CVE-2021-35331

Build-time code execution via uncontrolled printf format

Published Jul 5, 2021 · Updated Aug 4, 2024

A format-string flaw in Tcl 8.6.11 nmakehlp allows local users to execute code by supplying a crafted substitution file. The SubstituteFile function passes file-derived szBuffer content directly as printf's format string, allowing format directives to read or write process memory. Exploitation requires running the Windows-only build helper on attacker-controlled input; Tcl's maintainer disputes security significance because the builder already controls its inputs and output.

CVSS severity7.8
High
EPSS probability1.64%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

A format-string flaw in Tcl 8.6.11 nmakehlp allows local users to execute code by supplying a crafted substitution file. The SubstituteFile function passes file-derived szBuffer content directly as printf's format string, allowing format directives to read or write process memory. Exploitation requires running the Windows-only build helper on attacker-controlled input; Tcl's maintainer disputes security significance because the builder already controls its inputs and output.

The record

CVE
CVE-2021-35331
Published
Jul 5, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-134, T1203
Attack vector
local
Privileges
Unavailable

Timeline

How it unfolded

  1. Jul 5, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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 vectorlocal
Required privilegesUnavailable in this record.

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P
7.8NIST NVDCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-134Use of Externally-Controlled Format String
T1203Exploitation for Client Execution

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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