CVE-2026-48736: Symfony IpUtils::PRIVATE_SUBNETS Omits IPv6 Transition Forms Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-38cx-cq6f-5755",
  "modified" : "2026-06-15T17:31:28Z",
  "published" : "2026-06-15T17:31:28Z",
  "aliases" : [ "CVE-2026-48736" ],
  "summary" : "Symfony: IpUtils::PRIVATE_SUBNETS Omits IPv6 Transition Forms (6to4, NAT64, Teredo, IPv4-compatible): SSRF Bypass in NoPrivateNetworkHttpClient",
  "details" : "### Description\n\n`Symfony\\Component\\HttpClient\\NoPrivateNetworkHttpClient` is documented as a decorator that blocks requests to private networks by default. The list of blocked subnets (`Symfony\\Component\\HttpFoundation\\IpUtils::PRIVATE_SUBNETS` on 6.4+, a private constant in `NoPrivateNetworkHttpClient` on 5.4) enumerates RFC1918, loopback, link-local and IPv4-mapped IPv6 (`::ffff:0:0/96`) prefixes, but omits the remaining IPv6 transition forms that can embed a private IPv4 destination: 6to4 (`2002::/16`, RFC 3056), Teredo (`2001::/32`, RFC 4380), NAT64 (`64:ff9b::/96`, RFC 6052 and `64:ff9b:1::/48`, RFC 8215) and IPv4-compatible IPv6 (`::/96`, RFC 4291 §2.5.5.1).\n\n`IpUtils::checkIp6()` is a pure bitwise CIDR comparison against the constants list and never extracts the embedded IPv4, so an attacker who can supply a URL writes the loopback / RFC1918 IPv4 target as e.g. `http://[2002:7f00:1::]/` (6to4 → 127.0.0.1), `http://[64:ff9b::7f00:1]/` (NAT64 → 127.0.0.1), `http://[::7f00:1]/` (IPv4-compatible → 127.0.0.1) or `http://[2001::1]/` (Teredo). `IpUtils::isPrivateIp()` returns `false` and `NoPrivateNetworkHttpClient` dispatches the request.\n\nReal-world reachability of the embedded IPv4 depends on the deploy's IPv6 routing (6to4 tunnel interface, upstream NAT64 gateway, kernel handling of IPv4-compatible addresses), but the security boundary the decorator promises — the dispatch decision — is crossed regardless of whether the packet ultimately lands on the embedded IPv4.\n\n### Resolution\n\nThe private-subnet list now includes `::/96`, `2002::/16`, `2001::/32`, `64:ff9b::/96` and `64:ff9b:1::/48`. Blanket blocking of these prefixes matches the policy applied by Chromium and Mozilla's Private Network Access; server-side HTTPS APIs are not legitimately published on these prefixes.\n\nThe patches for this issue are available [here](https://github.com/symfony/symfony/commit/82765368cf74177c36613575182f168a2eb765b2) for branch 5.4 and [here](https://github.com/symfony/symfony/commit/85b831555be8ea1f43bf01078afe87bc4c92f65e) for branch 6.4 (and forward-ported to 7.4, 8.0 and 8.1).\n\n### Credits\n\nSymfony would like to thank tonghuaroot for reporting the issue and Nicolas Grekas for providing the fix.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "symfony/http-client"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "5.4.0"
      }, {
        "fixed" : "5.4.53"
      } ]
    } ]
  }, {
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      "ecosystem" : "Packagist",
      "name" : "symfony/http-foundation"
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    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "6.4.0"
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        "fixed" : "6.4.41"
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  }, {
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      "name" : "symfony/http-foundation"
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    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "7.0.0"
      }, {
        "fixed" : "7.4.13"
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  }, {
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      "events" : [ {
        "introduced" : "8.0.0"
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  }, {
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      "name" : "symfony/symfony"
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  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/symfony/symfony/security/advisories/GHSA-38cx-cq6f-5755"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/symfony/symfony/commit/82765368cf74177c36613575182f168a2eb765b2"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/symfony/symfony/commit/85b831555be8ea1f43bf01078afe87bc4c92f65e"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/http-client/CVE-2026-48736.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/http-foundation/CVE-2026-48736.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/symfony/CVE-2026-48736.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/symfony/symfony"
  }, {
    "type" : "WEB",
    "url" : "https://symfony.com/cve-2026-48736"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-184", "CWE-918" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-15T17:31:28Z",
    "nvd_published_at" : null
  }
}