When a server is configured to support unencrypted HTTP/2, it reads a few bytes from each new connection to see if they contain the HTTP/2 client preface. ReadHeaderTimeout is unexpectedly not being applied when doing this.

Enforce a recursion limit in Unmarshal to prevent stack exhaustion when parsing deeply-nested, recursive structures.

-------- Forwarded Message -------- Subject: [security] Go 1.26.6 and Go 1.25.13 are released Date: Thu, 13 Aug 2026 18:54:28 +0000 From: announce () golang org To: golang-nuts () googlegroups com

Hello gophers,

We have just released Go versions 1.26.6 and 1.25.13, minor point releases.

These releases include 10 security fixes following the security policy <https://go.dev/doc/security/policy>:

x/mod/sumdb/tlog: fix transparency log tile verification bypass

A malicious GOPROXY was previously capable of forging up to two sumdb tiles that allow for a requested module to bypass the GOSUMDB check and persist attacker-controlled module content to a local Go module cache.

This attack allows for a malicious GOPROXY to serve malicious module content that cannot be detected by evaluating the transparency log.

All tiles are now correctly verified against their parents.

In order to determine if you have been affected:

rm -r go.sum go.work.sum vendor/ && go mod tidy

Thanks to Filippo Valsorda (Geomys) for reporting this issue.

This is CVE-2026-56865 and Go issue https://go.dev/issue/80744 <https://go.dev/issue/80744>.

x/mod/sumdb: ignore unrelated, unauthenticated hashes in Lookup

A malicious GOSUMDB was capable of serving arbitrary module content not contained within the transparency log.

This attack allows for a coordinating GOPROXY and GOSUMDB to serve a client malicious module content that cannot be detected by evaluating the transparency log.

In order to determine if you have been affected:

rm -r go.sum go.work.sum vendor/ && go mod tidy

Thanks to mundur for reporting this issue.

This is CVE-2026-56864 and Go issue https://go.dev/issue/80745 <https://go.dev/issue/80745>.

encoding/xml: add recursion depth guard during decode

Previously, DecodeElement would reset the depth counter causing it to never fire; this could lead to stack exhaustion.

This is CVE-2026-56859 and Go issue https://go.dev/issue/80481 <https://go.dev/issue/80481>.

net/http: apply ReadHeaderTimeout when doing unencrypted HTTP/2 check

When a server is configured to support unencrypted HTTP/2, it reads a few bytes from each new connection to see if they contain the HTTP/2 client preface. Previously, this was being done with no timeout applied. ReadHeaderTimeout is now applied for this.

This is CVE-2026-56853 and Go issue https://go.dev/issue/80205 <https://go.dev/issue/80205>.

net/url: avoid quadratic complexity in resolvePath

Previously, resolving relative paths containing parent directory (|..|) segments performed string conversions and buffer rewrites on each step, resulting in quadratic time complexity and high memory allocation overhead.

Now, path resolution operates on a byte buffer using index-based backtracking for |..| segments, eliminating the quadratic time complexity and significantly reducing memory allocations.

This is CVE-2026-56860 and Go issue https://go.dev/issue/80494 <https://go.dev/issue/80494>.

golang.org/x/net/dns/dnsmessage: panic when parsing invalid SVCB record

Parsing an invalid SVCB or HTTPS RR can panic when the size of a parameter value overflows the message buffer.

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

This is CVE-2026-46600 and Go issue https://go.dev/issue/79795 <https://go.dev/issue/79795>.

crypto/tls: limit handshake messages we are willing to accept post-handshake

Previously, we always counted handshake messages, such as KeyUpdate, as state-advancing, regardless of whether a handshake has been completed or not. As a result, a malicious client can keep sending KeyUpdate messages to force the server to keep performing key derivation operations indefinitely.

Thanks to Qi Deng of Aurascape.ai for reporting this issue.

This is CVE-2026-56862 and Go issue https://go.dev/issue/80528 <https://go.dev/issue/80528>.

html/template: fix Javascript regexp context tracking

Previously, pathological inputs could close an unescaped |/| early, allowing for attack-controlled data to inject arbitrary content, potentially leading to XSS.

Thanks to Ali Sherif for reporting this issue.

