaw affecting XML processing libraries in instant messaging and communication platforms. This issue specifically targets the expat_erl.c component within ejabberd messaging server versions prior to 2.1.7 and 3.x versions before 3.0.0-alpha-3, as well as the exmpp library before version 0.9.7. The flaw stems from inadequate recursion detection mechanisms during entity expansion processes, creating a pathway for malicious actors to exploit the system through carefully crafted XML documents.
The technical implementation of this vulnerability involves the exploitation of XML entity expansion recursion without proper safeguards against infinite loops or excessive resource consumption. When a malicious XML document contains a large number of nested entity references, the vulnerable software fails to detect and terminate the recursive expansion process, leading to unbounded memory allocation and excessive cpu utilization. This behavior creates a classic resource exhaustion scenario where the system becomes unresponsive or crashes entirely due to the continuous consumption of computational resources.
From an operational perspective, this vulnerability poses significant risks to communication infrastructure and services that rely on XML processing for message handling and data exchange. The impact extends beyond simple service disruption to potentially affecting entire communication networks, particularly in environments where ejabberd or similar messaging systems serve as core infrastructure components. Attackers can leverage this vulnerability to perform resource exhaustion attacks that may affect availability of service for legitimate users, making it particularly dangerous in mission-critical communication environments.
The vulnerability aligns with CWE-400, which categorizes improper handling of resource exhaustion conditions, and demonstrates characteristics similar to CVE-2003-1564, indicating a recurring pattern in XML parser implementations where entity expansion recursion is not properly controlled. This flaw falls under the attack pattern taxonomy of resource exhaustion attacks and specifically relates to the technique of exploiting XML parsing vulnerabilities for denial of service purposes. The attack surface is particularly broad given the widespread adoption of ejabberd and similar messaging platforms in enterprise and open source communication environments, making this vulnerability particularly impactful across multiple deployment scenarios.
Mitigation strategies should include immediate patching of affected software versions to the corrected releases, implementing xml parser configuration limits on entity expansion depth and size, and deploying network level controls to monitor and restrict xml traffic patterns that exhibit recursive entity expansion behaviors. Organizations should also consider implementing rate limiting mechanisms and resource monitoring to detect and prevent exploitation attempts, while establishing proper xml validation and sanitization processes to prevent malformed documents from reaching vulnerable parsers. The fix typically involves implementing proper recursion depth checking and establishing maximum limits on entity expansion to prevent unbounded resource consumption during xml processing operations.