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CVE-2026-84971— Persistent client crash loop via undersized FLE2 insert-update ciphertext in decryption path

Quick assessment

Affected
MongoDB libmongocrypt
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

客户端加密库在解密路径中对意外值大小的处理不当，可能导致内部检查失败，从而终止使用该库的进程。攻击者若能将被适当构造的加密值放置到应用程序将要解密的位置，或能够控制应用程序所收到的响应，就可能导致该应用程序停止运行。

CVSS 6.5 · Medium

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 1

VendorProduct Version RangeStatus
MongoDB libmongocrypt 1.7.0< 1.20.4 affected
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I. Basic Information for CVE-2026-84971

Vulnerability Information
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Vulnerability Title
Persistent client crash loop via undersized FLE2 insert-update ciphertext in decryption path
Source: CVE Program / CVE List V5
Vulnerability Description
Improper handling of an unexpected value size in the decryption path of a client-side encryption library can cause a failed internal check that terminates the process using the library. A party able to place a suitably formed encrypted value where an application will decrypt it, or able to control the responses the application receives, may cause that application to stop running.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
可达断言
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
MongoDB libmongocrypt 1.7.0 ~ 1.20.4 -

II. Public POCs for CVE-2026-84971

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III. Intelligence Information for CVE-2026-84971

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Vendor Pages for CVE-2026-84971 (1)

Same Patch Batch · MongoDB · 2026-09-03 · 10 CVEs total

CVE-2026-84970 6.2 MEDIUM Heap over-read or silent misparse via 32-bit truncation of JSON length in BSON JSON parser
CVE-2026-84964 5.9 MEDIUM Heap corruption via OCSP request double free from crafted multi-URL certificate in TLS cli
CVE-2026-84963 5.3 MEDIUM Silent field truncation via unchecked int cast of huge JSON string values in JSON-to-BSON
CVE-2026-84968 5.3 MEDIUM Heap out-of-bounds read via corrupt nested BSON in field path error message
CVE-2026-84965 5.1 MEDIUM Heap write primitive via size round-up wrap during JSON parsing on 32-bit builds
CVE-2026-84966 5.1 MEDIUM BSON element injection via NUL-embedded document keys in builder append
CVE-2026-84967 4.3 MEDIUM Arbitrary command execution via shell-expanded connection string in Launch MongoDB Shell t
CVE-2026-84962 4.2 MEDIUM Authenticated KMS request forgery via CRLF injection in GCP key identifier strings
CVE-2026-84969 3.7 LOW Heap overflow via truncated base64 encoding of binary fields in length-limited JSON output

IV. Related Vulnerabilities

Same product: libmongocrypt
Same vendor: MongoDB
Same weakness: CWE-617

V. Comments for CVE-2026-84971

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