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RewindexCVEsLangChain (LangSmith)

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

Medium · CVSS 5.3 (CVSS 3.1)EPSS 0.26%, 18th percentilePublished Updated Assigner: GitHub_M

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.

Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N · CWE-1336

Tracked companies

LangChain (LangSmith)

Mapped from the CVE’s affected-product data, NVD CPE matches, vendor advisories or package feeds. A mapping means the vendor’s software is named, not that its hosted service was exposed.

Status incidents that name it

No tracked status incident names this CVE.

Affected vendors and products

langchain-ai: langchain

References

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