mod_mime_magic: Disable decompression by default, add config option

to enable it.

* modules/metadata/mod_mime_magic.c (magic_server_config_rec): Add                                     
  decompression_enabled field.                                                                         
  (create_magic_server_config, merge_magic_server_config): Initialize and                              
  merge decompression_enabled.                                                                         
  (set_decompression): New function.                                                                   
  (magic_cmds): Add MimeMagicDecompression directive.                                                  
  (tryit): Only decompress files when decompression is enabled.                                        
                                                                                                       
* docs/manual/mod/mod_mime_magic.xml: Document MimeMagicDecompression                                  
  directive.                                                                                          

PR: 69985
Submitted by: Gordon Messmer <gmessmer redhat.com>
Github: closes #625


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1932902 13f79535-47bb-0310-9956-ffa450edef68

Author: notroj

changes-entries/pr69985.txt

@@ -0,0 +1,4 @@
+  *) mod_mime_magic: Add MimeMagicDecompression directive to control
+     decompression of compressed files for MIME type detection.
+     Decompression is now disabled by default. PR 69985.
+     [Gordon Messmer <gmessmer redhat.com>]

docs/manual/mod/mod_mime_magic.xml

@@ -271,4 +271,64 @@ using the specified magic file</description>
 </usage>
 </directivesynopsis>
 
+<directivesynopsis>
+<name>MimeMagicDecompression</name>
+<description>Enable decompression of compressed files for MIME type detection</description>
+<syntax>MimeMagicDecompression On|Off</syntax>
+<default>MimeMagicDecompression Off</default>
+<contextlist><context>server config</context><context>virtual host</context>
+</contextlist>
+
+<usage>
+    <p>The <directive>MimeMagicDecompression</directive> directive controls
+    whether <module>mod_mime_magic</module> will attempt to decompress files
+    that appear to be compressed (gzip, compress, etc.) in order to determine
+    the MIME type of the uncompressed content. This feature is <strong>disabled
+    by default</strong> and should only be enabled if you understand the
+    significant drawbacks. It exists to maintain backward compatibility with
+    previous releases of httpd, but its use is discouraged.</p>
+
+    <note type="warning"><title>Security and Compatibility Issues</title>
+    <p>This feature has several serious flaws and is disabled by default:</p>
+    <ol>
+    <li><strong>Not RFC-compliant:</strong> Standards documents consistently
+    recommend against setting Content-Encoding for files that are already
+    compressed (such as .zip or .gz files). See
+    <a href="https://www.rfc-editor.org/rfc/rfc9110.html#name-content-encoding">RFC 9110</a>.</li>
+
+    <li><strong>Breaks content integrity:</strong> When Content-Encoding is set,
+    most HTTP clients will decompress the file before writing it to disk. This
+    causes the downloaded file to have a different size and checksum than the
+    original, breaking signature verification and checksum validation. Software
+    distribution sites will find this particularly problematic.</li>
+
+    <li><strong>Unpredictable behavior:</strong> This feature only applies to
+    files that don't match a MIME type via file extension. This can lead to
+    inconsistent behavior where some files in a directory are affected and
+    others are not, making problems difficult to diagnose.</li>
+
+    <li><strong>Performance impact:</strong> Decompression requires forking and
+    executing an external <code>gzip</code> process for each compressed file,
+    which adds significant overhead.</li>
+
+    <li><strong>Security risk:</strong> Passing untrusted uploaded file data to
+    an external binary (<code>gzip</code>) could potentially expose the server to
+    compression bombs, resource exhaustion, or remote code execution
+    vulnerabilities in the decompression tool.</li>
+    </ol>
+    </note>
+
+    <example><title>Example (not recommended)</title>
+    <highlight language="config">
+# Only enable if you fully understand the risks
+MimeMagicDecompression On
+    </highlight>
+    </example>
+
+    <p>In most cases, it is better to ensure files have proper extensions
+    that can be mapped via <module>mod_mime</module> rather than relying on
+    this feature.</p>
+</usage>
+</directivesynopsis>
+
 </modulesynopsis>

