CVE-2021-42574

An issue was discovered in the Bidirectional Algorithm in the Unicode Specification through 14.0. It permits the visual reordering of characters via control sequences, which can be used to craft source code that renders different logic than the logical ordering of tokens ingested by compilers and interpreters. Adversaries can leverage this to encode source code for compilers accepting Unicode such that targeted vulnerabilities are introduced invisibly to human reviewers. NOTE: the Unicode Consortium offers the following alternative approach to presenting this concern. An issue is noted in the nature of international text that can affect applications that implement support for The Unicode Standard and the Unicode Bidirectional Algorithm (all versions). Due to text display behavior when text includes left-to-right and right-to-left characters, the visual order of tokens may be different from their logical order. Additionally, control characters needed to fully support the requirements of bidirectional text can further obfuscate the logical order of tokens. Unless mitigated, an adversary could craft source code such that the ordering of tokens perceived by human reviewers does not match what will be processed by a compiler/interpreter/etc. The Unicode Consortium has documented this class of vulnerability in its document, Unicode Technical Report #36, Unicode Security Considerations. The Unicode Consortium also provides guidance on mitigations for this class of issues in Unicode Technical Standard #39, Unicode Security Mechanisms, and in Unicode Standard Annex #31, Unicode Identifier and Pattern Syntax. Also, the BIDI specification allows applications to tailor the implementation in ways that can mitigate misleading visual reordering in program text; see HL4 in Unicode Standard Annex #9, Unicode Bidirectional Algorithm.

CVSS: 8.3High
Exploited in the wild: Not knownUnvetted proof-of-concept code on GitHub; not in CISA KEV
EPSS: 12.9%chance of exploitation in the next 30 daystop 4% of scored CVEsFIRST score of 11 Nov 2026
Fix: Patch availablesee the advisories in the references
Published: 1 Nov 2021analysed here 12 Nov 2026

Summary

Triage

Attack surface: Server-side
Exploitable: At runtime
Attack vector: network
Weakness: CWE-94 Code Injection
Can a WAF block it?: Partly. See why

The summary, impact and action text on this page are written by our AI analysis pipeline, or taken from the NVD description when a CVE was imported from NVD. They are not the vendor's wording: check them against the sources. A longer AI executive summary and technical analysis are generated for signed-in accounts on the Max plan.

Affected products

Products named in our analysis

unicode:unicode
fedoraproject:fedora
starwindsoftware:starwind_virtual_san

Fix and mitigation

WAF and other mitigations

Can a WAF block it? Partly. Code injection payloads are often obfuscated, so they slip past generic WAF rules.

Also consider: remove eval sinks · RASP

Based on the weakness type only; no CVE-specific evidence yet. We check Cloudflare, AWS, Google Cloud Armor and Azure; for Akamai, Imperva, Fastly, F5, ask your vendor.

Remediation steps (AI-written)

Step-by-step remediation is generated for accounts on the Max plan, for CVEs that match a technology in their stack. Add this technology to your stack to see the fix checklist.

Exploitation evidence

CISA KEV catalog: Not listed as of our last sync.
Public exploit code: Unvetted proof-of-concept code on GitHub: 11 GitHub proofs of concept
CISA SSVC: exploitation poc, automatable no, technical impact totalAssessed by CISA 11 Jun 2024 (SSVC 2.0.3). Exploitation is none, poc (public proof of concept) or active.

GitHub proofs of concept

and 7 more repositories

Listed so you can judge urgency, not as a recommendation to run anything. GitHub “proofs of concept” are unvetted: some are fake or carry malware. Exploit-DB marks the entries its staff verified.

Timeline

1 Nov 2021First proof-of-concept repository on GitHub (unvetted)
1 Nov 2021Publishedsame day later
11 Jun 2024CISA SSVC assessment: exploitation poc953 days (about 3 years) later
4 Aug 2024CVE record last updated54 days later
12 Nov 2026Analysed by vulneraibility.com830 days (about 2 years) later

CVSS vector

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:H

Metric	Value	What it means
Attack vector	NetworkReachable over the internet or any routable network.	Reachable over the internet or any routable network.
Attack complexity	HighNeeds specific conditions, a race or a particular configuration.	Needs specific conditions, a race or a particular configuration.
Privileges required	NoneThe attacker needs no prior access.	The attacker needs no prior access.
User interaction	RequiredA victim must click, open or otherwise cooperate.	A victim must click, open or otherwise cooperate.
Scope	ChangedImpact crosses a security boundary into other components.	Impact crosses a security boundary into other components.
Confidentiality	HighTotal data disclosure possible.	Total data disclosure possible.
Integrity	HighComplete modification possible.	Complete modification possible.
Availability	HighFull outage possible.	Full outage possible.

Common questions

Is CVE-2021-42574 being exploited?

Not confirmed. CVE-2021-42574 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 13% probability of exploitation in the next 30 days.

How severe is CVE-2021-42574?

CVE-2021-42574 is rated High with a CVSS score of 8.3. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.

Is there a patch for CVE-2021-42574?

Yes. A fix has been recorded for CVE-2021-42574. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.

What does CVE-2021-42574 affect?

CVE-2021-42574 affects unicode:unicode, fedoraproject:fedora, starwindsoftware:starwind_virtual_san. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.

Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.

References

NVD CVE.org CVE Details Source

18 references in the CVE record, grouped by the tags the record gives them.

Advisories (5)

lists.fedoraproject.org /archives/list/package-announce%40lists.fedoraproject.org/message/QUPA37D57VPTDL
lists.fedoraproject.org /archives/list/package-announce%40lists.fedoraproject.org/message/LQNTFF24ROHLVP
kb.cert.org /vuls/id/999008
lists.fedoraproject.org /archives/list/package-announce%40lists.fedoraproject.org/message/IH2RG5YTR6ZZOL
security.gentoo.org /glsa/202210-09

Where the data on this page comes from

CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
NVD: CVSS and CPE data
CISA Vulnrichment (ADP): SSVC decision points
FIRST EPSS: exploit probability and percentile
PoC-in-GitHub: GitHub repositories that claim a proof of concept

Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.

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