# CVE-2026-38640: Redox OS relibc Denial of Service in __assert_fail

Source: https://junglewise.ai/threats/cve-2026-38640-redox-os-relibc-denial-of-service-in-assert-fail  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-25. Updated: 2026-06-25  
License: CC BY 4.0 with a link back

CVE-2026-38640 · Severity: info · CVSS 5.5 · Published 2026-06-25

A vulnerability in relibc, a standard C library used by the Redox operating system, can cause applications to crash unexpectedly. By providing specially crafted, non-UTF-8 text to certain system functions, a local user or malicious program can trigger a system panic. This results in a denial-of-service condition where the affected application or service becomes unavailable.

Technologies: [Redox OS Relibc](https://junglewise.ai/threats/technologies/relibc). Vendors: [Redox OS](https://junglewise.ai/threats/vendors/redox-os).

## Quick answers

**Is CVE-2026-38640 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-38640 being exploited in the wild.

**How severe is CVE-2026-38640?** Informational severity, with a CVSS score of 5.5, published 25 June 2026.

**Which products and versions are affected?** Redox OS Relibc commit 61f42d6b93adc552a9e8b6f207fca907e8ca8be2 and earlier.

**Is there a fix for CVE-2026-38640?** A patch was published on 2026-02-10: Merge request 986 submitted to fix the issue using to_string_lossy Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-38640 attacked?** It is attacked with local access, in a software library.

**Am I affected by CVE-2026-38640?** Check whether you run Redox OS Relibc at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A denial of service vulnerability exists in relibc's `__assert_fail` function located in `src/header/assert/mod.rs`. The implementation uses `.to_str().unwrap()` on C-style strings provided as arguments (function name, file name, or condition). If these strings contain invalid UTF-8 sequences (e.g., 0xFF), the Rust `unwrap()` call triggers a panic, crashing the process. An attacker can exploit this by executing a local program that passes malformed strings to `__assert_fail`. A fix has been proposed to use `to_string_lossy()` to safely handle invalid UTF-8 data.

## Affected products

- Redox OS relibc commit 61f42d6b93adc552a9e8b6f207fca907e8ca8be2 and earlier

## Timeline

- 2026-02-10: patched: Merge request 986 submitted to fix the issue using to_string_lossy
- 2026-06-25: disclosed: CVE-2026-38640 published

## References

- [https://github.com/Marsman1996/pocs/tree/master/redox/CVE-2026-38640](https://github.com/Marsman1996/pocs/tree/master/redox/CVE-2026-38640)
- [https://gitlab.redox-os.org/redox-os/relibc/-/issues/262](https://gitlab.redox-os.org/redox-os/relibc/-/issues/262)
- [https://gitlab.redox-os.org/redox-os/relibc/-/merge_requests/986](https://gitlab.redox-os.org/redox-os/relibc/-/merge_requests/986)
- [https://gitlab.redox-os.org/redox-os/relibc/-/work_items/262](https://gitlab.redox-os.org/redox-os/relibc/-/work_items/262)

## Related threats

- [relibc index out of bounds in with_argv](https://junglewise.ai/threats/cve-2026-38638-relibc-index-out-of-bounds-in-with-argv)
- [relibc seekdir() denial of service via invalid offset](https://junglewise.ai/threats/cve-2026-38636-relibc-seekdir-denial-of-service-via-invalid-offset)
- [Redox OS relibc denial of service in DSO::mmap_and_copy](https://junglewise.ai/threats/cve-2026-38641-redox-os-relibc-denial-of-service-in-dso-mmap-and-copy)
- [Redox OS relibc Denial of Service in strptime via multibyte characters](https://junglewise.ai/threats/cve-2026-38639-redox-os-relibc-denial-of-service-in-strptime-via-multibyte)
- [Redox OS relibc Denial of Service in pthread_rwlockattr_setpshared](https://junglewise.ai/threats/cve-2026-38637-redox-os-relibc-denial-of-service-in-pthread-rwlockattr)