diff --git a/src/cert.rs b/src/cert.rs
index 75e022a1..fce500b5 100644
--- a/src/cert.rs
+++ b/src/cert.rs
@@ -13,6 +13,8 @@
 // OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
 use crate::{der, signed_data, Error};
+#[cfg(feature = "std")]
+use std::collections::HashMap;
 
 pub enum EndEntityOrCa<'a> {
     EndEntity,
@@ -32,6 +34,8 @@ pub struct Cert<'a> {
     pub eku: Option<untrusted::Input<'a>>,
     pub name_constraints: Option<untrusted::Input<'a>>,
     pub subject_alt_name: Option<untrusted::Input<'a>>,
+    #[cfg(feature = "std")]
+    pub unrecognized_extensions: HashMap<&'a[u8], untrusted::Input<'a>>,
 }
 
 pub fn parse_cert<'a>(
@@ -93,6 +97,8 @@ pub(crate) fn parse_cert_internal<'a>(
             eku: None,
             name_constraints: None,
             subject_alt_name: None,
+            #[cfg(feature = "std")]
+            unrecognized_extensions: HashMap::new(),
         };
 
         // mozilla::pkix allows the extensions to be omitted. However, since
@@ -166,42 +172,64 @@ enum Understood {
 
 fn remember_extension<'a>(
     cert: &mut Cert<'a>,
-    extn_id: untrusted::Input,
+    extn_id: untrusted::Input<'a>,
     value: untrusted::Input<'a>,
 ) -> Result<Understood, Error> {
     // We don't do anything with certificate policies so we can safely ignore
     // all policy-related stuff. We assume that the policy-related extensions
     // are not marked critical.
-
-    // id-ce 2.5.29
-    static ID_CE: [u8; 2] = oid![2, 5, 29];
-
-    if extn_id.len() != ID_CE.len() + 1 || !extn_id.as_slice_less_safe().starts_with(&ID_CE) {
-        return Ok(Understood::No);
-    }
-
-    let out = match *extn_id.as_slice_less_safe().last().unwrap() {
+    const ID_CE_KEY_USAGE: &[u8] = &oid![2, 5, 29, 15];
+    const ID_CE_SUBJECT_ALT_NAME: &[u8] = &oid![2, 5, 29, 17];
+    const ID_CE_BASIC_CONSTRAINTS: &[u8] = &oid![2, 5, 29, 19];
+    const ID_CE_NAME_CONSTRAINTS: &[u8] = &oid![2, 5, 29, 30];
+    const ID_CE_EXT_KEY_USAGE: &[u8] = &oid![2, 5, 29, 37];
+
+    let extension_id = &*extn_id.as_slice_less_safe();
+    // note: the "strange" arrays below could be generated by the `oid!` macro. However,
+    // I can't find a way to make Rust let me put the output of that macro in a match expression.
+    // It looks like it's in progress, however (https://github.com/rust-lang/rust/issues/74446).
+    // Until then, I'm afraid we're left with this jankiness (suggestions appreciated)
+    let out = match extension_id {
         // id-ce-keyUsage 2.5.29.15. We ignore the KeyUsage extension. For CA
         // certificates, BasicConstraints.cA makes KeyUsage redundant. Firefox
         // and other common browsers do not check KeyUsage for end-entities,
         // though it would be kind of nice to ensure that a KeyUsage without
         // the keyEncipherment bit could not be used for RSA key exchange.
-        15 => {
+        ID_CE_KEY_USAGE => {
             return Ok(Understood::Yes);
         }
 
         // id-ce-subjectAltName 2.5.29.17
-        17 => &mut cert.subject_alt_name,
+        ID_CE_SUBJECT_ALT_NAME => &mut cert.subject_alt_name,
 
         // id-ce-basicConstraints 2.5.29.19
-        19 => &mut cert.basic_constraints,
+        ID_CE_BASIC_CONSTRAINTS => &mut cert.basic_constraints,
 
         // id-ce-nameConstraints 2.5.29.30
-        30 => &mut cert.name_constraints,
+        ID_CE_NAME_CONSTRAINTS => &mut cert.name_constraints,
 
         // id-ce-extKeyUsage 2.5.29.37
-        37 => &mut cert.eku,
-
+        ID_CE_EXT_KEY_USAGE => &mut cert.eku,
+
+        //This is not a recognized extension, add it to the unrecognized
+        // extension hash and return `Understood::No`
+        #[cfg(feature = "std")]
+        _ => { 
+            let value = value.read_all(Error::BadDER, |value| {
+                Ok(value.read_bytes_to_end())
+            })?;
+            match cert.unrecognized_extensions.insert(extension_id, value) {
+                Some(_) => {
+                    // There appears to be two unrecognized extensions with the same ID.
+                    return Err(Error::ExtensionValueInvalid);
+                }
+                None => {
+                    // Insertion was successful, and the extension is unique to the certificate
+                    return Ok(Understood::No)
+                },
+            }
+        }
+        #[cfg(not(feature = "std"))]
         _ => {
             return Ok(Understood::No);
         }