modules/metadata/mod_mime_magic.c

@@ -118,6 +118,8 @@
 
 #define MAXMIMESTRING        256
 
+#define UNSET -1
+
 /* HOWMANY must be at least 4096 to make gzip -dcq work */
 #define HOWMANY  4096
 /* SMALL_HOWMANY limits how much work we do to figure out text files */
@@ -456,6 +458,7 @@ typedef struct {
     const char *magicfile;    /* where magic be found */
     struct magic *magic;      /* head of magic config list */
     struct magic *last;
+    int decompression_enabled; /* whether to decompress files for content detection */
 } magic_server_config_rec;
 
 /* per-request info */
@@ -472,8 +475,11 @@ module AP_MODULE_DECLARE_DATA mime_magic_module;
 
 static void *create_magic_server_config(apr_pool_t *p, server_rec *d)
 {
+    magic_server_config_rec *conf;
     /* allocate the config - use pcalloc because it needs to be zeroed */
-    return apr_pcalloc(p, sizeof(magic_server_config_rec));
+    conf = apr_pcalloc(p, sizeof(magic_server_config_rec));
+    conf->decompression_enabled = UNSET;
+    return conf;
 }
 
 static void *merge_magic_server_config(apr_pool_t *p, void *basev, void *addv)
@@ -484,6 +490,8 @@ static void *merge_magic_server_config(apr_pool_t *p, void *basev, void *addv)
                             apr_palloc(p, sizeof(magic_server_config_rec));
 
     new->magicfile = add->magicfile ? add->magicfile : base->magicfile;
+    new->decompression_enabled = (add->decompression_enabled != UNSET) ?
+                                  add->decompression_enabled : base->decompression_enabled;
     new->magic = NULL;
     new->last = NULL;
     return new;
@@ -502,6 +510,16 @@ static const char *set_magicfile(cmd_parms *cmd, void *dummy, const char *arg)
     return NULL;
 }
 
+static const char *set_decompression(cmd_parms *cmd, void *dummy, int arg)
+{
+    magic_server_config_rec *conf = (magic_server_config_rec *)
+    ap_get_module_config(cmd->server->module_config,
+                      &mime_magic_module);
+
+    conf->decompression_enabled = arg;
+    return NULL;
+}
+
 /*
  * configuration file commands - exported to Apache API
  */
@@ -510,6 +528,13 @@ static const command_rec mime_magic_cmds[] =
 {
     AP_INIT_TAKE1("MimeMagicFile", set_magicfile, NULL, RSRC_CONF,
      "Path to MIME Magic file (in file(1) format)"),
+    AP_INIT_FLAG("MimeMagicDecompression", set_decompression, NULL, RSRC_CONF,
+     "Enable decompression of compressed files for content type detection "
+     "(Off by default). WARNING: This feature is NOT RFC-compliant, can be "
+     "unpredictable, breaks content integrity (clients will decompress files "
+     "causing checksum mismatches), impacts performance (fork/exec overhead), "
+     "and is unsafe (passes untrusted data to external gzip binary). "
+     "Use only if you understand these risks."),
     {NULL}
 };
 
@@ -878,10 +903,13 @@ static int magic_process(request_rec *r)
 static int tryit(request_rec *r, unsigned char *buf, apr_size_t nb,
                  int checkzmagic)
 {
+    magic_server_config_rec *conf = (magic_server_config_rec *)
+                ap_get_module_config(r->server->module_config, &mime_magic_module);
+
     /*
-     * Try compression stuff
+     * Try compression stuff (only if decompression is enabled)
      */
-    if (checkzmagic == 1) {
+    if (checkzmagic == 1 && conf && conf->decompression_enabled == 1) {
         if (zmagic(r, buf, nb) == 1)
             return OK;
     }