critical CVSS 9.8

CVE-2026-17543·PHP vulnerability

Improper escaping of backslashes in attacker-provided parameters would allow for trivial SQL injection in PHP versions from 8.2.* before 8.2.33, from 8.3.* before 8.3.33, from 8.4.* before 8.4.24, and from 8.5.* before 8.5.9.

Severity
critical
Software
PHP
Fixed in
8.5.9
Published
2026-07-30

Affected versions

From: 8.5.0

Until: 8.5.9

Fixed in: 8.5.9

How to fix this CVE

Update PHP to version 8.5.9 or later to patch a critical SQL injection vulnerability stemming from improper backslash escaping in user-supplied parameters. This flaw allows unauthenticated attackers to bypass input sanitization and execute arbitrary SQL commands. Prioritize this update immediately given the CVSS 9.8 severity score and the ease of exploitation.

sudo dnf update php

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST|GET requests containing multiple consecutive backslashes (\\+) in query parameters, cookie values, or POST data followed by SQL keywords (SELECT, UNION, WHERE, OR, AND); example: param=value\\' OR '1'='1

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Configure WAF rules to block requests containing unescaped or double-encoded backslashes followed by SQL metacharacters in URI query strings and POST bodies. Implement strict input validation to reject parameters with backslash sequences that do not conform to expected input patterns. Example ModSecurity rule: SecRule ARGS|HEADERS '@rx \\\\+.*?(UNION|SELECT|WHERE|OR|AND)' 'id:1000,phase:2,deny,status:403,msg:Potential SQL Injection via Backslash Bypass'

How to check if you are affected

  1. Check your current PHP version: php --version
  2. Verify which PHP package versions are installed: dpkg -l | grep php (Debian/Ubuntu) or rpm -qa | grep php (RHEL/CentOS)
  3. Review web server access and error logs for SQL syntax errors or unusual query patterns: grep -i 'syntax error\|backslash\|injection' /var/log/apache2/error.log /var/log/nginx/error.log
  4. After patching, confirm the update: php --version and verify it reports 8.5.9 or higher

FAQ

What is CVE-2026-17543?

This is a critical SQL injection vulnerability in PHP caused by improper escaping of backslashes in attacker-controlled parameters. An unauthenticated remote attacker can exploit this to bypass query filters and execute unauthorized database commands.

Is CVE-2026-17543 being actively exploited?

According to CISA's KEV catalog, there is no evidence of active exploitation in the wild at this time. However, the simplicity of the attack vector means exploitation could become prevalent quickly, so urgent patching is still recommended.

What versions of PHP are affected by CVE-2026-17543?

PHP 8.2.0–8.2.32, 8.3.0–8.3.32, 8.4.0–8.4.23, and 8.5.0–8.5.8 are vulnerable. The fix is available in 8.2.33, 8.3.33, 8.4.24, and 8.5.9 respectively.

How do I check if my server is vulnerable to CVE-2026-17543?

Run php --version and check if your installed version falls within the affected ranges. Cross-reference against the NVD advisory or use a vulnerability scanner to confirm.

Does Defensia detect CVE-2026-17543?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If PHP is installed on a monitored server, CVE-2026-17543 will appear in your dashboard with remediation steps.

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References

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