Vulnerability in guzzlehttp/psr7 (CVE-2026-55766)
Summary
Vulnerability in guzzlehttp/psr7 (CVE-2026-55766). Risk of unauthorized operations or information disclosure. Exploitable via Request. Mitigation: upgrade to 2.12.1 or later.
Original description
The description as published by the source.
guzzlehttp/psr7 is a PSR-7 HTTP message library implementation in PHP. Prior to 2.12.1, guzzlehttp/psr7 did not reject CR/LF characters in certain first-party HTTP start-line fields: the request method, protocol version, and response reason phrase. If an application placed attacker-controlled data into one of those fields and later serialized the PSR-7 message as raw HTTP/1.x, for example with Message::toString() or an equivalent serializer, the serialized message could contain attacker-controlled header lines. The issue can also be reached through Message::parseRequest() or Message::parseResponse() when malformed raw messages are parsed into first-party PSR-7 objects and then serialized again. Creating or modifying a Request, Response, or other PSR-7 object alone is not sufficient. The issue requires the malformed message to be serialized and written to the network, forwarded, replayed, or otherwise processed by software that does not independently reject the malformed start line. This vulnerability is fixed in 2.12.1.
Source: GitHub Security Advisories (Global) NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-55766 has been found in guzzlehttp/psr7.
Attackers can target a specific entry point like Request over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 4.8/10.
What to do: upgrade guzzlehttp/psr7 to 2.12.1 or later.
If unsure, ask your IT team or search "guzzlehttp/psr7 CVE-2026-55766" on the vendor's site.
CVE-2026-55766 (guzzlehttp/psr7) — CWE-113 / CVSS v3 4.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: Request / Response / REQUEST_METHOD / SERVER_PROTOCOL
Patched: 2.12.1 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-113) exists in guzzlehttp/psr7. Attackers reach the vulnerable code path via Request without authentication.
Check the vendor advisory for the affected versions of guzzlehttp/psr7. Attack surface: Request / Response / REQUEST_METHOD / SERVER_PROTOCOL.
Severity: Medium (CVSS 4.8/10). Risk of unauthorized operations or information disclosure.
Update to 2.12.1.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'guzzlehttp/psr7' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
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1Identify exposure identify
grep -r 'guzzlehttp/psr7' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
guzzlehttp/psr7を grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade guzzlehttp/psr7 to 2.12.1ステージング環境で 2.12.1 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Debian DEBIAN-CVE-2026-55766 php-guzzlehttp-psr7
- Ubuntu UBUNTU-CVE-2026-55766 php-guzzlehttp-psr7
References
- advisory github.com
- advisory github.com
- advisory nvd.nist.gov
- package github.com
- web github.com