Summary
guzzlehttp/psr7 has Host Confusion via Authority Reinterpretation
Workarounds
If you cannot upgrade immediately, validate Host values before passing untrusted request data to Message::parseRequest(), legacy 1.x parse_request(), ServerRequest::fromGlobals(), or ServerRequest::getUriFromGlobals().
Accept only uri-host [ ":" port ]. Reject values containing whitespace, control characters, userinfo (@), path (/ or \), query (?), fragment (#), malformed IP literals or bracket syntax, or invalid port syntax.
Do not validate Host by prefixing it with http:// and passing it to parse_url(), because that can reinterpret malformed values as URI userinfo and host.
References
Impact
guzzlehttp/psr7 improperly interpreted malformed Host header values when constructing request URIs from inbound request data. This issue concerns inbound request parsing and server request construction. It does not require serializing a PSR-7 request, and it is not part of the normal outbound request-sending path used by guzzlehttp/guzzle.
A vulnerable flow is:
- An attacker controls a raw HTTP request or server variable containing a
Hostvalue. - The
Hostvalue contains URI authority delimiters, such as[email protected]. guzzlehttp/psr7uses that value to construct a URI.- The URI parser treats the portion before
@as userinfo and the portion after@as the URI host. - The resulting PSR-7 request URI host differs from the original
Hostheader value.
For example, Host: [email protected] can result in a PSR-7 URI whose host is evil.example, while the original Host header value remains [email protected].
Applications are affected if they parse attacker-controlled raw HTTP requests with GuzzleHttp\Psr7\Message::parseRequest() or the legacy 1.x GuzzleHttp\Psr7\parse_request() function, or if they build server requests from attacker-controlled server variables with GuzzleHttp\Psr7\ServerRequest::fromGlobals() or GuzzleHttp\Psr7\ServerRequest::getUriFromGlobals(), and then rely on the resulting URI host for routing, allow-list checks, credential selection, or forwarding decisions. Applications using guzzlehttp/psr7 only through Guzzle's standard HTTP client APIs are not expected to be affected. In affected forwarding or gateway scenarios, this may cause requests or credentials to be sent to an unintended host.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
CVE-2026-48998 has a CVSS score of 5.3 (Medium). 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 (2.10.2); upgrading removes the vulnerable code path.
Affected versions
Security releases
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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The issue is patched in 2.10.2 and later. 1.x is end-of-life and will not receive a patch.
Frequently Asked Questions
- What is CVE-2026-48998? CVE-2026-48998 is a medium-severity improper input validation vulnerability in guzzlehttp/psr7 (composer), affecting versions < 2.10.2. It is fixed in 2.10.2. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- How severe is CVE-2026-48998? CVE-2026-48998 has a CVSS score of 5.3 (Medium). 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 versions of guzzlehttp/psr7 are affected by CVE-2026-48998? guzzlehttp/psr7 (composer) versions < 2.10.2 is affected.
- Is there a fix for CVE-2026-48998? Yes. CVE-2026-48998 is fixed in 2.10.2. Upgrade to this version or later.
- Is CVE-2026-48998 exploitable, and should I be worried? Whether CVE-2026-48998 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-2026-48998 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-2026-48998? Upgrade
guzzlehttp/psr7to 2.10.2 or later.