CVE-2025-9556 (CVSS 9.8):Critical Vulnerability in LangChainGo Puts LLM Apps at Risk

CVE-2025-9556 (CVSS 9.8) is a critical vulnerability in LangChainGo, the Go implementation of LangChain, that exposes LLM applications to server-side weaknesses. According to CERT/CC Vulnerability Note, the flaw allows arbitrary file reads through the Gonja template engine, enabling attackers to inject malicious prompt content and trigger a server-side template injection (SSTI) attack.

LangChainGo leverages Gonja, which supports Jinja2 syntax, and the advisory notes that directives such as {% include %}, {% from %}, or {% extends %} can be misused within LangChainGo. By embedding crafted prompts, an attacker could cause reads of sensitive files like /etc/passwd, turning ordinary prompts into exploitable vectors.

The maintainers have acted quickly and a patch introduces security features, including a new RenderTemplateFS function that blocks filesystem access by default and supports secure file template referencing, according to CERT/CC. Developers are urged to update to the latest version immediately or, if upgrading is not possible, restrict user input and monitor for suspicious prompt activity.