Vulnerability · Published Apr 9, 2026 · Assigned by GitHub_M
CVE-2026-40087LangChain has incomplete f-string validation in prompt templates
Medium 5.3
In plain English
A medium-severity security flaw in langchain from langchain-ai. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 20% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| PyPI | langchain-core | 0.3.84 | GHSA-926x-3r5x-gfhw |
| PyPI | langchain-core | 1.2.28 | GHSA-926x-3r5x-gfhw |
| PyPI | langchain-core | 0.3.84, 1.2.28 | PYSEC-2026-2563 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| langchain-ai | langchain | < 0.3.83>= 1.0.0a1, < 1.2.28 |
Weakness: CWE-1336 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
References
- https://github.com/langchain-ai/langchain/security/advisories/GHSA-926x-3r5x-gfhw x_refsource_CONFIRM
- https://github.com/langchain-ai/langchain/pull/36612 x_refsource_MISC
- https://github.com/langchain-ai/langchain/pull/36613 x_refsource_MISC
- https://github.com/langchain-ai/langchain/commit/6bab0ba3c12328008ddca3e0d54ff5a6151cd27b x_refsource_MISC
- https://github.com/langchain-ai/langchain/commit/af2ed47c6f008cdd551f3c0d87db3774c8dfe258 x_refsource_MISC
- https://github.com/langchain-ai/langchain/releases/tag/langchain-core%3D%3D0.3.84 x_refsource_MISC
- https://github.com/langchain-ai/langchain/releases/tag/langchain-core%3D%3D1.2.28 x_refsource_MISC