[security] Go 1.26.3 and Go 1.25.10 are released

[anno...@golang.org]

Hello gophers,

We have just released Go versions 1.26.3 and 1.25.10, minor point releases.

These releases include 11 security fixes following the security policy:

• cmd/go: malicious module proxy can bypass checksum database

  A malicious module proxy could exploit a flaw in the go command's
  validation of module checksums to bypass checksum database validation.

  This vulnerability affects any user using an untrusted module proxy
  (GOMODPROXY) or checksum database (GOSUMDB).

  A malicious module proxy can serve altered versions of the Go toolchain.
  When selecting a different version of the Go toolchain than the
  currently installed toolchain (due to the GOTOOLCHAIN environment variable,
  or a go.work or go.mod with a toolchain line), the go command will download
  and execute a toolchain provided by the module proxy. A malicious module
  proxy can bypass checksum database validation for this downloaded
  toolchain.

  Since this vulnerability affects the security of toolchain downloads,
  setting GOTOOLCHAIN to a fixed version is not sufficient. You must upgrade
  your base Go toolchain.

  The go tool always validates the hash of a toolchain before executing it,
  so fixed versions will refuse to execute any cached, altered versions of the
  toolchain.

  The go tool trusts go.sum files to contain accurate hashes of the current
  module's dependencies. A malicious proxy exploiting this vulnerability to
  serve an altered module will have caused an incorrect hash to be recorded
  in the go.sum. Users who have configured a non-trusted GOPROXY can determine
  if they have been affected by running "rm go.sum ; go mod tidy ; go mod verify",
  which will revalidate all dependencies of the current module.

  The specific flaw in more detail:

  The go command consults the checksum database to validate downloaded modules,
  when a module is not listed in the go.sum file. It verifies that the module hash
  reported by the checksum database matches the hash of the downloaded module.
  If, however, the checksum database returns a successful response that contains
  no entry for the module, the go command incorrectly permitted validation to succeed.

  A module proxy may mirror or proxy the checksum database, in which case the go
  command will not connect to the checksum database directly. Checksums reported
  by the checksum database are cryptographically signed, so a malicious proxy
  cannot alter the reported checksum for a module. However, a proxy which returns
  an empty checksum response, or a checksum response for an unrelated module,
  could cause the go command to proceed as if a downloaded module has been validated.

  The go command now properly checks checksum database responses to ensure
  that the expected module signature is present, not just that if a signature is
  present it matches the expectation.

  Thanks to Mundur (https://github.com/M0nd0R) for reporting this issue.

  This is CVE-2026-42501 and Go issue https://go.dev/issue/79070.

• net/http/httputil: ReverseProxy forwards queries with more than urlmaxqueryparams parameters

  When used with a Rewrite function, or a Director function which parses query parameters,
  ReverseProxy sanitizes the forwarded request to remove query parameters which are not
  parsed by url.ParseQuery. ReverseProxy did not take ParseQuery's limit on the total number
  of query parameters (controlled by GODEBUG=urlmaxqueryparams=N) into account.
  This could permit ReverseProxy to forward a request containing a query parameter
  that was not visible to the Rewrite function.

  For example, the query "a1=x&a2=x&...&a10000=x&hidden=y" could forward the parameter
  "hidden=y" while hiding it from the proxy's Rewrite function.

  ReverseProxy now avoids forwarding parameters that exceed the ParseQuery limit.

  This is CVE-2026-39825 and Go issue https://go.dev/issue/78948.

• net: panic in Dial and LookupPort when handling NUL byte on Windows

  The Dial and LookupPort functions would panic on Windows when provided
  with an input containing a NUL (0). These functions now return an error
  rather than panicking.

  This is CVE-2026-39836 and Go issue https://go.dev/issue/79006.

• net/mail: quadratic string concatenation in consumePhrase

  Pathological inputs could cause DoS through consumePhrase
  when parsing an email address according to RFC 5322.

  This is CVE-2026-42499 and Go issue https://go.dev/issue/78987.

• net/mail: quadratic string concatentation in consumeComment

  Well-crafted inputs reaching ParseAddress, ParseAddressList,
  and ParseDate were able to trigger excessive CPU exhaustion
  and memory allocations.

  This is CVE-2026-39820 and Go issue https://go.dev/issue/78566.

• cmd/go: "go bug" follows symlinks in predictable temporary filenames

  The "go bug" command wrote to two files with predictable names in
  the system temporary directory (for example, "/tmp").

  An attacker with access to the temporary directory could create a
  symlink in one of these names, causing "go bug" to overwrite the
  target of the symlink.

  The "go bug" command now uses os.MkdirTemp to create a safe
  working directory.

  Thanks to Harshit Gupta (Mr HAX) for reporting this issue.

  This is CVE-2026-39819 and Go issue https://go.dev/issue/78584.

• cmd/go: "go tool pack" does not sanitize output paths

  The "go tool pack" subcommand is a minimal version of the Unix ar utility.
  It is used by the compiler as an internal tool with known-good inputs.

  The "pack" subcommand did not sanitize output filenames.
  When invoked to extract a malicious archive file, it could write
  files to arbitrary locations on the filesystem.

  The "pack" subcommand now refuses to extract files with names
  containing any directory components.

  Thanks to Harshit Gupta (Mr HAX) for reporting this issue.

  This is CVE-2026-39817 and Go issue https://go.dev/issue/78778.

• net/http: infinite loop in HTTP/2 transport when given bad SETTINGS_MAX_FRAME_SIZE

  When processing HTTP/2 SETTINGS frames, transport will enter an infinite loop of
  writing CONTINUATION frames if it receives a SETTINGS_MAX_FRAME_SIZE with a
  value of 0.

  This allows potential DoS against a client by a malicious server. HTTP/2
  transport now properly checks that the received SETTINGS_MAX_FRAME_SIZE is
  valid.

  Thanks to Marwan Atia (marwans...@gmail.com) for reporting this issue.

  This is CVE-2026-33814 and Go issue https://go.dev/issue/78476.

• html/template: escaper bypass leads to XSS

  If a trusted template author were to write a
  tag containing an empty type attribute or a type
  attribute with an ASCII whitespace, the execution of
  the template would incorrectly escape any data passed
  into the block.

  Thanks to Mundur (https://github.com/M0nd0R) for reporting this issue.

  This is CVE-2026-39826 and Go issue https://go.dev/issue/78981.

• net: crash when handling long CNAME response

  When using LookupCNAME with the cgo DNS resolver,
  a very long CNAME response could trigger a double-free of C memory
  and a crash. The double-free has been fixed.

  Thanks to hamayanhamayan for reporting this issue.

  This is CVE-2026-33811 and Go issue https://go.dev/issue/78803.

• html/template: bypass of meta content URL escaping causes XSS

  CVE-2026-27142 fixed a vulnerability in which URLs were not
  correctly escaped inside of a tag's attribute.
  If the URL content were to insert ASCII whitespaces around the
  = rune inside of the attribute, the escaper would
  fail to similarly escape it, leading to XSS.

  Dynamic inputs to a tag's attribute are now
  whitespace sanitized prior to escaping.

  Thanks to Samy Ghannad for reporting this issue.

  This is CVE-2026-39823 and Go issue https://go.dev/issue/78913.

View the release notes for more information:
https://go.dev/doc/devel/release#go1.26.3

You can download binary and source distributions from the Go website:
https://go.dev/dl/

To compile from source using a Git clone, update to the release with
git checkout go1.26.3 and build as usual.

Thanks to everyone who contributed to the releases.

Cheers,
Cherry and Michael for the Go team