PHPApplication
CVE-2026-6104
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedIn PHP versions 8.4.* before 8.4.21 and 8.5.* before 8.5.6, when an encoding name containing an embedded NUL byte is passed to mb_convert_encoding() or related mbstring functions, the code incorrectly assumes that when strncasecmp() returns 0 it means the strings have the same length. This can lead to out-of-bounds read of global memory, potentially causing a crash or information disclosure or crash. Affected functions include mb_convert_encoding(), mb_detect_encoding(), mb_convert_variables(), and mb_detect_order(), as well as the mbstring.detect_order and mbstring.http_output INI settings.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceIn PHP 8.4.x before 8.4.21 and 8.5.x before 8.5.6, the mbstring extension incorrectly handles encoding names containing embedded NUL bytes. The code assumes strncasecmp() returning 0 indicates equal-length strings, but NUL bytes truncate comparisons, leading to out-of-bounds memory reads. Affected functions include mb_convert_encoding(), mb_detect_encoding(), mb_convert_variables(), and mb_detect_order(), plus related INI settings.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
PHPApplication
>= 8.4.0, < 8.4.21>= 8.5.0, < 8.5.6CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check installed PHP versionRun `php -v` and examine the version number in the first line output (for example: PHP 8.4.15 or PHP 8.5.2)Affected if Version is 8.4.0 through 8.4.20, or 8.5.0 through 8.5.5 (vulnerable range)
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Verify mbstring extension is loadedRun `php -m` and check if mbstring appears in the module list, or run `php -i | grep -i mbstring` to confirm the extension is availableAffected if mbstring extension is enabled and loaded
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Inspect mbstring.detect_order INI settingRun `php -r "echo ini_get('mbstring.detect_order');"` or check phpinfo() output for the mbstring.detect_order valueAffected if This configuration setting is used and could accept or be manipulated to contain NUL bytes in encoding names
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Search for use of affected functions in application codeSearch source code files for calls to mb_convert_encoding, mb_detect_encoding, mb_convert_variables, or mb_detect_order, particularly where encoding parameters accept user input or configuration valuesAffected if The application code uses any of these four functions with encoding names that could contain NUL bytes
You are affected if your PHP version falls within 8.4.0-8.4.20 or 8.5.0-8.5.5, the mbstring extension is enabled, and your code or configuration uses mb_convert_encoding, mb_detect_encoding, mb_convert_variables, or mb_detect_order with encoding names that could contain NUL bytes.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped8.4.218.5.6
Upgrade PHP to version 8.4.21+/8.5.6+ to patch the mbstring vulnerability. As a defense-in-depth measure, implement input validation to reject encoding names containing NUL bytes before passing them to mbstring functions.
PHP 8.4.21 or PHP 8.5.6 (whichever branch matches your deployment)
- 1. Identify the currently installed PHP version by running `php -v` or checking your application's dependency files
- 2. If running PHP 8.4.x versions before 8.4.21, plan upgrade to PHP 8.4.21 or later
- 3. If running PHP 8.5.x versions before 8.5.6, plan upgrade to PHP 8.5.6 or later
- 4. For systems using package managers (apt, yum, dnf), update via: `sudo apt update && sudo apt upgrade php` or `sudo dnf update php`
- 5. For systems using package managers, ensure you install the mbstring extension alongside the core PHP update
- 6. For manual/source builds, download PHP 8.4.21 or 8.5.6 from php.net and rebuild with mbstring enabled
- 7. After upgrade, verify the fix by testing mbstring functions with encoding names containing embedded NUL bytes (e.g., using strings like "UTF-8\0")
- 8. Restart any affected PHP-FPM, Apache, or other web server processes to load the new PHP binaries
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in PHP
Scoped from the published advisory
- Consultation2.0 h
- Implementation4.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,088.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-6104 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-6104 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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