CVE-2025-3857 is a vulnerability identified in the Amazon Ion Dotnet library, a .NET implementation for reading and writing Amazon Ion data, a richly-typed, self-describing data serialization format. The issue resides in the RawBinaryReader class responsible for deserializing binary Ion data streams. When the library reads Ion data, it fails to check the number of bytes actually read from the underlying stream during deserialization. If the Ion data is malformed or truncated—such as incomplete or corrupted binary Ion input—the reader enters an infinite loop due to an unreachable exit condition in the loop logic. This infinite loop consumes CPU resources indefinitely, causing a denial of service by making the application unresponsive or significantly degraded. The vulnerability is classified under CWE-835 (Loop with Unreachable Exit Condition) and CWE-502 (Deserialization of Untrusted Data). The CVSS 4.0 base score is 8.7, indicating high severity, with an attack vector of network (AV:N), no required privileges (PR:N), no user interaction (UI:N), and a high impact on availability (VA:H). No known exploits have been reported yet, but the vulnerability is critical for any system processing untrusted Ion data streams. The recommended remediation is to upgrade to Amazon.IonDotnet version 1.3.1, which includes the necessary fixes to prevent the infinite loop condition. Organizations maintaining forks or derivative versions of the library must also apply equivalent patches to mitigate the risk.

For European organizations, the primary impact of CVE-2025-3857 is a denial of service condition that can disrupt applications or services relying on Amazon Ion Dotnet for data serialization and deserialization. This can affect availability of critical systems, especially those processing large volumes of Ion data or handling untrusted input from external sources. Industries such as finance, telecommunications, cloud service providers, and government agencies that use Amazon Ion for structured data exchange may experience service outages or degraded performance. The infinite loop can lead to resource exhaustion, potentially cascading into broader system instability or downtime. While confidentiality and integrity are not directly impacted, the availability disruption could have significant operational and reputational consequences. Since no authentication or user interaction is required, attackers can exploit this remotely by sending malformed Ion data streams, increasing the risk of widespread disruption. Organizations with automated data pipelines or APIs accepting Ion data are particularly vulnerable.

European organizations should immediately upgrade all instances of Amazon.IonDotnet to version 1.3.1 or later to incorporate the fix that prevents the infinite loop condition. For any internally maintained forks or derivative libraries based on Amazon Ion Dotnet, equivalent patches must be applied to ensure the vulnerability is remediated. Additionally, implement input validation and sanity checks on Ion data streams before deserialization to detect and reject malformed or truncated data early. Employ rate limiting and anomaly detection on network endpoints that accept Ion data to mitigate potential denial of service attempts. Monitor application logs and system metrics for signs of high CPU usage or unresponsive behavior indicative of infinite loops. Where feasible, isolate services handling Ion data in containerized or sandboxed environments to limit impact scope. Finally, maintain an inventory of all software components using Amazon Ion Dotnet to ensure comprehensive patch coverage and reduce attack surface.