Adminer before 5.5.1 X-Forwarded-Prefix Backslash Bypass (CVE-2026-16434) | HOL Guard CVE

Unknown severity · Source severity not reportedCVE-2026-16434

Adminer before 5.5.1 X-Forwarded-Prefix Backslash Bypass

Answer in brief

CVE-2026-16434 records a Unknown severity vulnerability in Adminer before 5.5.1 X-Forwarded-Prefix Backslash Bypass. The current sources do not mark it as known exploited. The current feed maps vrana/adminer (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Aug 25, 2026Updated Aug 25, 2026Source checked Aug 25, 2026First seen by HOL Aug 25, 2026Material review Aug 25, 2026

Upstream Advisory

Key facts

Risk: Unknown severitySource severity not reported
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

A CVSS score is not reported in the current record. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps vrana/adminer (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
vrana/adminergeneric	>=0 <5.5.1	5.5.1

Recommended response

Check inventory. Check lockfiles and deployed manifests for vrana/adminer.
Review the reported fix. Update vrana/adminer to 5.5.1 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Aug 25, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Aug 25, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Aug 25, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.