diff --git a/src/commitSha.ts b/src/commitSha.ts
index c48ba8e..2fd8aa4 100644
--- a/src/commitSha.ts
+++ b/src/commitSha.ts
@@ -3,6 +3,7 @@ import * as github from '@actions/github'
 
 import {Env} from './env'
 import {Inputs} from './inputs'
+import { isSafeGitArg } from './utils'
 import {
   canDiffCommits,
   cleanShaInput,
@@ -343,6 +344,31 @@ export const getSHAForPullRequestEvent = async ({
 }: SHAForPullRequestEvent): Promise<DiffResult> => {
   let targetBranch = github.context.payload.pull_request?.base?.ref
   const currentBranch = github.context.payload.pull_request?.head?.ref
+
+  // Validate branch/ref names and any gitFetchExtraArgs that may be tainted
+  import { isSafeGitArg } from './utils'
+  const argListToValidate = [
+    ...(Array.isArray(gitFetchExtraArgs) ? gitFetchExtraArgs : []),
+    remoteName
+  ]
+  if (currentBranch && !isSafeGitArg(currentBranch)) {
+    throw new Error(
+      `Unsafe branch name detected in PR head.ref: "${currentBranch}". Possible injection attempt.`
+    )
+  }
+  if (targetBranch && !isSafeGitArg(targetBranch)) {
+    throw new Error(
+      `Unsafe branch name detected in PR base.ref: "${targetBranch}". Possible injection attempt.`
+    )
+  }
+  for (const a of argListToValidate) {
+    if (!isSafeGitArg(a)) {
+      throw new Error(
+        `Unsafe extra git argument detected: "${a}". Possible injection attempt.`
+      )
+    }
+  }
+
   if (inputs.sinceLastRemoteCommit) {
     targetBranch = currentBranch
   }
diff --git a/src/utils.ts b/src/utils.ts
index 5ca3bde..0f980ad 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -284,6 +284,28 @@ export const submoduleExists = async ({
   return stdout.trim() !== ''
 }
 
+/**
+ * Checks if a string is safe to use as a git argument (prevents option injection)
+ * @param val - the argument to check
+ * @returns true if safe, false otherwise
+ */
+export function isSafeGitArg(val: string): boolean {
+  // Disallow args that start with dash, are empty, or obviously suspicious
+  if (!val || typeof val !== 'string') return false
+  if (val.startsWith('-')) return false
+  // Disallow sequences forbidden in git refs: spaces, control chars, "..", ".lock", etc.
+  if (/[ \t\n\r]/.test(val)) return false
+  if (val.includes('..')) return false
+  if (val.endsWith('.lock')) return false
+  if (val.includes('@{')) return false
+  if (val.includes('\\')) return false
+  if (val.startsWith('/')) return false
+  if (val.endsWith('/')) return false
+  if (val.includes('//')) return false
+  if (val.match(/[\x00-\x1F\x7F]/)) return false
+  return true
+}
+
 /**
  * Fetches the git repository
  * @param args - arguments for fetch command