CVE-2021-27378Network attack vector

Insufficient RNG entropy via permissive buffer check

Published Feb 18, 2021 · Updated Aug 3, 2024

Insufficient randomness in Rust rand_core 0.6.0 through 0.6.1 allows attackers to seed some downstream RNGs with short keys. The read_u32_into and read_u64_into functions multiply the source byte count instead of the destination word count in length assertions, so undersized sources pass and leave destination words unfilled. Exploitation requires downstream code to seed an affected RNG such as Hc128Rng through SeedableRng::from_seed; ChaCha RNGs are not affected by this behavior.

CVSS severity9.8
Critical
EPSS probability1.24%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Insufficient randomness in Rust rand_core 0.6.0 through 0.6.1 allows attackers to seed some downstream RNGs with short keys. The read_u32_into and read_u64_into functions multiply the source byte count instead of the destination word count in length assertions, so undersized sources pass and leave destination words unfilled. Exploitation requires downstream code to seed an affected RNG such as Hc128Rng through SeedableRng::from_seed; ChaCha RNGs are not affected by this behavior.

The record

CVE
CVE-2021-27378
Published
Feb 18, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-131, CWE-330
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 18, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8RustSec Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
9.8NIST National Vulnerability DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
9.8GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST National Vulnerability DatabaseAV:N/AC:L/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-131Incorrect Calculation of Buffer Size
CWE-330Use of Insufficiently Random Values

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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