CVE-2026-40087
Description
A flaw was found in LangChain. A missing validation of f-string prompt templates in some classes, specifically in DictPromptTemplate and ImagePromptTemplate, can cause the evaluation of attribute access or indexing expressions during template formatting. Also, f-string validation based on parsed top-level field names did not reject nested replacement fields inside format specifiers. A remote attacker can exploit this issue by providing a specially crafted prompt template, potentially leading to information disclosure.
Statement
To exploit this vulnerability, an attacker needs to supply a specially crafted input to the DictPromptTemplate or ImagePromptTemplate classes. The payload must contain Python attribute access expressions (e.g., accessing internal object classes), which bypass the template validator and get executed by the underlying f-string formatter, potentially causing information disclosure. There is no memory corruption or arbitrary code execution. Due to these reasons, this flaw has been rated with a moderate severity.
Mitigation
To mitigate this vulnerability, validate and sanitize any user-supplied input before it is passed into a DictPromptTemplate or ImagePromptTemplate. Reject any input containing curly braces unless they are strictly necessary and controlled. Furthermore, strip or reject input strings that attempt to use nested replacement fields (e.g., fields hidden within format specifiers like {value:{nested_format}}).
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.3 | N/A | N/A |
| Attack Vector | Network | N/A | N/A |
| Attack Complexity | Low | N/A | N/A |
| Privileges Required | None | N/A | N/A |
| User Interaction | None | N/A | N/A |
| Scope | Unchanged | N/A | N/A |
| Confidentiality | Low | N/A | N/A |
| Integrity Impact | None | N/A | N/A |
| Availability Impact | None | N/A | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Understanding the Weakness (CWE)
Integrity
Technical Impact: Execute Unauthorized Code or Commands
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