Vulnerability · Published Aug 25, 2026 · Assigned by VulnCheck
CVE-2026-16434Adminer before 5.5.1 X-Forwarded-Prefix Backslash Bypass
Low 2.3
In plain English
A low-severity improper input validation in adminer from vrana. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 33% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
Adminer 4.6.0 through 5.5.0 (fixed in 5.5.1) contains an incomplete fix for a prior X-Forwarded-Prefix vulnerability (GHSA-8478-xrj3-h9c2). The validation guard (bootstrap.inc.php) only rejects prefixes matching ^/[^/], blocking //evil.com but allowing values such as /\evil.com whose second character is a backslash. Because browsers normalize backslash to forward slash, a network-path reference survives into REQUEST_URI and reaches cookie_path(), affecting the Set-Cookie Path attribute. Exploitation requires that clients can set the X-Forwarded-Prefix header (a misconfigured or absent reverse proxy). Impact is limited to anomalous cookie-path scoping.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | adminer | no fix listed | DEBIAN-CVE-2026-16434 |
| Debian:13 | adminer | no fix listed | DEBIAN-CVE-2026-16434 |
| Debian:14 | adminer | 6.0.0-1 | DEBIAN-CVE-2026-16434 |
| Ubuntu:22.04:LTS | adminer | no fix listed | UBUNTU-CVE-2026-16434 |
| Ubuntu:24.04:LTS | adminer | no fix listed | UBUNTU-CVE-2026-16434 |
| Ubuntu:26.04:LTS | adminer | no fix listed | UBUNTU-CVE-2026-16434 |
Affected products (as reported)
Weakness: CWE-20 · Vector: CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N
References
- GitHub Security Advisory (GHSA-fr74-9mf9-gf44) vendor-advisory
- VulnCheck Advisory: Adminer before 5.5.1 X-Forwarded-Prefix Backslash Bypass third-party-advisory