# CVE-2026-49953: Discuz! X5.0 CAPTCHA bypass via predictable character set

Source: https://junglewise.ai/threats/cve-2026-49953-discuz-x5-0-captcha-bypass-via-predictable-character-set  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-15. Updated: 2026-06-15  
License: CC BY 4.0 with a link back

CVE-2026-49953 · Severity: medium · CVSS 6.5 · Published 2026-06-15

Discuz! X5.0, a popular forum and community platform, contains a security weakness in its CAPTCHA system. This system is designed to distinguish human users from automated bots during registration and login. Because the CAPTCHA images are too simple and predictable, attackers can use automated tools to solve them, allowing for large-scale automated abuse such as account takeovers, spam, or fake account creation.

Technologies: [Discuz! X](https://junglewise.ai/threats/technologies/discuz-x). Vendors: [Discuz!](https://junglewise.ai/threats/vendors/discuz).

## Quick answers

**Is CVE-2026-49953 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-49953 being exploited in the wild.

**How severe is CVE-2026-49953?** Medium severity, with a CVSS score of 6.5, published 15 June 2026.

**Which products and versions are affected?** Discuz! X 20260320 through 20260610.

**Is there a fix for CVE-2026-49953?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-49953 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-49953?** Check whether you run Discuz! X 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 CAPTCHA bypass vulnerability exists in Discuz! X5.0 (releases 20260320 through 20260610) due to the use of a predictable character set and limited image complexity. This weakness allows an unauthenticated remote attacker to collect CAPTCHA samples and train a custom Optical Character Recognition (OCR) model to reliably solve the challenges. By bypassing these controls, attackers can automate interactions with protected endpoints such as login, registration, and posting forms. This vulnerability was notably used as a component in a larger exploit chain to automate a race condition attack leading to remote code execution. As of the advisory date, no official patch has been released.

## Affected products

- Discuz! Discuz! X5.0 20260320 through 20260610

## Timeline

- 2026-03-20: other: Discuz! X5.0 initial release date
- 2026-04-27: other: Vendor first contacted by researcher
- 2026-05-09: other: Vendor acknowledges OCR-based bypass as a known issue
- 2026-06-09: other: CVE identifier assigned
- 2026-06-13: disclosed: Public disclosure at hackmeeting 0x1D
- 2026-06-15: advisory: Publication of security advisory KIS-2026-10

## References

- [https://karmainsecurity.com/KIS-2026-10](https://karmainsecurity.com/KIS-2026-10)
- [https://karmainsecurity.com/chaining-bugs-in-discuz-from-race-condition-to-rce](https://karmainsecurity.com/chaining-bugs-in-discuz-from-race-condition-to-rce)
- [https://www.vulncheck.com/advisories/discuz-x5-0-captcha-bypass-via-predictable-character-set](https://www.vulncheck.com/advisories/discuz-x5-0-captcha-bypass-via-predictable-character-set)

## Related threats

- [Discuz! X5.0 local file inclusion in plugin management](https://junglewise.ai/threats/cve-2026-49954-discuz-x5-0-local-file-inclusion-in-plugin-management)
- [Discuz! X5.0 authentication bypass in dbbak.php via token reuse](https://junglewise.ai/threats/cve-2026-49952-discuz-x5-0-authentication-bypass-in-dbbak-php-via-token-reuse)