CVE-2021-41253: Zydis is an x86/x86-64 disassembler library. Users of Zydis versions v3.2.0 and older that use the string functions provided in `zycore` in order to a...

Description

Zydis is an x86/x86-64 disassembler library. Users of Zydis versions v3.2.0 and older that use the string functions provided in `zycore` in order to append untrusted user data to the formatter buffer within their custom formatter hooks can run into heap buffer overflows. Older versions of Zydis failed to properly initialize the string object within the formatter buffer, forgetting to initialize a few fields, leaving their value to chance. This could then in turn cause zycore functions like `ZyanStringAppend` to make incorrect calculations for the new target size, resulting in heap memory corruption. This does not affect the regular uncustomized Zydis formatter, because Zydis internally doesn't use the string functions in zycore that act upon these fields. However, because the zycore string functions are the intended way to work with the formatter buffer for users of the library that wish to extend the formatter, we still consider this to be a vulnerability in Zydis. This bug is patched starting in version 3.2.1. As a workaround, users may refrain from using zycore string functions in their formatter hooks until updating to a patched version.

Metrics

CVSS 3.1

8.1/10

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

EPSS Probability

1.80%

75.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Zyantific	Zydis	<= 3.2.0

References

https://github.com/zyantific/zydis/commit/55dd08c210722aed81b38132f5fd4a04ec1943b5Patch, Third Party Advisory
https://github.com/zyantific/zydis/security/advisories/GHSA-q42v-hv86-3m4gThird Party Advisory
https://huntr.dev/bounties/96b0a482-7041-45b1-9327-c6a4a8f32d3aExploit, Third Party Advisory
https://huntr.dev/bounties/d2536d7d-36ce-4723-928c-98d1ee039784Exploit, Patch, Third Party Advisory
https://github.com/zyantific/zydis/commit/55dd08c210722aed81b38132f5fd4a04ec1943b5Patch, Third Party Advisory
https://github.com/zyantific/zydis/security/advisories/GHSA-q42v-hv86-3m4gThird Party Advisory
https://huntr.dev/bounties/96b0a482-7041-45b1-9327-c6a4a8f32d3aExploit, Third Party Advisory
https://huntr.dev/bounties/d2536d7d-36ce-4723-928c-98d1ee039784Exploit, Patch, Third Party Advisory

Timeline

Published: Nov 8, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-41253?

Zydis is an x86/x86-64 disassembler library. Users of Zydis versions v3.2.0 and older that use the string functions provided in `zycore` in order to append untrusted user data to the formatter buffer within their custom formatter hooks can run into heap buffer overflows. Older versions of Zydis failed to properly initialize the string object within the formatter buffer, forgetting to initialize a few fields, leaving their value to chance. This could then in turn cause zycore functions like `ZyanStringAppend` to make incorrect calculations for the new target size, resulting in heap memory corruption. This does not affect the regular uncustomized Zydis formatter, because Zydis internally doesn't use the string functions in zycore that act upon these fields. However, because the zycore string functions are the intended way to work with the formatter buffer for users of the library that wish to extend the formatter, we still consider this to be a vulnerability in Zydis. This bug is patched starting in version 3.2.1. As a workaround, users may refrain from using zycore string functions in their formatter hooks until updating to a patched version.

How severe is CVE-2021-41253?

CVE-2021-41253 has a CVSS score of 8.1/10 (HIGH severity). The EPSS model estimates a 1.80% probability of exploitation in the next 30 days.

How do I fix CVE-2021-41253?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.