CVE-2021-45960 is a high-severity vulnerability affecting versions of the Expat XML parsing library prior to 2.4.3. The flaw arises from an improper left bit-shift operation by 29 or more places within the storeAtts function in the xmlparse.c source file. This operation can cause realloc misbehavior, such as allocating insufficient memory or only freeing memory without proper reallocation. Expat is a widely used open-source XML parser implemented in C, commonly embedded in numerous software products and systems for XML processing. The vulnerability is categorized under CWE-682 (Incorrect Calculation), indicating a logic error in memory size calculation. Exploiting this flaw could lead to memory corruption, potentially resulting in denial of service (application crashes), or in some cases, arbitrary code execution if an attacker can control the input XML data. The CVSS v3.1 base score is 8.8, reflecting high impact on confidentiality, integrity, and availability, with network attack vector, low attack complexity, requiring privileges but no user interaction, and unchanged scope. No known exploits are currently reported in the wild, and no vendor patches are explicitly linked, though upgrading to Expat 2.4.3 or later is the recommended remediation. Given Expat's integration in many software stacks, the vulnerability poses a significant risk where vulnerable versions are used, especially in network-facing applications parsing untrusted XML data.

For European organizations, the impact of CVE-2021-45960 can be substantial depending on their reliance on software components embedding vulnerable versions of Expat. Potential impacts include service disruption due to application crashes, data breaches from memory corruption leading to unauthorized data access, and in worst cases, full system compromise if arbitrary code execution is achieved. Critical infrastructure sectors such as finance, healthcare, telecommunications, and government services often process XML data and may embed Expat in middleware, web services, or enterprise applications. An attacker exploiting this vulnerability remotely could disrupt business operations, compromise sensitive personal or corporate data, and undermine trust in digital services. The requirement for some level of privileges to exploit may limit exposure but does not eliminate risk, especially in multi-tenant or cloud environments where privilege boundaries can be complex. The lack of known exploits suggests limited active targeting currently, but the high CVSS score warrants proactive mitigation to prevent future attacks.

European organizations should take the following specific steps: 1) Inventory all software and systems that use Expat, including embedded devices, middleware, and third-party applications, to identify vulnerable versions prior to 2.4.3. 2) Prioritize upgrading Expat to version 2.4.3 or later, ensuring that all dependent software components are updated accordingly. 3) Where immediate upgrades are not feasible, implement compensating controls such as input validation and sanitization of XML data to reduce the risk of malicious payloads triggering the vulnerability. 4) Employ runtime protections like memory corruption mitigations (e.g., ASLR, DEP) and application sandboxing to limit exploitation impact. 5) Monitor network traffic and application logs for anomalous XML parsing errors or crashes that could indicate exploitation attempts. 6) Collaborate with software vendors to obtain patches or mitigations if embedded Expat cannot be updated independently. 7) Integrate this vulnerability into vulnerability management and incident response workflows to ensure timely detection and remediation.