How severe is CVE-2021-31386?

CVE-2021-31386 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity none, and availability none.

Is CVE-2021-31386 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (48th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-31386?

CVE-2021-31386 primarily affects Juniper Junos. In total, 227 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-31386?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2021-31386 published?

CVE-2021-31386 was published on 2021-10-19 and last updated on 2026-06-17.

CVE-2021-31386

CVE-2021-31386 is a medium-severity vulnerability in Juniper Junos with a CVSS 3.x base score of 5.3. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-300.

Key facts

Severity: Medium (CVSS 3.x base score 5.3)
CVSS v2: 2.6
EPSS exploit prediction: 1% (48th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-300
Affected product: Juniper Junos
Published: 2021-10-19
Last modified: 2026-06-17

Description

A Protection Mechanism Failure vulnerability in the J-Web HTTP service of Juniper Networks Junos OS allows a remote unauthenticated attacker to perform Person-in-the-Middle (PitM) attacks against the device. This issue affects: Juniper Networks Junos OS 12.3 versions prior to 12.3R12-S20; 15.1 versions prior to 15.1R7-S11; 18.3 versions prior to 18.3R3-S6; 18.4 versions prior to 18.4R3-S10; 19.1 versions prior to 19.1R3-S7; 19.2 versions prior to 19.2R3-S4; 19.3 versions prior to 19.3R3-S4; 19.4 versions prior to 19.4R3-S6; 20.1 versions prior to 20.1R3-S2; 20.2 versions prior to 20.2R3-S3; 20.3 versions prior to 20.3R3-S1; 20.4 versions prior to 20.4R3; 21.1 versions prior to 21.1R3; 21.2 versions prior to 21.2R2.

Frequently asked questions

What is CVE-2021-31386?: A Protection Mechanism Failure vulnerability in the J-Web HTTP service of Juniper Networks Junos OS allows a remote unauthenticated attacker to perform Person-in-the-Middle (PitM) attacks against the device. This issue affects: Juniper Networks Junos OS 12.3 versions prior to 12.3R12-S20; 15.1 versions prior to 15.1R7-S11; 18.3 versions prior to 18.3R3-S6; 18.4 versions prior to 18.4R3-S10; 19.1 versions prior to 19.1R3-S7; 19.2 versions prior to 19.2R3-S4; 19.3 versions prior to 19.3R3-S4; 19.4 versions prior to 19.4R3-S6; 20.1 versions prior to 20.1R3-S2; 20.2 versions prior to 20.2R3-S3; 20.3 versions prior to 20.3R3-S1; 20.4 versions prior to 20.4R3; 21.1 versions prior to 21.1R3; 21.2 versions prior to 21.2R2.
How severe is CVE-2021-31386?: CVE-2021-31386 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity none, and availability none.
Is CVE-2021-31386 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (48th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-31386?: CVE-2021-31386 primarily affects Juniper Junos. In total, 227 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-31386?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2021-31386 published?: CVE-2021-31386 was published on 2021-10-19 and last updated on 2026-06-17.

References

https://kb.juniper.net/JSA11254

Affected products (227)

cpe:2.3:o:juniper:junos:12.3:-:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r1:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r10:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r10-s1:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r10-s2:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r11:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s1:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s10:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s11:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s12:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s13:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s14:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s15:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s16:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s17:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s18:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s19:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s3:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s4:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s6:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r12-s8:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r2:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r3:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r4:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r5:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r6:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r7:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r8:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:12.3:r9:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:15.1:-:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:15.1:a1:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:15.1:f:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:15.1:f1:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:15.1:f2:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:15.1:f2-s1:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:15.1:f2-s2:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:15.1:f2-s3:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:15.1:f2-s4:*:*:*:*:*:*
cpe:2.3:o:juniper:junos:15.1:f3:*:*:*:*:*:*

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