How severe is CVE-2021-31875?

CVE-2021-31875 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-31875 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (80th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-31875?

CVE-2021-31875 affects Cesanta Mongooseos Mjs. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-31875?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2021-31875 published?

CVE-2021-31875 was published on 2021-04-29 and last updated on 2026-06-17.

CVE-2021-31875

CVE-2021-31875 is a critical-severity vulnerability in Cesanta Mongooseos Mjs with a CVSS 3.x base score of 9.8. 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-193.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 2% (80th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-193
Affected product: Cesanta Mongooseos Mjs
Published: 2021-04-29
Last modified: 2026-06-17

Description

In mjs_json.c in Cesanta MongooseOS mJS 1.26, a maliciously formed JSON string can trigger an off-by-one heap-based buffer overflow in mjs_json_parse, which can potentially lead to redirection of control flow. NOTE: the original reporter disputes the significance of this finding because "there isn’t very much of an opportunity to exploit this reliably for an information leak, so there isn’t any real security impact."

Frequently asked questions

What is CVE-2021-31875?: In mjs_json.c in Cesanta MongooseOS mJS 1.26, a maliciously formed JSON string can trigger an off-by-one heap-based buffer overflow in mjs_json_parse, which can potentially lead to redirection of control flow. NOTE: the original reporter disputes the significance of this finding because "there isn’t very much of an opportunity to exploit this reliably for an information leak, so there isn’t any real security impact."
How severe is CVE-2021-31875?: CVE-2021-31875 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-31875 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (80th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-31875?: CVE-2021-31875 affects Cesanta Mongooseos Mjs. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-31875?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2021-31875 published?: CVE-2021-31875 was published on 2021-04-29 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:cesanta:mongooseos_mjs:1.26:*:*:*:*:*:*:*

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