LangChain has incomplete f-string validation in prompt templates

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

EPSS

• CVE-2026-40087: 0.26% estimated exploitation activity in the next 30 days; percentile 18.02 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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. Examples of the affected shape include: ```python "{message.additional_kwargs[secret]}" "https://example.com/{image.class.name}.png" ``` Second, f-string validation based on parsed top-level field names did not reject nested replacement fields inside format specifiers. For example: ```python "{name:{name.class.name}}" ``` 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…

Published by the source

8 Apr 2026, 21:51 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

24 Oct 2026, 04:13 UTC

1. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-926x-3r5x-gfhw

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# Patch Next advisory brief: CVE-2026-40087

Advisory: LangChain has incomplete f-string validation in prompt templates
IDs: CVE-2026-40087, GHSA-926x-3r5x-gfhw
Record: https://patchnext.com/advisories/cve-2026-40087 (revision 1, source checked 24 Oct 2026, 04:13 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: langchain-core 1.2.28, 0.3.84. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI langchain-core: affected < 0.3.84; >= 1.0.0a1, < 1.2.28; fixed in 0.3.84, 1.2.28

## Suggested inspection (read-only)
- `python -m pip show langchain-core`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on langchain-core.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-926x-3r5x-gfhw (OSV record GHSA-926x-3r5x-gfhw lists affected ranges: PyPI langchain-core >=0 =1.0.0a1 <1.2.28 (fixed))

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