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CVE-2026-40087— LangChain has incomplete f-string validation in prompt templates

Quick assessment

Affected
langchain-ai langchain
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

LangChain是LangChain开源的一个用于开发由大型语言模型 （LLM） 提供支持的应用程序的框架。 LangChain 0.3.84之前版本和1.2.28之前版本存在安全漏洞，该漏洞源于f字符串提示模板验证不完整，可能导致属性访问或索引表达式在格式化时被求值。

CVSS 5.3 · Medium EPSS 0.45% · P37

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application
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I. Basic Information for CVE-2026-40087

Vulnerability Information
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Vulnerability Title
LangChain has incomplete f-string validation in prompt templates
Source: CVE Program / CVE List V5
Vulnerability Description
LangChain is a framework for building agents and LLM-powered applications. Prior to 0.3.84 and 1.2.28, LangChain's f-string prompt-template validation was incomplete in two respects. First, some prompt template classes accepted f-string templates and formatted them without enforcing the same attribute-access validation as PromptTemplate. In particular, DictPromptTemplate and ImagePromptTemplate could accept templates containing attribute access or indexing expressions and subsequently evaluate those expressions during formatting. Second, f-string validation based on parsed top-level field names did not reject nested replacement fields inside format specifiers. In this pattern, the nested replacement field appears in the format specifier rather than in the top-level field name. As a result, earlier validation based on parsed field names did not reject the template even though Python formatting would still attempt to resolve the nested expression at runtime. This vulnerability is fixed in 0.3.84 and 1.2.28.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
CWE-1336
Source: CVE Program / CVE List V5
Vulnerability Title
LangChain 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
LangChain是LangChain开源的一个用于开发由大型语言模型 （LLM） 提供支持的应用程序的框架。 LangChain 0.3.84之前版本和1.2.28之前版本存在安全漏洞，该漏洞源于f字符串提示模板验证不完整，可能导致属性访问或索引表达式在格式化时被求值。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
langchain-ai langchain < 0.3.83 -

II. Public POCs for CVE-2026-40087

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III. Intelligence Information for CVE-2026-40087

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Patches & Fixes for CVE-2026-40087 (3)

Vendor Advisories for CVE-2026-40087 (1)

IV. Related Vulnerabilities

Same product: langchain
Same vendor: langchain-ai
Same weakness: CWE-1336

V. Comments for CVE-2026-40087

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