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CVE-2026-84971

Persistent client crash loop via undersized FLE2 insert-update ciphertext in decryption path

7.1 HighFix available

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.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
Low
User interaction
None

What an attacker can affect

Confidentiality
None
Integrity
None
Availability
High

Exploit likelihood

0.26%Up 0.1 points since Sep 4, 2026
90%50%10%1%

What each source reports

CVE List Score used 7.1

Persistent client crash loop via undersized FLE2 insert-update ciphertext in decryption path

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.

Record
CVE-2026-84971
CVSS 4.0
7.1 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-617
Updated
2026-11-02
EUVDEUVD-2026-70507 7.1

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.

Record
EUVD-2026-70507
CVSS 4.0
7.1 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-pxxc-vwf4-r3mr 7.1

Improper handling of an unexpected value size in the decryption path of a client-side encryption...

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.

Record
GHSA-pxxc-vwf4-r3mr
CVSS 4.0
7.1 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-617
Updated
2026-11-02
NVD 7.1

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.

Record
CVE-2026-84971
CVSS 4.0
7.1 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-617
Updated
2026-11-02
OSV 7.1

Persistent client crash loop via undersized FLE2 insert-update ciphertext in decryption path

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.

Record
CVE-2026-84971
CVSS 4.0
7.1 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Weakness
CWE-617
Updated
2026-11-02
OSVDEBIAN-CVE-2026-84971 7.1

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.

Record
DEBIAN-CVE-2026-84971
CVSS 4.0
7.1 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Updated
2026-11-02
OSVUBUNTU-CVE-2026-84971 7.1

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.

Record
UBUNTU-CVE-2026-84971
CVSS 4.0
7.1 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
CVSS 3.1
6.5 MediumCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
libmongocrypt
Debian:12
 All versions, plus listed versions
1.10.0-11.10.1-11.11.0-11.12.0-11.13.0-1
Show 18 moreShow less
1.13.1-11.13.2-11.16.0-11.17.0-11.17.3-11.18.1-11.18.2-11.19.0-11.19.1-11.20.0-11.20.1-11.20.2-11.20.3-11.20.4-11.7.2-11.8.2-11.8.4-11.9.1-1
 None known
libmongocrypt
Debian:13
 All versions, plus listed versions
1.13.2-11.13.2-1+deb13u11.16.0-11.17.0-11.17.3-1
Show 9 moreShow less
1.18.1-11.18.2-11.19.0-11.19.1-11.20.0-11.20.1-11.20.2-11.20.3-11.20.4-1
 None known
libmongocrypt
Debian:14
 before 1.20.4-1, plus listed versions
1.13.2-11.16.0-11.17.0-11.17.3-11.18.1-1
Show 7 moreShow less
1.18.2-11.19.0-11.19.1-11.20.0-11.20.1-11.20.2-11.20.3-1
1.20.4-1
libmongocrypt
Ubuntu:22.04:LTS
 All versions, plus listed versions
1.2.1-11.2.2-11.3.0-11.3.0-1ubuntu1
 None known
libmongocrypt
Ubuntu:24.04:LTS
 All versions, plus listed versions
1.7.2-11.8.2-11.8.4-11.8.4-1build21.8.4-1build3
 None known
libmongocrypt
Ubuntu:26.10
 All versions, plus listed versions
1.17.0-11.19.0-11.19.1-11.20.0-11.20.1-1build11.20.2-1
 None known
libmongocrypt
Ubuntu:26.04:LTS
 All versions, plus listed versions
1.13.2-11.17.0-1
 None known

History

History
RecordedChangedSources
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-84971 is a high vulnerability (CVSS 7.1) in Mongodb Libmongocrypt, published September 3, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.26%, the 17th percentile. Fixed in libmongocrypt 1.20.4-1.

References

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