This is CVE-2026-56858 and Go issue https://go.dev/issue/80435 <https://go.dev/issue/80435>.

x/net/idna: failure to reject ASCII-only Punycode-encoded labels

The ToASCII and ToUnicode functions incorrectly accepted Punycode-encoded labels that decode to an ASCII-only label. For example, ToUnicode("xn--example-.com") incorrectly returned the name "example.com" rather than an error.

The idna package implements the processing algorithm from UTS 46. Older versions of UTS 46 included a specification bug which permitted multiple ASCII labels to decode to the same Unicode label. UTS 46 revision 33 fixed the specification bug. The idna package now implements the updated specification.

This behavior can lead to privilege escalation in programs using the idna package. For example, a program which performs privilege checks on the ASCII hostname may reject "example.com" but permit "xn--example-.com". If that program subsequently converts the ASCII hostname to Unicode, it will inadvertently permits access to the Unicode name "example.com".

Thanks to KC1zs4 (https://github.com/KC1zs4 <https://github.com/KC1zs4>) for reporting this issue.

This is CVE-2026-39821 and Go issue https://go.dev/issue/78760 <https://go.dev/issue/78760>.

encoding/asn1: enforce maximum recursion depth

Enforce a recursion limit in Unmarshal to prevent stack exhaustion when parsing deeply-nested, recursive structures.

Thanks to Marwan Atia (marwansamir688 () gmail com <mailto:marwansamir688 () gmail com>) for reporting this issue.

This is CVE-2026-33818 and Go issue https://go.dev/issue/80405 <https://go.dev/issue/80405>.

View the release notes for more information: https://go.dev/doc/devel/release#go1.26.6 <https://go.dev/doc/devel/release#go1.26.6>

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

To compile from source using a Git clone, update to the release with |git checkout go1.26.6| and build as usual.

Thanks to everyone who contributed to the releases.

Cheers, Dmitri and Mark for the Go team

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On Unix systems, opening a file in an os.Root improperly follows symlinks to locations outside of the Root when the final path component of the a path is a symbolic link and the path ends in /. For example, 'root.Open("symlink/")' will open "symlink" even when "symlink" is a symbolic link pointing outside of the root.

(x509.Certificate).VerifyHostname previously called matchHostnames in a loop over all DNS Subject Alternative Name (SAN) entries. This caused strings.Split(host, ".") to execute repeatedly on the same input hostname. With a large DNS SAN list, verification costs scaled quadratically based on the number of SAN entries multiplied by the hostname's label count. Because x509.Verify validates hostnames before building the certificate chain, this overhead occurred even for untrusted certificates.

Decoding a maliciously-crafted MIME header containing many invalid encoded-words can consume excessive CPU.

https://groups.google.com/g/golang-announce/c/qcCIEXso47M announces: 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 SETTINGSMAXFRAMESIZE

When processing HTTP/2 SETTINGS frames, transport will enter an infinite loop of writing CONTINUATION frames if it receives a SETTINGSMAXFRAMESIZE 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 SETTINGSMAXFRAMESIZE 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

ReverseProxy can forward queries containing parameters not visible to Rewrite functions. 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 does not take ParseQuery's limit on the total number of query parameters (controlled by GODEBUG=urlmaxqueryparams=N) into account. This can permit ReverseProxy to forward a request containing a query parameter that is not visible to the Rewrite function. For example, the query "a1=x&a2=x&...&a10000=x&hidden=y" can forward the parameter "hidden=y" while hiding it from the proxy's Rewrite function.

The "go tool pack" subcommand (usually used only by the compiler as an internal tool with known-good inputs) does not sanitize output filenames. Extracting a malicious archive file with the "pack" subcommand can write files to arbitrary locations on the filesystem.

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

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

The "go tool pack" subcommand (usually used only by the compiler as an internal tool with known-good inputs) does not sanitize output filenames. Extracting a malicious archive file with the "pack" subcommand can write files to arbitrary locations on the filesystem.

The "go bug" command writes to two files with predictable names in the system temporary directory (for example, "/tmp"). An attacker with access to the temporary directory can create a symlink in one of these names, causing "go bug" to overwrite the target of the symlink.

