How severe is CVE-2026-48979?

CVE-2026-48979 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-48979?

CVE-2026-48979 affects the following packages: php-standard-library/h2 (Packagist), php-standard-library/h2 (Packagist), php-standard-library/php-standard-library (Packagist), php-standard-library/php-standard-library (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2026-48979?

Update php-standard-library/h2 to 6.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48979 is resolved across your whole dependency graph.

How do I detect CVE-2026-48979 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for php-standard-library/h2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-48979 if there is no patch (or I can't update yet)?

Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Is CVE-2026-48979 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-48979 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-48979?

CVE-2026-48979 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 37th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-48979?

CVE-2026-48979 is classified as CWE-444 (CWE-444). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-48979 published, and has it been updated?

CVE-2026-48979 was published on June 17, 2026 and was last updated on August 12, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘

🐘 Packagist

Not in CISA KEV

HIGH severity

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

HIGH

CVE-2026-48979 is a high-severity (CVSS 7.5) CWE-444 vulnerability in php-standard-library/h2. A fix is available for php-standard-library/h2 — see the affected versions and patch details below.

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

Also known asGHSA-pw9p-jvrm-f7rm

Published

Jun 17, 2026

Updated

Aug 12, 2026

Affected

4 pkgs

Patched

4 / 4

Exploits

None indexed

Exploitation data as of Sep 26, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-48979.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs37th percentile — riskier than 37% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2026-48979 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 379,842 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

4 pkgs affected

🐘php-standard-library/h2🐘php-standard-library/h2🐘php-standard-library/php-standard-library🐘php-standard-library/php-standard-library

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

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.

Patches

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.

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

Affected Packages

4 total 4 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`php-standard-library/h2`	≥ 6.1.0&&< 6.1.2	6.1.2`composer require php-standard-library/h2:^6.1.2`
🐘Packagist	`php-standard-library/h2`	≥ 6.2.0&&< 6.2.1	6.2.1`composer require php-standard-library/h2:^6.2.1`
🐘Packagist	`php-standard-library/php-standard-library`	≥ 6.1.0&&< 6.1.2	6.1.2`composer require php-standard-library/php-standard-library:^6.1.2`
🐘Packagist	`php-standard-library/php-standard-library`	≥ 6.2.0&&< 6.2.1	6.2.1`composer require php-standard-library/php-standard-library:^6.2.1`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for php-standard-library/h2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update php-standard-library/h2 to 6.1.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-48979 is resolved across your whole dependency graph.
Workarounds
Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Frequently Asked Questions

## 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 h