CVE-2026-40087

MEDIUMNVD 5.35.3—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 5.3 under NVD CVSS v3. EPSS exploit probability: 0.3%, top 82% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
5.3EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 5.3Exploit: None knownExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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.

CVSS v3
5.3
EG Score
5.3(medium)
EG Risk
39(Track)
EG Risk 39/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity53% × 45%
Exploitation0% × 40%
Automatability100% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
18%
KEV
Not listed

Published

April 9, 2026

Last Modified

April 16, 2026

Advisory Details (7)

Auto-updated Aug 2, 2026
Patch available. Sources: github_release, github, github_commit, github_pr.
github Patch Available

Incomplete f-string validation in prompt templates · Advisory · langchain-ai/langchain · GitHub

https://github.com/langchain-ai/langchain/security/advisories/GHSA-926x-3r5x-gfhw
github_release Patch Available

langchain-core==1.2.28

Patch available: langchain-ai/langchain langchain-core==1.2.28

https://github.com/langchain-ai/langchain/releases/tag/langchain-core%3D%3D1.2.28
github_release Patch Available

langchain-core==0.3.84

Patch available: langchain-ai/langchain langchain-core==0.3.84

https://github.com/langchain-ai/langchain/releases/tag/langchain-core%3D%3D0.3.84
github_pr

fix(core): sanitize prompts more

Fix merged in langchain-ai/langchain PR #36613 on 2026-04-08 — awaiting tagged release

https://github.com/langchain-ai/langchain/pull/36613
github_pr Patch Available

fix(core): add more sanitization to templates

Patch available: langchain-ai/langchain langchain-openai==1.3.3 (PR #36612 merged 2026-04-08)

https://github.com/langchain-ai/langchain/pull/36612
github_commit Patch Available

commit af2ed47c6f00 (langchain-ai/langchain)

Patch available: langchain-ai/langchain langchain-openai==1.3.3 (contains commit af2ed47c6f00)

https://github.com/langchain-ai/langchain/commit/af2ed47c6f008cdd551f3c0d87db3774c8dfe258
github_commit

commit 6bab0ba3c123 (langchain-ai/langchain)

Fix landed in langchain-ai/langchain commit 6bab0ba3c123 — awaiting tagged release

https://github.com/langchain-ai/langchain/commit/6bab0ba3c12328008ddca3e0d54ff5a6151cd27b

Affected Packages

(1 across 1 ecosystem)
PyPI(1)
PackageVulnerable rangeFixed inDependents
langchain-core0.0.1 ... 1.2.9 (266 versions)1.2.28—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 7× in last 7d / 26× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 150 total refreshes for this CVE.

  1. 2026-09-15 03:11 UTCEPSS rescore
  2. 2026-09-13 16:47 UTCEPSS rescore
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  1. 2026-08-16 14:56 UTCEPSS rescore
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  40. 2026-07-20 20:32 UTCOSV refresh
  41. 2026-07-20 17:08 UTCEPSS rescore
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  75. 2026-06-21 14:56 UTCEPSS rescore

Frequently asked(5)

What is CVE-2026-40087?
CVE-2026-40087 is a medium vulnerability published on April 9, 2026. 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…
When was CVE-2026-40087 disclosed?
CVE-2026-40087 was first published in the National Vulnerability Database on April 9, 2026, with the most recent update on April 16, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-40087 actively exploited?
CVE-2026-40087 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 82.0% of all scored CVEs.
What is the CVSS score of CVE-2026-40087?
CVE-2026-40087 has a CVSS v3 base score of 5.3 (NVD).
How do I remediate CVE-2026-40087?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-40087, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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