Forbid path traversal ('.' and '..') as @path parameters.

They're likely to have the unintended effect. For example, passing ".."
to @delete /account/book/{isbn}/ yields @delete /account/.

retrofit/src/main/java/retrofit2/RequestBuilder.java

@@ -16,6 +16,7 @@
 package retrofit2;
 
 import java.io.IOException;
+import java.util.regex.Pattern;
 import javax.annotation.Nullable;
 import okhttp3.FormBody;
 import okhttp3.Headers;
@@ -32,6 +33,21 @@ final class RequestBuilder {
       { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };
   private static final String PATH_SEGMENT_ALWAYS_ENCODE_SET = " \"<>^`{}|\\?#";
 
+  /**
+   * Matches strings that contain {@code .} or {@code ..} as a complete path segment. This also
+   * matches dots in their percent-encoded form, {@code %2E}.
+   *
+   * <p>It is okay to have these strings within a larger path segment (like {@code a..z} or {@code
+   * index.html}) but when alone they have a special meaning. A single dot resolves to no path
+   * segment so {@code /one/./three/} becomes {@code /one/three/}. A double-dot pops the preceding
+   * directory, so {@code /one/../three/} becomes {@code /three/}.
+   *
+   * <p>We forbid these in Retrofit paths because they're likely to have the unintended effect.
+   * For example, passing {@code ..} to {@code DELETE /account/book/{isbn}/} yields {@code DELETE
+   * /account/}.
+   */
+  private static final Pattern PATH_TRAVERSAL = Pattern.compile("(.*/)?(\\.|%2e|%2E){1,2}(/.*)?");
+
   private final String method;
 
   private final HttpUrl baseUrl;
@@ -91,7 +107,13 @@ void addPathParam(String name, String value, boolean encoded) {
       // The relative URL is cleared when the first query parameter is set.
       throw new AssertionError();
     }
-    relativeUrl = relativeUrl.replace("{" + name + "}", canonicalizeForPath(value, encoded));
+    String replacement = canonicalizeForPath(value, encoded);
+    String newRelativeUrl = relativeUrl.replace("{" + name + "}", replacement);
+    if (PATH_TRAVERSAL.matcher(newRelativeUrl).matches()) {
+      throw new IllegalArgumentException(
+          "@Path parameters shouldn't perform path traversal ('.' or '..'): " + value);
+    }
+    relativeUrl = newRelativeUrl;
   }
 
   private static String canonicalizeForPath(String input, boolean alreadyEncoded) {

retrofit/src/test/java/retrofit2/RequestFactoryTest.java

@@ -878,6 +878,88 @@ Call<ResponseBody> method(@Path(value = "ping", encoded = true) String ping) {
     assertThat(request.body()).isNull();
   }
 
+  @Test public void pathParametersAndPathTraversal() {
+    class Example {
+      @GET("/foo/bar/{ping}/") //
+      Call<ResponseBody> method(@Path(value = "ping") String ping) {
+        return null;
+      }
+    }
+
+    assertMalformedRequest(Example.class, ".");
+    assertMalformedRequest(Example.class, "..");
+
+    assertThat(buildRequest(Example.class, "./a").url().encodedPath())
+        .isEqualTo("/foo/bar/.%2Fa/");
+    assertThat(buildRequest(Example.class, "a/.").url().encodedPath())
+        .isEqualTo("/foo/bar/a%2F./");
+    assertThat(buildRequest(Example.class, "a/..").url().encodedPath())
+        .isEqualTo("/foo/bar/a%2F../");
+    assertThat(buildRequest(Example.class, "../a").url().encodedPath())
+        .isEqualTo("/foo/bar/..%2Fa/");
+    assertThat(buildRequest(Example.class, "..\\..").url().encodedPath())
+        .isEqualTo("/foo/bar/..%5C../");
+
+    assertThat(buildRequest(Example.class, "%2E").url().encodedPath())
+        .isEqualTo("/foo/bar/%252E/");
+    assertThat(buildRequest(Example.class, "%2E%2E").url().encodedPath())
+        .isEqualTo("/foo/bar/%252E%252E/");
+  }
+
+  @Test public void encodedPathParametersAndPathTraversal() {
+    class Example {
+      @GET("/foo/bar/{ping}/") //
+      Call<ResponseBody> method(@Path(value = "ping", encoded = true) String ping) {
+        return null;
+      }
+    }
+
+    assertMalformedRequest(Example.class, ".");
+    assertMalformedRequest(Example.class, "%2E");
+    assertMalformedRequest(Example.class, "%2e");
+    assertMalformedRequest(Example.class, "..");
+    assertMalformedRequest(Example.class, "%2E.");
+    assertMalformedRequest(Example.class, "%2e.");
+    assertMalformedRequest(Example.class, ".%2E");
+    assertMalformedRequest(Example.class, ".%2e");
+    assertMalformedRequest(Example.class, "%2E%2e");
+    assertMalformedRequest(Example.class, "%2e%2E");
+    assertMalformedRequest(Example.class, "./a");
+    assertMalformedRequest(Example.class, "a/.");
+    assertMalformedRequest(Example.class, "../a");
+    assertMalformedRequest(Example.class, "a/..");
+    assertMalformedRequest(Example.class, "a/../b");
+    assertMalformedRequest(Example.class, "a/%2e%2E/b");
+
+    assertThat(buildRequest(Example.class, "...").url().encodedPath())
+        .isEqualTo("/foo/bar/.../");
+    assertThat(buildRequest(Example.class, "a..b").url().encodedPath())
+        .isEqualTo("/foo/bar/a..b/");
+    assertThat(buildRequest(Example.class, "a..").url().encodedPath())
+        .isEqualTo("/foo/bar/a../");
+    assertThat(buildRequest(Example.class, "a..b").url().encodedPath())
+        .isEqualTo("/foo/bar/a..b/");
+    assertThat(buildRequest(Example.class, "..b").url().encodedPath())
+        .isEqualTo("/foo/bar/..b/");
+    assertThat(buildRequest(Example.class, "..\\..").url().encodedPath())
+        .isEqualTo("/foo/bar/..%5C../");
+  }
+
+  @Test public void dotDotsOkayWhenNotFullPathSegment() {
+    class Example {
+      @GET("/foo{ping}bar/") //
+      Call<ResponseBody> method(@Path(value = "ping", encoded = true) String ping) {
+        return null;
+      }
+    }
+
+    assertMalformedRequest(Example.class, "/./");
+    assertMalformedRequest(Example.class, "/../");
+
+    assertThat(buildRequest(Example.class, ".").url().encodedPath()).isEqualTo("/foo.bar/");
+    assertThat(buildRequest(Example.class, "..").url().encodedPath()).isEqualTo("/foo..bar/");
+  }
+
   @Test public void pathParamRequired() {
     class Example {
       @GET("/foo/bar/{ping}/") //
@@ -2783,4 +2865,12 @@ static <T> Request buildRequest(Class<T> cls, Object... args) {
 
     return buildRequest(cls, retrofitBuilder, args);
   }
+
+  static void assertMalformedRequest(Class<?> cls, Object... args) {
+    try {
+      Request request = buildRequest(cls, args);
+      fail("expected a malformed request but was " + request);
+    } catch (IllegalArgumentException expected) {
+    }
+  }
 }