diff --git a/crates/oxide/src/scanner/mod.rs b/crates/oxide/src/scanner/mod.rs
index 30724f162188..9097bdd94554 100644
--- a/crates/oxide/src/scanner/mod.rs
+++ b/crates/oxide/src/scanner/mod.rs
@@ -18,6 +18,7 @@ use ignore::{gitignore::GitignoreBuilder, WalkBuilder};
 use init_tracing::{init_tracing, SHOULD_TRACE};
 use rayon::prelude::*;
 use std::collections::{BTreeMap, BTreeSet};
+use std::fs;
 use std::path::{Path, PathBuf};
 use std::sync::{Arc, Mutex};
 use std::time::SystemTime;
@@ -646,6 +647,11 @@ fn create_walker(sources: &Sources) -> Option<WalkBuilder> {
                 } else {
                     other_roots.insert(base);
                 }
+                if is_ignored_by_allowlist_gitignore(base) {
+                    let patterns = ignores.entry(base).or_default();
+                    patterns.insert("!/**/*".to_string());
+                    patterns.insert(BINARY_EXTENSIONS_GLOB.clone());
+                }
             }
             SourceEntry::Pattern { base, pattern } => {
                 let mut pattern = pattern.to_string();
@@ -848,6 +854,64 @@ fn create_walker(sources: &Sources) -> Option<WalkBuilder> {
     Some(builder)
 }
 
+/// Returns whether a path is ignored by a root `.gitignore` that starts as an allow-list.
+fn is_ignored_by_allowlist_gitignore(path: &Path) -> bool {
+    let Some(root) = path.ancestors().find(|parent| parent.join(".git").exists()) else {
+        return false;
+    };
+
+    let ignore_file = root.join(".gitignore");
+    if !ignore_file.exists() {
+        return false;
+    }
+
+    let Ok(ignore_content) = fs::read_to_string(&ignore_file) else {
+        return false;
+    };
+    if !gitignore_is_allowlist(&ignore_content) {
+        return false;
+    }
+
+    let mut builder = GitignoreBuilder::new(root);
+    for line in ignore_content.lines() {
+        if builder.add_line(Some(ignore_file.clone()), line).is_err() {
+            return false;
+        }
+    }
+
+    let Ok(ignore) = builder.build() else {
+        return false;
+    };
+    let matched = ignore.matched_path_or_any_parents(path, true);
+    matched.is_ignore()
+}
+
+/// Returns whether a `.gitignore` uses a bare `*` followed by negated allow-list rules.
+fn gitignore_is_allowlist(contents: &str) -> bool {
+    let mut has_star = false;
+    for line in contents.lines() {
+        let line = line.trim();
+        if line.is_empty() || line.starts_with('#') {
+            continue;
+        }
+
+        if line.starts_with('!') {
+            if has_star {
+                return true;
+            }
+
+            continue;
+        }
+
+        if line == "*" {
+            has_star = true;
+            continue;
+        }
+    }
+
+    false
+}
+
 #[cfg(test)]
 mod tests {
     use super::{ChangedContent, Scanner};
diff --git a/crates/oxide/tests/scanner.rs b/crates/oxide/tests/scanner.rs
index 4003ff253048..3138c9de10a5 100644
--- a/crates/oxide/tests/scanner.rs
+++ b/crates/oxide/tests/scanner.rs
@@ -1412,6 +1412,33 @@ mod scanner {
         assert_eq!(candidates, vec!["content-['index.html']"]);
     }
 
+    #[test]
+    fn test_explicit_source_can_override_allowlist_with_unwhitelisted_vendor() {
+        let ScanResult {
+            candidates, files, ..
+        } = scan_with_globs(
+            &[
+                (".gitignore", "*\n!/app\n!/app/design/**"),
+                ("app/design/index.html", "content-['app/design/index.html']"),
+                (
+                    "vendor/acme/theme/templates/component.phtml",
+                    "content-['vendor/acme/theme/templates/component.phtml']",
+                ),
+                (
+                    "vendor/acme/theme/templates/image.png",
+                    "content-['vendor/acme/theme/templates/image.png']",
+                ),
+            ],
+            vec!["@source './vendor/acme/theme'"],
+        );
+
+        assert_eq!(
+            candidates,
+            vec!["content-['vendor/acme/theme/templates/component.phtml']"]
+        );
+        assert_eq!(files, vec!["vendor/acme/theme/templates/component.phtml"]);
+    }
+
     #[test]
     fn test_allow_explicit_node_modules_paths() {
         // Create a temporary working directory