diff --git a/include/xrpl/basics/base64.h b/include/xrpl/basics/base64.h
index ed30e40a36..6b4cd26604 100644
--- a/include/xrpl/basics/base64.h
+++ b/include/xrpl/basics/base64.h
@@ -39,6 +39,35 @@
 
 namespace xrpl {
 
+namespace base64 {
+
+/**
+ * Returns the maximum number of characters needed to base64-encode @p nBytes bytes.
+ *
+ * @param nBytes Number of input bytes.
+ * @return Size of the encoded string, including padding.
+ */
+constexpr std::size_t
+encodedSize(std::size_t const nBytes)
+{
+    return 4 * ((nBytes + 2) / 3);
+}
+
+/**
+ * Returns the maximum number of bytes a base64 string of @p nChars characters
+ * decodes to.
+ *
+ * @param nChars Number of base64 characters.
+ * @return Upper bound on the number of decoded bytes.
+ */
+constexpr std::size_t
+decodedSize(std::size_t const nChars)
+{
+    return ((nChars / 4) * 3) + 2;
+}
+
+}  // namespace base64
+
 std::string
 base64Encode(std::uint8_t const* data, std::size_t len);
 
diff --git a/include/xrpl/server/Manifest.h b/include/xrpl/server/Manifest.h
index 70fc8f6f8f..fe5f1db985 100644
--- a/include/xrpl/server/Manifest.h
+++ b/include/xrpl/server/Manifest.h
@@ -1,10 +1,16 @@
 #pragma once
 
 #include <xrpl/basics/UnorderedContainers.h>
+#include <xrpl/basics/base64.h>
+#include <xrpl/basics/base_uint.h>
 #include <xrpl/beast/utility/Journal.h>
 #include <xrpl/protocol/PublicKey.h>
 #include <xrpl/protocol/SecretKey.h>
 
+#include <atomic>
+#include <cstddef>
+#include <cstdint>
+#include <functional>
 #include <optional>
 #include <shared_mutex>
 #include <string>
@@ -135,15 +141,47 @@ struct Manifest
 std::string
 to_string(Manifest const& m);
 
-/** Constructs Manifest from serialized string
+/**
+ *Largest a valid manifest can be, in decoded bytes.
+ *
+ *   A manifest has a fixed set of fields. Each is serialized as a field header
+ *   (1-2 bytes), an optional length prefix (1 byte for these sizes), and the
+ *   field body. Taking every field at its largest gives the maximum below, so
+ *   anything larger cannot be a valid manifest.
+ *
+ *       Field               header + length + body = bytes
+ *       sfVersion   (U16)        2          0     2     4
+ *       sfSequence  (U32)        1          0     4     5
+ *       sfPublicKey (33)         1          1    33    35
+ *       sfSigningPubKey (33)     1          1    33    35
+ *       sfSignature (72)         1          1    72    74
+ *       sfMasterSignature (72)   2          1    72    75
+ *       sfDomain    (128)        1          1   128   130
+ *                                                    -----
+ *                                                      358
+ */
+constexpr std::size_t kMaxManifestBytes = 358;
 
-    @param s Serialized manifest string
+/**
+ * Largest a valid manifest can be, in base64 characters.
+ *
+ *   base64 encodes 3 bytes as 4 characters, so this is the encoded form of
+ *   @ref kMaxManifestBytes. Callers that receive a base64 manifest should
+ *   reject anything longer than this before decoding, to avoid allocating
+ *   memory for an oversized input.
+ */
+constexpr std::size_t kMaxManifestBase64 = base64::encodedSize(kMaxManifestBytes);
 
-    @return `std::nullopt` if string is invalid
-
-    @note This does not verify manifest signatures.
-          `Manifest::verify` should be called after constructing manifest.
-*/
+/**
+ * Constructs Manifest from serialized string
+ *
+ * @param s Serialized manifest string
+ *
+ * @return `std::nullopt` if string is invalid
+ *
+ * @note This does not verify manifest signatures.
+ *       `Manifest::verify` should be called after constructing manifest.
+ */
 /** @{ */
 std::optional<Manifest>
 deserializeManifest(Slice s, beast::Journal journal);
diff --git a/src/libxrpl/basics/base64.cpp b/src/libxrpl/basics/base64.cpp
index 541ddd0839..7772752f40 100644
--- a/src/libxrpl/basics/base64.cpp
+++ b/src/libxrpl/basics/base64.cpp
@@ -76,20 +76,6 @@ getInverse()
     return &kTab[0];
 }
 
-/// Returns max chars needed to encode a base64 string
-constexpr std::size_t
-encodedSize(std::size_t n)
-{
-    return 4 * ((n + 2) / 3);
-}
-
-/// Returns max bytes needed to decode a base64 string
-constexpr std::size_t
-decodedSize(std::size_t n)
-{
-    return ((n / 4) * 3) + 2;
-}
-
 /** Encode a series of octets as a padded, base64 string.
 
     The resulting string will not be null terminated.
diff --git a/src/libxrpl/server/Manifest.cpp b/src/libxrpl/server/Manifest.cpp
index b18fdadbe2..b34955dc28 100644
--- a/src/libxrpl/server/Manifest.cpp
+++ b/src/libxrpl/server/Manifest.cpp
@@ -62,6 +62,11 @@ deserializeManifest(Slice s, beast::Journal journal)
     if (s.empty())
         return std::nullopt;
 
+    // A valid manifest has a fixed maximum size, so reject anything larger
+    // before parsing it.
+    if (s.size() > kMaxManifestBytes)
+        return std::nullopt;
+
     static SOTemplate const kManifestFormat{
         // A manifest must include:
         // - the master public key
diff --git a/src/xrpld/app/misc/detail/ValidatorList.cpp b/src/xrpld/app/misc/detail/ValidatorList.cpp
index ce07e0ec77..5c95eb553c 100644
--- a/src/xrpld/app/misc/detail/ValidatorList.cpp
+++ b/src/xrpld/app/misc/detail/ValidatorList.cpp
@@ -1130,6 +1130,15 @@ ValidatorList::applyList(
 
     json::Value list;
     auto const& manifest = localManifest ? *localManifest : globalManifest;
+    // Reject an oversized manifest before decoding it, so we do not allocate
+    // memory for an input that cannot be a valid manifest. deserializeManifest
+    // also enforces the decoded-byte limit, but checking here avoids the
+    // base64 decode entirely.
+    if (manifest.size() > kMaxManifestBase64)
+    {
+        JLOG(j_.warn()) << "UNL manifest exceeds maximum size";
+        return PublisherListStats{ListDisposition::Invalid};
+    }
     auto m = deserializeManifest(base64Decode(manifest));
     if (!m)
     {