What is CVE-2026-48979?

CVE-2026-48979 is a high-severity security vulnerability in php-standard-library/h2 (composer), affecting versions >= 6.1.0, < 6.1.2. It is fixed in 6.1.2, 6.2.1.

How severe is CVE-2026-48979?

CVE-2026-48979 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Which packages are affected by CVE-2026-48979?

php-standard-library/h2 (composer) versions >= 6.1.0, = 6.1.0, < 6.1.2.

Is there a fix for CVE-2026-48979?

Yes. CVE-2026-48979 is fixed in 6.1.2, 6.2.1. Upgrade to this version or later.

Is CVE-2026-48979 exploitable, and should I be worried?

Whether CVE-2026-48979 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

What actually determines whether CVE-2026-48979 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

How do I fix CVE-2026-48979?

Upgrade: php-standard-library/h2 to 6.1.2 or later, php-standard-library/h2 to 6.2.1 or later, php-standard-library/php-standard-library to 6.1.2 or later, php-standard-library/php-standard-library to 6.2.1 or later.

CVE-2026-48979 in php-standard-library/h2

Q: How severe is CVE-2026-48979?

CVE-2026-48979 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Q: Which packages are affected by CVE-2026-48979?

php-standard-library/h2 (composer) versions >= 6.1.0, = 6.1.0, < 6.1.2.

Q: Is there a fix for CVE-2026-48979?

Yes. CVE-2026-48979 is fixed in 6.1.2, 6.2.1. Upgrade to this version or later.

Q: Is CVE-2026-48979 exploitable, and should I be worried?

Whether CVE-2026-48979 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

Q: What actually determines whether CVE-2026-48979 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

Q: How do I fix CVE-2026-48979?

Upgrade: php-standard-library/h2 to 6.1.2 or later, php-standard-library/h2 to 6.2.1 or later, php-standard-library/php-standard-library to 6.1.2 or later, php-standard-library/php-standard-library to 6.2.1 or later.

Summary

PHP Standard Library: HTTP/2 server-side missing content-length validation enables request smuggling

Workarounds

None at the protocol layer. Applications using Psl\H2\ServerConnection directly should upgrade.

Resources

RFC 9113 §8.1.1 (HTTP/2 request/response exchange)
RFC 9110 §8.6 (content-length header)
https://github.com/php-standard-library/php-standard-library/releases/tag/6.1.2
https://github.com/php-standard-library/php-standard-library/releases/tag/6.2.1

Impact

Psl\H2\ServerConnection does not validate that the total bytes received in DATA frames match the content-length header declared in the HEADERS frame, in violation of RFC 9113 §8.1.1.

A malicious client can:

Send more DATA bytes than declared, smuggling additional content past application-level size limits.
Send fewer DATA bytes than declared and close the stream early, causing applications that trust the declared length to behave incorrectly.

The vulnerability is only reachable for consumers using Psl\H2\ServerConnection directly to accept untrusted client traffic. The high-level Psl\HTTP\Server is in active development and was not yet released at the time of this advisory; consumers of documented high-level PSL APIs are not affected.

CVE-2026-48979 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.1.2, 6.2.1); upgrading removes the vulnerable code path.

Affected versions

php-standard-library/h2 (>= 6.1.0, < 6.1.2) php-standard-library/h2 (>= 6.2.0, < 6.2.1) php-standard-library/php-standard-library (>= 6.1.0, < 6.1.2) php-standard-library/php-standard-library (>= 6.2.0, < 6.2.1)

Security releases

php-standard-library/h2 → 6.1.2 (composer) php-standard-library/h2 → 6.2.1 (composer) php-standard-library/php-standard-library → 6.1.2 (composer) php-standard-library/php-standard-library → 6.2.1 (composer)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Fixed in 6.1.2 and 6.2.1.

Parses and validates the content-length header on incoming HEADERS (server-side only, clients do not enforce this per RFC 9110 §9.3.2).
Tracks cumulative DATA frame payload length per stream.
Throws StreamException on mismatch or overflow.

Regression tests landed in #781, 9 of the new tests fail against the pre-fix code, proving the validation boundary is enforced.

Frequently Asked Questions

What is CVE-2026-48979? CVE-2026-48979 is a high-severity security vulnerability in php-standard-library/h2 (composer), affecting versions >= 6.1.0, < 6.1.2. It is fixed in 6.1.2, 6.2.1.
How severe is CVE-2026-48979? CVE-2026-48979 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
Which packages are affected by CVE-2026-48979?
- php-standard-library/h2 (composer) (versions >= 6.1.0, < 6.1.2)
- php-standard-library/php-standard-library (composer) (versions >= 6.1.0, < 6.1.2)
Is there a fix for CVE-2026-48979? Yes. CVE-2026-48979 is fixed in 6.1.2, 6.2.1. Upgrade to this version or later.
Is CVE-2026-48979 exploitable, and should I be worried? Whether CVE-2026-48979 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
What actually determines whether CVE-2026-48979 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
How do I fix CVE-2026-48979?
- Upgrade php-standard-library/h2 to 6.1.2 or later
- Upgrade php-standard-library/h2 to 6.2.1 or later
- Upgrade php-standard-library/php-standard-library to 6.1.2 or later
- Upgrade php-standard-library/php-standard-library to 6.2.1 or later