How severe is CVE-2026-21428?

CVE-2026-21428 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability none.

Is CVE-2026-21428 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (29th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-21428?

CVE-2026-21428 affects Yhirose Cpp-httplib. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-21428?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-21428 have an EU (EUVD) identifier?

Yes. CVE-2026-21428 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-0026.

When was CVE-2026-21428 published?

CVE-2026-21428 was published on 2026-01-01 and last updated on 2026-06-17.

CVE-2026-21428

CVE-2026-21428 is a high-severity vulnerability in Yhirose Cpp-httplib with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-93.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v4: 7.7
EPSS exploit prediction: 0% (29th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-0026
Weakness: CWE-93
Affected product: Yhirose Cpp-httplib
Published: 2026-01-01
Last modified: 2026-06-17

Description

cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to version 0.30.0, the ``write_headers`` function does not check for CR & LF characters in user supplied headers, allowing untrusted header value to escape header lines. This vulnerability allows attackers to add extra headers, modify request body unexpectedly & trigger an SSRF attack. When combined with a server that supports http1.1 pipelining (springboot, python twisted etc), this can be used for server side request forgery (SSRF). Version 0.30.0 fixes this issue.

Frequently asked questions

What is CVE-2026-21428?: cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to version 0.30.0, the ``write_headers`` function does not check for CR & LF characters in user supplied headers, allowing untrusted header value to escape header lines. This vulnerability allows attackers to add extra headers, modify request body unexpectedly & trigger an SSRF attack. When combined with a server that supports http1.1 pipelining (springboot, python twisted etc), this can be used for server side request forgery (SSRF). Version 0.30.0 fixes this issue.
How severe is CVE-2026-21428?: CVE-2026-21428 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability none.
Is CVE-2026-21428 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (29th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-21428?: CVE-2026-21428 affects Yhirose Cpp-httplib. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-21428?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-21428 have an EU (EUVD) identifier?: Yes. CVE-2026-21428 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-0026.
When was CVE-2026-21428 published?: CVE-2026-21428 was published on 2026-01-01 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:yhirose:cpp-httplib:*:*:*:*:*:*:*:*

More vulnerabilities in Yhirose Cpp-httplib

CVE-2025-66570 — Critical (CVSS 10.0): cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to 0.27.0, a vulnerability…
CVE-2026-45372 — Critical (CVSS 9.9): cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to 0.44.0, when cpp-httplib's…
CVE-2025-53628 — High (CVSS 8.8): cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to 0.20.1, cpp-httplib does not…
CVE-2026-32627 — High (CVSS 8.7): cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to 0.37.2, when a cpp-httplib…
CVE-2026-46527 — High (CVSS 7.5): cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to 0.44.0, When the server has…
CVE-2026-31870 — High (CVSS 7.5): cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to 0.37.1, when a cpp-httplib…

All CVEs affecting Yhirose Cpp-httplib →

Other CWE-93 (CRLF Injection) vulnerabilities

CVE-2024-51501 — Critical (CVSS 10.0): Refit is an automatic type-safe REST library for .NET Core, Xamarin and .NET The various header-related Refit…
CVE-2026-45372 — Critical (CVSS 9.9): cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to 0.44.0, when cpp-httplib's…
CVE-2026-11362 — Critical (CVSS 9.8): DataDog::DogStatsd versions through 0.07 for Perl allow metric injections from event tags. DataDog::DogStatsd does not…
CVE-2025-40671 — Critical (CVSS 9.3): SQL injection vulnerability in AES Multimedia's Gestnet v1.07. This vulnerability allows an attacker to retrieve,…
CVE-2026-11373 — Critical (CVSS 9.1): Net::Statsite::Client versions through 1.1.0 for Perl allow metric injections. Net::Statsite::Client is a client for…
CVE-2026-50638 — Critical (CVSS 9.1): Metrics::Any::Adapter::DogStatsd versions before 0.04 for Perl does not protect against metric injections. The statsd…

Browse all CWE-93 (CRLF Injection) vulnerabilities →