ReverseProxy can forward queries containing parameters not visible to Rewrite functions. 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 does not take ParseQuery's limit on the total number of query parameters (controlled by GODEBUG=urlmaxqueryparams=N) into account. This can permit ReverseProxy to forward a request containing a query parameter that is not visible to the Rewrite function. For example, the query "a1=x&a2=x&...&a10000=x&hidden=y" can forward the parameter "hidden=y" while hiding it from the proxy's Rewrite function.

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

Let's Encrypt client and ACME library written in Go (Lego). Prior to 4.34.0, the webroot HTTP-01 challenge provider in lego is vulnerable to arbitrary file write and deletion via path traversal. A malicious ACME server can supply a crafted challenge token containing ../ sequences, causing lego to write attacker-influenced content to any path writable by the lego process. This vulnerability is fixed in 4.34.0.

During chain building, the amount of work that is done is not correctly limited when a large number of intermediate certificates are passed in VerifyOptions.Intermediates, which can lead to a denial of service. This affects both direct users of crypto/x509 and users of crypto/tls.

When verifying a certificate chain containing excluded DNS constraints, these constraints are not correctly applied to wildcard DNS SANs which use a different case than the constraint. This only affects validation of otherwise trusted certificate chains, issued by a root CA in the VerifyOptions.Roots CertPool, or in the system certificate pool.

Validating certificate chains which use policies is unexpectedly inefficient when certificates in the chain contain a very large number of policy mappings, possibly causing denial of service. This only affects validation of otherwise trusted certificate chains, issued by a root CA in the VerifyOptions.Roots CertPool, or in the system certificate pool.

During chain building, the amount of work that is done is not correctly limited when a large number of intermediate certificates are passed in VerifyOptions.Intermediates, which can lead to a denial of service. This affects both direct users of crypto/x509 and users of crypto/tls.

Arithmetic over induction variables in loops were not correctly checked for underflow or overflow. As a result, the compiler would allow for invalid indexing to occur at runtime, potentially leading to memory corruption.

If one side of the TLS connection sends multiple key update messages post-handshake in a single record, the connection can deadlock, causing uncontrolled consumption of resources. This can lead to a denial of service. This only affects TLS 1.3.

SWIG file names containing 'cgo' and well-crafted payloads could lead to code smuggling and arbitrary code execution at build time due to trust layer bypass.

Case-sensitive excludedSubtrees name constraints cause Auth Bypass in crypto/x509

On Linux, if the target of Root.Chmod is replaced with a symlink while the chmod operation is in progress, Chmod can operate on the target of the symlink, even when the target lies outside the root. The Linux fchmodat syscall silently ignores the ATSYMLINKNOFOLLOW flag, which Root.Chmod uses to avoid symlink traversal. Root.Chmod checks its target before acting and returns an error if the target is a symlink lying outside the root, so the impact is limited to cases where the target is replaced with a symlink between the check and operation.

url.Parse insufficiently validated the host/authority component and accepted some invalid URLs.

url.Parse insufficiently validated the host/authority component and accepted some invalid URLs.

The Go MCP SDK used Go's standard encoding/json.Unmarshal for JSON-RPC and MCP protocol message parsing in versions prior to 1.3.1. Go's standard library performs case-insensitive matching of JSON keys to struct field tags — a field tagged json:"method" would also match "Method", "METHOD", etc. This violated the JSON-RPC 2.0 specification, which defines exact field names. A malicious MCP peer may have been able to send protocol messages with non-standard field casing that the SDK would silently accept. This had the potential for bypassing intermediary inspection and coss-implementation inconsistency. Go's standard JSON unmarshaling was replaced with a case-sensitive decoder in commit 7b8d81c. Users are advised to update to v1.3.1 to resolve this issue.

During session resumption in crypto/tls, if the underlying Config has its ClientCAs or RootCAs fields mutated between the initial handshake and the resumed handshake, the resumed handshake may succeed when it should have failed. This may happen when a user calls Config.Clone and mutates the returned Config, or uses Config.GetConfigForClient. This can cause a client to resume a session with a server that it would not have resumed with during the initial handshake, or cause a server to resume a session with a client that it would not have resumed with during the initial handshake.

The html.Parse function in golang.org/x/net/html has an infinite parsing loop when processing certain inputs, which can lead to denial of service (DoS) if an attacker provides specially crafted HTML content.