CVE-2026-27795: The Chain Breaker: Bypassing LangChain's SSRF Guards

Source: Dev.to

Vulnerability Overview

Vulnerability ID: CVE-2026-27795
CVSS Score: 4.1 (Medium)
Published: 2026-02-25

A sophisticated Server-Side Request Forgery (SSRF) bypass was discovered in the @langchain/community package, specifically within the RecursiveUrlLoader. The component performed a safety check on the URL before fetching it, but allowed the underlying fetch client to automatically follow redirects. An attacker could supply a benign, validated URL that redirected to sensitive internal resources (e.g., AWS metadata service at 169.254.169.254), bypassing the initial security checks. This is a classic “Check‑Then‑Act” race condition.

TL;DR

LangChain’s URL loader validated a URL before fetching it, but the fetch client automatically followed redirects to unsafe destinations. An attacker could use a “safe” URL that redirects to 169.254.169.254 to steal cloud credentials.

⚠️ Exploit Status: Proof‑of‑Concept (PoC)

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Technical Details

• CWE ID: CWE-918 (Server‑Side Request Forgery)
• Attack Vector: Network
• Impact: Information Disclosure
• Exploit Status: PoC Available
• Patch Status: Released (v1.1.18)

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Affected Systems

• @langchain/community < 1.1.18 (fixed in 1.1.18)

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Code Analysis

Commit: 2812d2b – Fix handles redirects manually to prevent SSRF bypass.

fetch(currentUrl, { redirect: "manual" })

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Exploit Details

The unit tests added in the fix commit serve as a functional PoC for the redirect bypass. See the commit for the PoC implementation.

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Mitigation Strategies

• Upgrade @langchain/community to version 1.1.18 or later.
• Network‑level egress filtering to block access to metadata services (e.g., 169.254.169.254).
• Isolation: Run LLM workers in isolated containers or VPCs.

Remediation Steps

1. Identify projects using @langchain/community < 1.1.18.
2. Run npm install @langchain/community@latest or yarn upgrade @langchain/community.
3. Verify the installed version (node_modules/@langchain/community/package.json) is 1.1.18 or newer.
4. Review firewall rules to ensure 169.254.169.254 is blocked for application containers.

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References

• GitHub Security Advisory (GHSA-mphv-75cg-56wg)
• CWE‑918: Server‑Side Request Forgery (SSRF)