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TI-2026-21978 Aliases CVE-2026-27590GHSA-5r3v-vc8m-m96g

Improper input validation in caddyserver caddy

Priority HIGH 80CWE-180Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Attack requirements
None
Impact type
—
Confidentiality · integrity · availability
High · High · High
Fix status
Fixed

Attack profile from CVSS 4.0 · CNA. Fix status from CNA and GitHub.

Summary

Caddy is an extensible server platform that uses TLS by default. Prior to version 2.11.1, Caddy's FastCGI path splitting logic computes the split index on a lowercased copy of the request path and then uses that byte index to slice the original path. This is unsafe for Unicode because `strings.ToLower()` can change UTF-8 byte length for some characters. As a result, Caddy can derive an incorrect `SCRIPT_NAME`/`SCRIPT_FILENAME` and `PATH_INFO`, potentially causing a request that contains `.php` to execute a different on-disk file than intended (path confusion). In setups where an attacker can control file contents (e.g., upload features), this can lead to unintended PHP execution of non-.php files (potential RCE depending on deployment). Version 2.11.1 fixes the issue.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00614 49th percentileScored 2026-11-07
Public exploit Yes 1 exploit referenceTagged by NVD
Fix Fixed caddy (caddyserver): 2.11.1github.com/caddyserver/caddy/v2 (Go): 2.11.1

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
Yes
Technical impact
Total

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
caddyservercaddybefore 2.11.12.11.1Patch availableCNA
—github.com/caddyserver/caddy/v2 (Go)before 2.11.12.11.1Patch availableGHSA-5r3v-vc8m-m96g
  • caddy Vendorcaddyserver Affected before 2.11.1 Fixed in 2.11.1 Patch available CNA
  • github.com/caddyserver/caddy/v2 (Go) Vendor— Affected before 2.11.1 Fixed in 2.11.1 Patch available GHSA-5r3v-vc8m-m96g

CPE configurations NVD · 1

CPEVersionsRoleVendor
cpe:2.3:a:caddyserver:caddy:*:*:*:*:*:*:*:*before 2.11.1Vulnerablecaddyserver
  • cpe:2.3:a:caddyserver:caddy:*:*:*:*:*:*:*:* Versions before 2.11.1 Vulnerable ·caddyserver

Remediation

  • Fix · CNA caddy (caddyserver) Fixed in 2.11.1
  • Fix · GHSA-5r3v-vc8m-m96g github.com/caddyserver/caddy/v2 (Go) Fixed in 2.11.1 Advisory →

Vendor assessments

Red Hat

Red Hat's page for this CVE →

Not affected: 1 product
  • Red Hat

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-27590
  2. 2026-10-03 Alias added: GHSA-5r3v-vc8m-m96g
  3. 2026-10-25 EPSS moved to the below the 50th percentile

References

Exploit 1

Advisory 2

Other 3

  • TI-2023-23979 The HTTP/2 protocol allows a denial of service (server resource consumption) because… KEVCVSS 3.1HIGH 7.5
  • TI-2026-21974 Caddy's mTLS client authentication silently fails open when CA certificate file is missing or malformed CVSS 4.0HIGH 8.8
  • TI-2026-23680 Improper authentication in caddyserver caddy CVSS 3.1HIGH 8.8
  • TI-2026-21976 Caddy: MatchHost becomes case-sensitive for large host lists (>100), enabling host-based route/auth bypass CVSS 4.0HIGH 7.7
  • TI-2026-21975 Caddy: MatchPath %xx (escaped-path) branch skips case normalization, enabling path-based route/auth bypass CVSS 4.0HIGH 7.7
  • TI-2026-37952 Caddy: Windows `file_server` path authorization bypass via encoded backslash CVSS 3.1HIGH 7.5
See all 19 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences